Titanium Alloys: Russian Aircraft and Aerospace Applications

Titanium Alloys Russian Aircraft and Aerospace Applications

Advances in Metallic Alloys A series edited by J.N. Fridlyander, All-Russian Institute of Aviation Materials, Moscow, Russia and D.G. Eskin, Netherlands Institute for Metals Research, Delft, The Netherlands Volume 1 Liquid Metal Processing: Applications to Aluminum Alloy Production I.G. Brodova, P.S. Popel and G.I. Eskin Volume 2 Iron in Aluminum Alloys: Impurity and Alloying Elment N.W. Belov, A.A. Aksenov and D.G. Eskin Volume 3 Magnesium Alloy Containing Rare Earth Metals: Structure and Properties L.L. Rokhlin Volume 4 Phase Transformations of Elements Under High Pressure E.Yu Tonkov and E.G. Ponyatovsky Volume 5 Titanium Alloys: Russian Aircraft and Aerospace Applications Valentin N. Moiseyev

Titanium Alloys Russian Aircraft and Aerospace Applications

Valentin N. Moiseyev

Boca Raton London New York Singapore

A CRC title, part of the Taylor & Francis imprint, a member of the Taylor & Francis Group, the academic division of T&F Informa plc.

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Published in 2006 by CRC Press Taylor & Francis Group 6000 Broken Sound Parkway NW, Suite 300 Boca Raton, FL 33487-2742 © 2006 by Taylor & Francis Group, LLC CRC Press is an imprint of Taylor & Francis Group No claim to original U.S. Government works Printed in the United States of America on acid-free paper 10 9 8 7 6 5 4 3 2 1 International Standard Book Number-10: 0-8493-3273-7 (Hardcover) International Standard Book Number-13: 978-0-8493-3273-9 (Hardcover) Library of Congress Card Number 2005041889 This book contains information obtained from authentic and highly regarded sources. Reprinted material is quoted with permission, and sources are indicated. A wide variety of references are listed. Reasonable efforts have been made to publish reliable data and information, but the author and the publisher cannot assume responsibility for the validity of all materials or for the consequences of their use. No part of this book may be reprinted, reproduced, transmitted, or utilized in any form by any electronic, mechanical, or other means, now known or hereafter invented, including photocopying, microfilming, and recording, or in any information storage or retrieval system, without written permission from the publishers. For permission to photocopy or use material electronically from this work, please access www.copyright.com (http://www.copyright.com/) or contact the Copyright Clearance Center, Inc. (CCC) 222 Rosewood Drive, Danvers, MA 01923, 978-750-8400. CCC is a not-for-profit organization that provides licenses and registration for a variety of users. For organizations that have been granted a photocopy license by the CCC, a separate system of payment has been arranged. Trademark Notice: Product or corporate names may be trademarks or registered trademarks, and are used only for identification and explanation without intent to infringe.

Library of Congress Cataloging-in-Publication Data Moiseyev, Valentin N. Titanium alloys in Russia / Valentin N. Moiseyev. p. cm. -- (Advances in metallic alloys ; v. 5) Includes bibliographical references and index. ISBN 0-8493-3273-7 (alk. paper) 1. Titanium alloys--Russia (Federation) I. Title. II. Series. TN693.T5M58 2005 620.1'89322--dc22

2005041889

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Contents Introduction

1

Chapter 1 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8

Structure of Titanium Alloys Effect of impurities on the properties of titanium Interaction of titanium with other elements Solid α- and β-solutions in titanium alloys Chemical compounds in titanium alloys Stability of solid α- and β-solutions Phase transformations in titanium alloys Enhancement of mechanical strength of titanium alloys Enhancement of high-temperature strength of titanium alloys References

5 5 9 13 17 22 25 29 35 42

Chapter 2 2.1 2.2 2.3 2.4

Structural Titanium Alloys General characteristics High-ductility low-strength alloys Medium-strength alloys High-strength alloys References

47 47 51 67 84 116

Chapter 3 3.1 3.2 3.3

High-Temperature Titanium Alloys General characteristics Martensite-type high-temperature alloys High-temperature pseudo-α-alloys References

119 119 121 141 147

Chapter 4 4.1 4.2 4.3

Functional-Purpose Titanium Alloys Titanium alloys operated at low temperatures Special-purpose corrosion-resistant titanium alloys Intermetallics-based titanium alloys 4.3.1 Titanium aluminides 4.3.2 Titanium nickelide 4.3.3 Eutectoid-based alloys 4.4 Titanium alloys for production of cast shapes References

149 149 150 156 157 159 161 162 167

Chapter 5 Technological Properties of Titanium Alloys 5.1 Formation of the structure and properties in hot deformation of titanium alloys 5.2 Heat treatment of titanium alloys

169 169 174

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THE RISE OF THE SUPERCONDUCTORS

5.2.1 Annealing 5.2.2 Hardening heat treatment (quenching and aging) 5.3 Welding of titanium alloys 5.4 Surface engineering of titanium alloys 5.4.1 Interaction of titanium with atmospheric gases during heating 5.4.2 Effect of mechanical treatment on titanium alloy castings 5.4.3 Surface hardening of titanium alloy items References Chapter 6 6.1 6.2 6.3 6.4 6.5

Subject Index

Applications of Titanium and Titanium Alloys Titanium alloys in the aircraft industry Titanium in engine manufacturing Titanium in rocket manufacturing Titanium in shipbuilding Titanium in the chemical engineering industry and other fields References

176 180 181 186 186 188 189 191 195 196 199 201 202 202 205 207

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Introduction Titanium is a rather new metal and is, probably, the last addition to the comparatively small group of structural materials for large-capacity constructions. Along with iron, aluminum, magnesium, copper, and nickel, it becomes one of the essential metal materials for modern machine building, as its reserves in the Earth’s crust are rather big. The advantages of titanium as a structural material are well known. The major stimulus for titanium to be used in various engineering fields are its high specific strength and high-temperature strength within a broad temperature range, and also a high corrosion resistance in most aggressive media. Titanium used in industries is usually not technical-grade titanium but its alloys, which exceed the nonalloyed metal by mechanical strength, high-temperature strength and other useful properties. The temperature range of the best application of titanium alloys is from the deep-freeze temperatures (cryogenic alloys) up to 500–600°C (high-temperature alloys). A characteristic feature of titanium and its alloys is high sensitivity to impurities, especially atmospheric oxygen and nitrogen. Oxygen, nitrogen and other impurities form alloys of the type of interstitial solid solutions or metallide phases with titanium and significantly affect the properties of metal when present even in minor amounts (decimal and sometimes even hundredth fractions of a percent). This explains the large number of grades of initial spongy titanium cast to produce semiproducts. Russian state standard 17303-72 establishes the following grades and chemical compositions for initial spongy titanium (Table 1). Comparison of titanium with other structural metals shows it to be the most refractory and to have lower values of thermal conductivity, electrical resistance and ooooooooooo Table 1 Grades and chemical compositions of spongy titanium. Grade

TG-90 TG-100 TG-110 TG-120 TG-130 TG-150

Maximal hardness, HB (10/1500/30)

N2

C

Cl

Fe

90 100 110 120 130 150

0.02 0.02 0.02 0.03 0.03 0.04

0.02 0.03 0.03 0.04 0.04 0.05

0.08 0.08 0.08 0.08 0.10 0.12

0.06 0.07 0.09 0.01 0.13 0.20

Chemical composition, %, no more than Si

Ni

0.01 0.02 0.03 0.03 0.04 0.04

0.05 0.05 0.03 0.05 0.05 0.05

O2 0.04 0.04 0.05 0.06 0.08 0.10

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VALENTIN N. MOISEYEV

Table 2 Physical properties of titanium and other metals. Properties Melting temperature, ° C Density, g/cm3 Thermal conductivity, Cal/cm s deg Electrical resistance, µΩ cm Heat capacity, Cal/g deg Coefficient of thermal expansion, ×106, deg–1 Young’s modulus, kgf/mm2

Ti

Mg

1665 650 4.51 1.74 0.0407 0.35 55.4 4.40 0.126 0.245 8.9 25.7 11200 4500

Al 660 2.70 0.57 2.68 0.211 24.0 7250

Fe

Cu

1535 1083 7.86 8.94 0.17 0.92 10.0 1.72 0.109 0.093 11.9 16.4 20000 12250

and thermal expansion. By density, titanium is attributed to light metals and occupies an intermediate position between aluminum and iron (Table 2). Similar to iron, titanium is a polymorphic metal and has a phase transformation at a temperature of 882°C. Below this temperature, the hexagonal close-packed lattice of α-titanium is stable; above it, the body-centered cubic lattice of β-titanium. There is a great variety of commercial titanium alloys which differ by their structures, physical, chemical, and mechanical properties, and applications. All titanium alloys are divided into three groups by the type of structure: (i) alloys based on solid α- and β-solutions, (ii) alloys based on solid solutions with some amount of a chemical compound and (iii) alloys based on a chemical compound. The most numerous and traditional group are titanium alloys which represent solid solutions. As a rule, they are structural alloys with a high ratio of strength and ductility, satisfactory fusion weldability, capability of hardening heat treatment, good thermal stability, and other properties required for modern structural materials. Solid-solution titanium alloys retain high strength at temperatures up to 350–450°C. Solid-solution alloys with chemical compounds are also widespread. This is a class of titanium alloys based on α-, (α+β) , and β-solid solutions with an amount of disperse formations of a chemical compound, which ensures a significant enhancement of strength and high-temperature strength. Commercial titanium alloys contain a minor amount of a chemical compound or the initial stage of its formation in the α- or (α+β) -matrix. Aluminum (Ti3Al), silicon (Ti5Si3), carbon (TiC), boron (TiB), etc., are used as alloying elements forming chemical compounds in titanium. Other, more complex chemical compounds can be formed in multicomponent alloys. Development of solid solution-based alloys with a chemical compound made it possible to increase the operational temperatures up to 500–600°C. Titanium alloys based on chemical compounds have been developed comparatively recently and quickly became the focus of attention owing to their properties, unique in some cases. There are at least three types of such materials, which are of commercial interest. These are heat-resistant alloys based on titanium aluminides Ti3Al (α2-phase), TiAl (γ -phase); shape memory alloys based on titanium nickelides (TiNi) and fire-safe alloys based on the eutectoid (α+Ti2Cu and α+TiCr). The latter type can only conditionally be attributed to alloys based on a chemical compound; however, their functional properties are determined by eutectoid which includes the compound.

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Alloys based on aluminides can be operated at temperatures up to 700–800°C. In Russian practice, there are over 30 commercial titanium alloys that are divided into the following groups by their preferable applications: structural alloys, high-strength thermally hardened alloys, high-temperature alloys, alloys for fabrication of cast shapes, alloys with special properties (for cryogenic temperatures, for very aggressive media, shape memory alloys, etc.). The monograph considers titanium alloys with respect to these groups and applications. Along with the description of the physical and mechanical properties of various-purpose alloys, we discuss the general regularities in the change of alloys’ structure and properties depending on their chemical composition and heat treatment. The data in Introduction make it possible to predict the behavior of various alloys at all stages of production, and also in operation of fabricated products at elevated temperatures and stresses. We present the major parameters of titanium alloys’ treatment, taking into consideration their structural features and the requirements to their physical and mechanical properties. The chapter on special-purpose titanium alloys discusses (besides alloys for cryogenic temperatures, with decreased tendency to oxidation, corrosion-resistant and shape memory alloys) a new direction in developing and using titanium alloys based on chemical compounds and solid solution-based alloys with intermetallic compounds. In conclusion, we look at the experience and efficiency of using titanium alloys in various fields of machine building. The monograph is intended for metallurgists, physical metallurgists, design and process engineers, and metallurgical students.

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Structure of Titanium Alloys

1.1 EFFECT OF IMPURITIES ON THE PROPERTIES OF TITANIUM Titanium exists in two allotropic modifications: high-temperature (with the bodycentered crystalline (bcc) lattice) and low-temperature (with the hexagonal closepacked (hcp) lattice). α-Titanium exists at temperatures below 882°C, and β-titanium at higher temperatures up to the melting point. The lattice parameters of α-titanium are as follows: a = 2.9504 Å, c = 4.683 Å; at 25°C (as obtained by extrapolation) a = 3.282 Å. The properties of titanium and its alloys depend to a great extent on the content of inevitable gas impurities, which get into the metal from the initial raw material, spongy titanium. Impurities, mainly oxygen, determine the useful property of titanium – its high mechanical strength. Indeed, titanium free of impurities is of no interest as a structural material. For instance, titanium refined using the iodide process has the following mechanical properties: ultimate strength, 250 MPa; yield strength, 106 MPa; relative elongation, 72%; transverse contraction, 86%; Vickers hardness, 83.4 kgf/mm2. In this metal the content of oxygen is less than 0.01%; of nitrogen, less than 0.008%; other impurities are mainly traces. For comparison, technical-grade titanium VT1-00 has ultimate strength of 300–450 MPa, and VT1-0 of 400–550 MPa. The strength of technicalgrade titanium is enhanced mainly due to oxygen and, to a lower extent, due to nitrogen, carbon, silicon and iron. Thus, technical-grade titanium is, in fact, a complex alloy containing elements, which have their own effects on the temperature of polymorphic transformations of titanium. The essence of these effects is that technical-grade titanium has no definite transformation point, but has a temperature range of transformation. In pure titanium, the α-modification transforms into the β-modification at a temperature of 882.5°C. In technical-grade metal, transformation begins at a lower temperature and ends at a higher temperature than in pure titanium. The range of transformation for technicalgrade titanium is 865 to 920°C (at the combined oxygen and nitrogen contents no higher than 0.15%). The difference in microstructure between pure and technical-grade titanium is mainly determined by oxygen impurity.

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VALENTIN N. MOISEYEV

O, at.% 0

0.50

1.50

1.00

2.00

σ0.2

σB

45

σ0.2

350 300

HV 15

250 200

HV

40

150

δ,%

δ (l = 12.7 mm)

HV, kgf/mm2

σB, σ0.2, kgf/mm2

σB 75

100 20 δ (l = 25.4 mm) 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 O, wt.%

Figure 1 Effect of oxygen impurity on the mechanical properties of titanium (from various sources). N2, at.% 0.75

σB, σ0.2, kgf/mm2

75

1.75

1.25

2.25 400

σB σ0.2

45

HV

350 300 250

15

200 HV

150

40

100

δ (l = 12.7 mm)

δ,%

HV, kgf/mm2

0.25

20 δ (l = 25.4 mm)

0

0.1

0.2 0.3

0.4

0.5 0.6

0.7

N2, wt.%

Figure 2 Effect of nitrogen impurity on the mechanical properties of titanium (from various sources).

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C, at.% 0

0.5

1.0

1.5

2.5

2.0

3.0

3.5

56

σ0.2 28

HV 150

14

100

50 δ (l = 12.7 mm)

40 δ,%

200

HV, kgf/mm2

σB, σ0.2, kgf/mm2

σB 42

30 δ (l = 25.4 mm)

20 10 0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

C, wt.%

Figure 3 Effect of carbon impurity on the mechanical properties of titanium (from various sources).

The high-temperature β-modification in nonalloyed titanium could not be fixed by quenching even at the highest rates of cooling. During quenching, the β-modification passes into the α-modification by a martensite-type instantaneous transformation. The structure formed is designated α′ and is a common polyhedral structure of pure metal. It is approximately the same both for pure and technical-grade titanium. In quenching from the β-region, the microstructural changes of pure titanium are insignificant and are characterized by the emergence of jagged grain boundaries. In the case of technical-grade titanium, the structure after quenching sharply changes from polyhedral to needle-like. If quenching is performed from temperatures inside the transformation range, i.e., from the two-phase region, then various ratios between the equilibrium (primary) and martensite (secondary) α-structures can be obtained. Effects of oxygen, nitrogen and carbon impurities on the mechanical properties of titanium are shown in Figs. 1–3. Graphical interpolation can establish the following approximate coefficients of hardening caused by impurities within the ranges of their content in technical-grade titanium: 0.05% oxygen increases ultimate strength by 60 MPa; 0.05% nitrogen, by 125 MPa, and 0.05% carbon, by 35 MPa. For instance, 0.05% iron increases ultimate strength of titanium by 10 MPa. Interestingly, iron forms substitution solid solutions with titanium, not interstitial solid solutions as oxygen, nitrogen and carbon do.

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The combined effect of the four impurities on the properties of titanium can be estimated to accuracy sufficient for practice. Brown proposed the following formula to calculate the hardness: HB = 196 %N 2 – 158 %O 2 – 45 %C – 20 %Fe – 57 , where 57 is the value of hardness of purest titanium and %%N2, O2 are the percentages of these elements. The effect of silicon impurities on the properties of pure titanium should also be considered. According to Goldhoff and coworkers, Vickers hardness of alloys increases almost linearly from 150 up to 645 kgf/mm2 as the content of silicon rises from zero up to 16%. If one takes the coefficient of 0.28 to convert from Vickers hardness to ultimate strength of technical-grade titanium, then 1% silicon should increase the ultimate strength by about 100 MPa; and 0.05%, by 5 MPa. However, it was found that 0.5% silicon increased the ultimate strength of titanium by 130 MPa and, therefore, the increment of strength at 0.05% silicon would be 12 MPa. Therefore, the combined effect of the impurities on the mechanical properties of titanium are rather significant even at their comparatively small amounts given by specifications. Comparison of the hardening effect of various impurities on titanium shows that oxygen can be considered not only a harmful impurity but also a useful alloying additive. Nitrogen, though a more potent strengthener than oxygen, evokes embrittlement of titanium, so alloys with more than 0.05% nitrogen are of no practical significance. Carbon is a comparatively poor strengthener, but at over 0.2% in the alloy it causes the emergence of a brittle and solid carbide phase. Yet, alloying with oxygen up to 0.5% preserves a satisfactory ductility (δ5 > 15%) at a significant increment of strength (σB = 300–800 MPa). Studies of the effect of oxygen impurities on the cold brittleness of titanium showed that titanium with an oxygen content of 0.13–0.15% (σB = 500 MPa) was virtually noncold brittle even at liquid-nitrogen temperature (–196°C), i.e., impact strength decreased by only 15% as compared with the value at 20°C (from 164 down to 140 MPa, respectively). Titanium with an oxygen content of 0.3% (σB = 670 MPa) showed under the same conditions a decrease of impact viscosity by 55%. Specimens used for the experiments were annealed in vacuum and the content of hydrogen in them did not exceed 0.005%. Hydrogen was not taken into account in the estimation of the hardening effect of impurities, because its influence as a strengthener is negligible within the limits of its content in technical-grade titanium. However, being insignificant as a strengthener, hydrogen causes a brittle failure of titanium due to the formation of a hydride phase. Because of this, hydrogen is considered to be one of the most undesirable impurities in titanium. The presence of oxygen enhances the harmful effect of other impurities. There is an opinion that the purity of initial titanium should be constantly increased. If there is a necessity to increase the strength of technical-grade titanium, it is much better to add a strictly definite amount of oxygen than to use low-grade titanium with a large scatter of properties. This is the way it is done, for instance, in the U.K. and the U.S. where three out of four grades of technical-grade titanium contain a deliberately introduced addition of oxygen to ensure the strength required.

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One of the tendencies in the development of modern titanium alloys is to increase the extent of alloying. In all alloys with the ultimate strength of about 1000 MPa the sum total of alloying elements is 7– 10% (alloys VT6, VT14, VT3-1, VT22, etc.). Alloys are developed in which the sum total of alloying components reaches 18–40% (alloys VT32, VT35). Alloying increases strength but reduces ductility of alloys. Therefore, the initial material, in this case titanium, should have maximum ductility. The more alloying is to be done, the more ductile and, thus, more free of impurities, the initial titanium should be. Usually, it is not advantageous to increase strength by using impurities, as it is accompanied by a considerable loss of ductility, because the major impurities – oxygen, nitrogen and carbon form interstitial solid solutions with titanium. Besides, these impurities have an adverse effect on other important characteristics of titanium alloys – thermal stability, creep resistance and notch sensitivity. Notch sensitivity is characterized by the ratio of ultimate strength of a notched specimen to ultimate strength of a smooth specimen. For brittle materials this value is less than unity; for ductile materials, more than unity; i.e., in the latter case the notch acts to strengthen the specimen. The effect of an oxygen impurity on the notch sensitivity of an alloy VT6 for round specimens with the radius of 0.1 mm at the basis of the notch and the notch angle of 60° gave the following results: the oxygen contents of 0.10%, 0.22% and 0.40% yield the ratio σBH/σB of 1.71, 1.63 and 1.47, respectively. This dependence of notch sensitivity on the oxygen content creates the danger of premature breakdown, in particular, of bolted joints. An increase of the purity of the initial titanium would make it possible not only to improve the quality of the existing alloys, but also to develop new alloys with good ductile properties along with high strength.

1.2 INTERACTION OF TITANIUM WITH OTHER ELEMENTS Research and development of commercial metal alloys, including titanium alloys, is determined by the character of interaction of the base with elements of the Periodic Table and also by the regularities of the change of physical, chemical, mechanical and other properties in various systems. The character of interaction of titanium with alloying elements and impurities depends to a significant extent on the atomic radii, position of metals in the electromotive series, valence, ionization potential and some other characteristics. Consideration of these factors enables a prediction of the results of interaction of various elements. Based on the size and structure of titanium and alloyed elements, the phase diagrams of alloys can be represented as shown in Fig. 4. Elements with the difference in atomic diameters of no more than 15%, i.e., with the ratio of atomic diameters of the alloying elements to the atomic diameter of titanium from 0.88 to 1.15, can form (based on the Hume-Rothery rule) substitution solid solutions.

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VALENTIN N. MOISEYEV

1.7

5

1.6 1.5 1.4 4 1.2 1.1 1.0

3 Substitution solid solutions No solid solutions

0.9 0.8 0.7

2

Ratio of diameters

Goldschmidt atomic diameters, kX

1.3 No solid solutions

0.6 0.5 0.4 1

Interstitial solid solutions

0.3 0.2 0.1

0

0

Figure 4 Position of the atomic diameter of titanium with respect to the atomic diameters of alloying elements.

Based on the Hegg rule, formation of interstitial solid solutions is feasible when the ratio of atomic diameters of the interacting elements is less than 0.59. The elements forming chemical compounds with titanium are between the elements forming substitution and interstitial solutions. Most alloying elements are capable of forming, within a broad concentration range, substitution and interstitial solutions with one of the two or with both modifications of titanium. Interstitial solid solutions are formed with elements with the smallest ratio of their atomic diameter to that of titanium. Among other factors affecting the formation of solid solution, of great importance is the valence of solvent metal and dissolved metal. Titanium is capable of forming solid solutions with transition metals of similar electronic configuration, which contain unpaired α-electrons. These metals are zirconium, hafnium, vanadium, niobium, tantalum, chromium, molybdenum and tungsten. Within a smaller range of concentrations, titanium alloys form solid solutions with other transition metals, which contain paired α-electrons. They include such metals as manganese, iron, cobalt, nickel, palladium and platinum.

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The type of lattice of the metal dissolved is of great significance. Transition elements with a body-centered lattice stabilize the high-temperature body-centered cubic modification of β-titanium. Titanium-based interstitial solid solutions form alloys with hydrogen, oxygen, nitrogen and carbon – the elements with small atomic diameters with respect to titanium. The size and structure factors are determined to a considerable degree by the position of alloying elements in Mendeleyev’s Periodic Table. The structure of electronic shells defines the character of interatomic bonds and specifies the chemical essence of the alloying processes. The basis for considering of the structure of titanium alloys with groups of elements can be the character of the phase diagrams. The binary phase diagrams can be classified into groups by the character of the liquidus lines near the titanium ordinate, and the shape of a segment characterizing the secondary transformation in the systems can be used to divide the groups into subgroups. With more complex phase diagrams, this classification has the form presented in Fig. 5. This classification of the phase diagrams is most convenient for analyzing the formation of solid α- and β-solutions of titanium with various alloying elements and impurities. Consideration of the phase diagrams of titanium with Periodic Table elements shows that at room temperature only some of them form rather broad regions of solid α-, α+β- or β-solutions. Table 3 shows some characteristics of low-temperature segments of the phase diagrams of titanium with elements, which are of greatest interest as alloying elements in the development of commercial alloys based on solid α- and β-solutions or are impurities in titanium. Analysis of the phase diagrams in the region of the existence of solid α- and β-solutions of titanium with Periodic Table elements shows that the choice of elements suitable for developing commercial alloys is limited. Zirconium gives a continuous series of solid solutions with titanium. Molybdenum and vanadium form continuous solid solutions with β-titanium and are restrictedly dissolved in α-titanium. Niobium and tantalum form similar phase diagrams with titanium. Chemical compounds were found in the region of high concentrations in some alloys of titanium with this group of elements. Aluminum and tin are distinguished by a significant solubility in α-titanium, which makes it possible to consider them as important alloying elements for commercial titanium alloys. Aluminum, being a rather potent strengthener, is of greatest interest in this respect. Chromium, manganese, iron, cobalt, nickel, copper and tungsten, which dissolve little in α-titanium are restrictedly used as alloying elements due to the possibility of eutectoid transformation. The use of these elements in commercial titanium alloys is determined by the rate of eutectoid transformation and the effect of eutectoid formed on the physical and mechanical properties. The value of an alloying element in the development of commercial titanium alloys is determined by such factors as the properties of solid α- and β-solutions of

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Ia

Ib l

l

l l+β

l+β

I

Ic

β

β

β

α+β

α+β α

α+β

α Zr, Hf

α+β α

III

α+β α

O, N, Sn

β-Ti + α-Th α-Ti + α-Th Th

IIIb

l+α l+β

β + Tin Xm

B, Ce, La, Ge

IIIa

β

l

α + Tin Xm

α α + Tin Xm H, Si, Mn, Fe, Co, Ni, Cu, Ag, Au l

IIc

l+β

β

β + Tin Xm

α + Tin Xm Cr, U

l

l+β

β α+β

β + Tin Xm

IIb

l

l+β

α

V, Nb, Ta, Mo, Re

IIa

II

l+β

l

l + Tin Xm

IIIc

l+β

l+β

β

β + Tin Xm α+β α

α + Tin Xm C, Al

l+W

l

β

β+W

α+β α

α+β W, Pb

Figure 5 Phase diagrams of titanium binary alloys with various elements.

elements with titanium, by the ultimate solubility of elements in α- and β-titanium and by the effect of an intermetallic component on the properties of the alloy. A predominant number of titanium-alloying element phase diagram has been plotted for an equilibrium achieved at a given temperature within several hundred or, at best, several thousand hours, whereas under real conditions titanium alloys are at operational temperatures for tens of thousands of hours. Besides, under real operational conditions the metal experiences stresses which can exert a significant effect on the character of phase transformations in the metal. The regions of the existence of solid-solution phases in various titanium alloys are given in Table 3.

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Table 3 Regions of existence of solid-solution phases in various titanium alloys. Alloys Type of phase diagram

Alloying element, wt. % α

α+β

β

At temperature, °C

Zr Hf

Ia Ia

0–100 0–100

– –

– –

20 20

V Nb Ta Mo

Ib Ib Ib Ib

0–2 0–4 0–9 0–0.5

2–30 4–50 9–70 0.5–30

30–100 50–100 70–100 30–100

20 20 20 20

Cr

Ic

0–0.5

–

–

670

H Si Mn Fe Co Ni Cu Sn O N

IIa IIa IIa IIa IIa IIa IIa IIIa IIIa IIIa

0–0.2 0–0.3 0–0.5 0–0.5 0–1.0 0–0.2 0–2.1 0–18.6 0–3.2 0–2.5

– – – – – – – – – –

– – – – – – – – – –

320 600 550 615 685 770 798 865 400 600

C Al

IIIb IIIb

0–0.5 0–7

– –

– –

920 20

W

IIIc

0–0.8

–

–

715

1.3 SOLID α- AND β-SOLUTIONS IN TITANIUM ALLOYS A feature of the interaction of titanium with other alloying elements is their influence on the temperature of the allotropic transformation of titanium. This factor determines to a considerable degree the diversity of properties of titanium alloys and the possibility of changing them by heat treatment. Alloying elements increasing the statistical significance of atomic stable configurations d5 decrease the temperature of polymorphic transformation. Alloying elements in titanium dissolve in those phases, in which they increase forces of atom–atom interaction and enhance their relative stability. Thus, aluminum increases the elastic constants of solid α-solution and expands the temperature interval of the existence of the α-phase. Molybdenum and vanadium, which stabilize the β-phase, decrease the elastic constants of α-solution, thus increasing these characteristics for β-solution.

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°C 940 Al

β

920

α

900 880

+

α-Al β-Sn β-Mo

β

α+β

860

Sn α-Al α-Sn β-Mo

840 α

α+β

820

Μο

800 0

2 4 6 8 10 Percentage of alloying addition

Figure 6 Three types of titanium phase diagrams. T, °C

2

Tn2 Tn1

1

T1 α Ti

β

α+β C1α

C1

C1β C2β

β-Stabilizing element, %

Figure 7 A scheme of “titanium–β-stabilizing element” phase diagram illustrating the effect of the α-stabilizing element on the boundaries of phase regions in binary (1) and ternary (2) alloys.

With respect to titanium, alloying elements are divided into elements stabilizing the α-phase and those stabilizing the β-phase in titanium. Elements having little effect on the temperature of allotropic transformation in titanium (tin, zirconium) are singled out into a group of neutral strengtheners (Fig. 6). Elements of this type have an effect on the structure and properties of alloys, which differs from that of typical α- or β-stabilizing elements.

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A generalized titanium–β-stabilizing element phase diagram, which covers α-, α+β- and β-regions in solid state (at low temperatures) is given in Fig. 7. This diagram is valid for elements isomorphic to β-titanium, such as vanadium, niobium, tantalum and molybdenum. It could also be used in consideration of titanium alloys with eutectoid-forming elements, if solid β-solution does not undergo eutectoid transformation. A generalized diagram consists of two curves plotted from a common origin, which corresponds to the temperature of titanium allotropic transformation. The lower curve restricts the region of the existence of solid α-solution, and the crosssection of this curve with the abscissa axis in point Cα corresponds to the ultimate concentration of solid solution at room temperature. The upper curve determines the boundary between the α+β- and α-regions, and the point of its intersection with the abscissa axis Cβ corresponds to the minimum required concentration of the β-stabilizing component to form solid β-solution stable within the entire temperature range. This diagram is valid both for binary alloys of titanium with a β-stabilizing element and for alloys with several alloying elements stabilizing the β-phase in titanium. At a certain temperature (for instance, T1), irrespective of the content of the β-stabilizing element in the alloy, its content in the α- and β-phases is constant (C1α and C1β, respectively). An increase in the content of the β-stabilizing element in the alloy with titanium at a given temperature in the α+β-region is accompanied by an increase in the amount of the β-phase, without a change of its chemical composition. The physical and mechanical properties of titanium alloys, representing solid αand β-solutions, are determined by the properties of these solutions. Table 4 presents the physical and mechanical properties of a solid α-solution, as determined during the tension of annealed titanium alloys cantoning alloying elements within the range of solubility in α-titanium. The properties of a solid β-solution were determined on alloys quenched from the β-region and having a mechanically stable β-phase, i.e., the β-phase which does not undergo any transformations distorting the true properties of the phase during the tension test. As it follows from Table 4, the ultimate strength of saturated solid α-solutions in all alloys of titanium with β-alloying elements are small as compared with the strength of non-alloyed titanium. It varies within the range of 47 to 59 kgf/mm2. At the same time, solid α-solutions with such elements as Al, Sn, Zr and O have a higher ultimate strength. The ultimate strength of investigated solid β-solutions in binary titanium alloys with various β-stabilizing elements are considerably larger – from 54 up to 108 kgf/mm2 in alloys of titanium with niobium or cobalt. As a rule, segments of multicomponent phase diagrams in the region of solid α-solution are plotted by extrapolation with insufficient significance. Nevertheless, data available suggest that the solubility of β-stabilizing elements in α-titanium should decrease proportionally to the amount of elements occurring in the alloy simultaneously. Therefore, there is no sufficient reason to assume that the strength of solid α-solution could be considerably increased by alloying with several β-stabilizing elements. From this point of view, hardening of solid α-solution by

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β- and α-stabilizing elements or by neutral strengtheners simultaneously is of great interest. In this case, as it follows from the ternary “titanium–β-stabilizing element– α-stabilizing element or neutral strengthener” phase diagrams, the solubility of β-stabilizing elements in α-titanium does not decrease; in some cases it even increases. Table 4 Some physical and mechanical properties of solid α- and β-solutions of binary titanium alloys. Element

Content, %

Properties σB, kgf/mm2 σ0.2, kgf/mm2

E, kgf/mm2

δ5, %

42 50 42 46 45 48 43 49 44 72 40 57 38 41 41 45

11000 11050 11000 11000 11100 11050 11050 11050 11070 11200 11300 11250 11050 11050 11050 11070

40 38 45 44 33 27 30 24 25 19 42 36 32 28 35 28

77 91 45 65 98 97 100 101 98

10100 10200 10000 10100 10200 10000 10000 10000 10300

24 18 19 20 13 18 15 14 16

α-alloys Mo V Nb Ta Fe Cr Mn Co W Al O O Zr Zr Sn Sn

0.5 2.0 4.0 9.0 0.5 0.5 0.7 0.7 0.8 7.5 0.15 0.30 3.0 6.0 2.0 4.0

53 58 47 50 59 53 59 57 59 83 48 64 48 58 48 53 β-alloys

Mo V Nb Ta Fe Cr Mn Co W

18 20 50 50 9 12 13 9 30

87 95 54 72 101 101 106 108 100

As for solid β-solutions, in the interaction of titanium with two or more β-stabilizing elements the phase fields are mainly additive to the binary systems of titanium with each β-stabilizing element. In multicomponent systems of titanium with β- and α-stabilizing elements the region of the existence of α+β-alloys slightly expands, and the β-region becomes

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narrower. The character of interaction of α- and β-stabilizing elements in titanium is schematically shown in Fig. 7. Besides boundary 1, which separates the (α+β)- and β-regions, there is boundary 2, which corresponds to alloys additionally containing an α-stabilizing element (for instance, aluminum or oxygen), which increases the → β transformation. As it follows from the diagram, the alloy temperature of α+β ← containing no α-stabilizing element (of concentration C1) has a β-phase with the concentration C1β of the β-stabilizing element at a temperature T1. In the alloy additionally containing an α-stabilizing element, the concentration of the β-stabilizing element in the β-phase would be higher (C2β), although the amount of β-phase in the alloy decreases. All this provided that the solubility of the β-stabilizing element in α-titanium remains the same.

1.4 CHEMICAL COMPOUNDS IN TITANIUM ALLOYS Intermetallic compounds in titanium alloys are numerous and diverse. Intermetallics formed in the regions adjacent to solid α- and β-solutions are of commercial interest for developing new alloys. Of special interest are intermetallic compounds formed in alloys of titanium with aluminum, because aluminum occurs in almost all commercial alloys as an alloying additive. In Ti–Al alloys, solid α-solution is formed at a temperature of 1020–1100°C in the peritectoid reaction. As the temperature decreases, the solubility of aluminum in α-titanium goes down, being about 6.5% at room temperature. Solid α-solution oversaturated with aluminum tends to breakdown to form the α-phase at a slow cooling or isothermal heating. The α-phase has the stoichiometric composition of the type of Ti3Al. Formation of this phase during aging is accompanied by a considerable loss of ductility without any noticeable increase of hardness or strength of the alloy. As in some titanium alloys the content of aluminum reaches 7% or more, the breakdown of the solid α-solution in Ti–Al alloys is the phenomenon to be taken into account. At an aluminum content of up to 7%, annealed alloys (800°C, 1 h, cooling on the air) and after heating up to 500°C for 100 h have a satisfactory ductility. As the content of aluminum is increased further, the ductility characteristics decrease after a prolonged heating, which is due to the formation of the α2-phase in ever-increasing amounts. An alloy containing 9% Al has a low ductility in annealed and aged state (δ5 = 2.5%, ψ = 10%). At the same time, alloys quenched from 800°C in water or air have a good ductility at an aluminum content of up to 10%. In this case, an accelerated cooling makes it possible to suppress α → α2 transformation which decreases the ductility of the alloy (Fig. 8). The alloys containing up to 7% Al, quenched and aged for 100 h at temperatures from 400 to 500°C do not change their mechanical properties. As a result of aging at 450 and 500°C the alloys containing over 7% Al, become embrittled and their

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σB, MPa

1000 Annealing

2

900

3

800 Quenching 700

Annealing

60 15

2

10

δ, %

1

1

Quenching

5

2

3

Annealing

0 1 ψ, %

30

Quenching

20

3

10 0

2 5

6

7

8

9

10

Al, %

Figure 8 Mechanical properties of Ti–Al alloys quenched in water from 900°C and aged at 400°C (1), 450°C (2) and 500°C (3) for 100 h as compared with annealed state (800°C, for 1 h; cooling at a rate of 3°C/min). Al, % 8 α + α2 + β 7

1

6

2

5 0

2

4

6

8

10

12 Mn; Mo, %

Figure 9 The interphase boundary between the regions α+β/α+β+α2 in Ti–Al–Mo (1) and Ti–Al–Mn (2) systems.

ductility proves the same as after annealing and slow cooling. The temperature of 400°C is insufficient for intensive breakdown of the oversaturated α-phase and the alloys preserve their properties which are close to the quenched state. It should be noted that an alloy with 10% Al showed a noticeable decrease of ductility after aging at 400°C. Evidently, the more oversaturated the α-phase is by aluminum, the more apt to breakdown it is. Aging of alloys for 1 h shows that a sharp decrease of ductility is observed after heating at temperatures over 480–500°C. Addition of a third element to binary Ti–Al alloys considerably affects the interface of the α/α+α2 phase regions. Contradictory data on the effect of various

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t, °C 3

500 400

1

2

α + β + TiMn

α+β

300 500

19

(a) 1

2

Α

400 3

300 200 0

2

4

6 Mn, %

8

(b) 10

Figure 10 The interphase boundary between the regions α+β/α+β+α2 in Ti–Al–Mn systems.

elements on the position of the interface of the phase fields have been published. Nevertheless, it could be maintained that most elements narrows down the region of the existence of the α-phase. Figures 9 and 10 give as an example the interphase boundary between the regions α+β/α+β+α2 in titanium alloys with an isomorphic β-stabilizing element, molybdenum, and an eutectoid-forming element, manganese, which were annealed at a temperature of 800°C for 1 h and cooled at room temperature at 2–4°C/min. Mechanical properties of alloys – hardness, ultimate strength, reduction of area, impact strength, impact strength of a specimen with a crack – are more sensitive characteristics to fix the initiation of α2-phase formation, which give this phenomenon a greater practical importance. By their effect on the contractions of the region of solid α+β-solutions, β-stabilizing elements can be arranged into a series by the decrease of solubility in α-titanium: zirconium, tin, vanadium, molybdenum, manganese, chromium, iron – the same series as their stabilizing effect on the β-phase. Most transition elements, such as chromium, manganese, iron, cobalt, nickel, copper, silver, silicon, beryllium, bismuth, lead, and others dissolve in α-titanium insignificantly but form solid solutions with β-titanium. These solid solutions decompose by the eutectoid reaction to form solid α-solution and an intermetallic component. In binary alloys of titanium with copper, silver and gold the eutectoid decomposition of the β-phase is intensive. In alloys of titanium with chromium, manganese, iron and cobalt the eutectoid process is slack. Nickel occupies an intermediate position. Depending on the β-stabilizing element, eutectoid transformation proceeds by the reaction β → α + chemical compound or α1 → α + chemical compound. The rate of eutectoid transformation determines an increase of eutectoid transformation temperature (Table 5). Assessment of the effect of eutectoid on the mechanical properties during the heating of an as-quenched alloy up to the eutectoid transformation temperature is

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impossible due to the formation of intermediate metastable phases or states – ω-phase and dispersed formations of α- and β-phases. To exclude the side metastable transformations of the β-phase, in assessment of the effect of eutectoid on the mechanical properties of Ti–Fe, Ti–Cr, Ti–Mn, Ti–Co and Ti–W alloys they were subjected before heating to a stabilizing annealing, which included slow cooling from the annealing temperature. Then the alloys were heated near the eutectoid transformation temperature for various times. The phase composition of the alloys after a prolonged heating was determined by the X-ray method under an optical microscope and an electron microscope. Table 5 Comparative rate of the eutectoid reaction in alloys of titanium with various elements. Element

Critical concentration, wt. %

Eutectoid composition, wt. %

Eutectoid formation temperature, °C

Manganese Iron Chromium Cobalt Nickel Copper Iron Silver Silicon

6.5 4.0 8.0 7.0 8.0 13.0 β-phase not fixed β-phase not fixed β-phase not fixed

20 15 15 9 7 7 16 22.8 0.9

550 600 675 685 770 790 830 855 860

Figure 11 shows the phase regions of the existence of the eutectoid component in Ti–Mn and the change of ductility depending on the manganese content, temperature and duration of heating the alloys preannealed at 800°C for 1 h and cooled at a rate of 2–4°C/min. t, °C 1

500

α + β +TiCr2

2 3

400

α+β

300 500

(a) Α

1

400

2 3

300

(b)

200 0

2

4

6 Cr, %

8

10

Figure 11 The change of position of the interphase boundary between the regions α+β/α+β+TiMn and a related decrease of plasticity (reduction of area by 25%) in the Ti–Mn system as a function of temperature and time of heating.

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21

α'(α'')

882 T1

α'(α'') + β β+ω

Tcr T2

α+β

α C1

Cα

Ti

β

Mo

Me

β Ccr C2

Cβ

β-Stabilizing element, %

Figure 12 The change of position of the interphase boundary between the regions α+β/α+β+TiCr and a related decrease of plasticity (reduction of area by 25%) in the Ti–Cr system as a function of temperature and time of heating.

Figure 12 shows the same for Ti–Cr alloys. In binary alloys of titanium with iron and tungsten, an increase of the duration of heating at certain temperatures leads to the coagulation of the intermetallic component, to a decrease of strength and a rise of ductility. This phenomenon is rather pronounced in Ti–Fe alloys (Table 6) and, to a smaller degree, in Ti–W alloys. In such alloys, the formation of eutectoid is not accompanied by any significant deterioration of mechanical properties. Table 6 Change of the mechanical properties of annealed Ti–Fe alloys depending on the duration of heating at 500°C (rod, 12-mm dia). 500°C, h

100 1000 10000

2% Fe

4% Fe

6% Fe

σB, kgf/mm2

δ5, %

ψ, %

σB, kgf/mm2

δ5, %

ψ, %

σB, kgf/mm2

δ5, %

ψ, %

60 60 59 52

20 20 20 21

56 55 55 53

73 70 66 52

20 19 19 23

45 42 40 42

81 80 76 58

17 16 15 19

30 26 24 33

Addition of a β-titanium isomorphic element into alloys of titanium with eutectoid elements expands the region of the existence of solid α+β-solutions. Addition of a β-titanium isomorphic element (Mo) into alloys of titanium with an eutectoid element (Cr) stabilizes the β-phase and the eutectoid transformation is considerably slowed down. Still, it is not suppressed and completes given enough time for the stabilization (Table 7). Table 7 gives the phase composition and the ratio of the transverse contraction of a specimen, subjected to isothermal heating, to the transverse contraction of the initial specimen, depending on the time of heating the alloys.

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Table 7 Phase composition and ductility (% of the initial value) of Ti–4% Cr–Mo alloys depending on the Mo content and duration of heating at 500°C (rod, 12 mm dia; annealing at 800°C for 1 h, cooling at 2–4°C/min). Holding time at 500°C, h Initial 100 1000 10000

Content, wt. % 0

2

4

8

α+β (100%) α + β + TiCr2 (80%) α + β + TiCr2 (65%) α + β + TiCr2 (35%)

α+β (100%) α+β (80%) α + β + TiCr2 (70%) α + β + TiCr2 (32%)

α+β (100%) α+β (85%) α + β + TiCr2 (70%) α + β + TiCr2 (33%)

α+β (100%) α+β (87%) α+β (75%) α + β + TiCr2 (31%)

Heating at a temperature of 500°C for 1000 h, irrespective of the Mo content in the alloys, leads to a ductility decrease due to eutectoid transformation. Addition of up to 6% Al, which is a stabilizer, or of neutral strengtheners (up to 6% Zr or 4% Sn) have no effect on the character of eutectoid transformation. Its kinetics is significantly affected by α-stabilizing interstitial elements, oxygen and nitrogen. A higher oxygen content contributes to a decrease in the temperature of the onset of eutectoid transformation and makes the process more complete (Table 8). Table 8 Phase composition of a Ti–4% Cr alloy depending on the oxygen content and the temperature of isothermal heating for 1000 h. Oxygen content, wt. %

350°C

400°C

0.005 0.010 0.015 0.020 0.025

α+β α+β α+β α+β α + β + (TiCr2)*

α+β α+β α+β α+β α + β + TiCr2

Phase composition after heating for 1000 h at temperature 450°C

500°C

α+β α+β α+β α + β + (TiCr2)* α + β + (TiCr2)* α + β + (TiCr2)* α + β + TiCr2 α + β + TiCr2 α + β + TiCr2 α + β + TiCr2

* traces

1.5 STABILITY OF SOLID α- AND β-SOLUTIONS This section considers the stability of solid α- and β-solutions in multicomponent systems depending on temperature, stress and duration of these factors by example of various commercial titanium alloys. Of greatest interest in this respect are those concentration regions of commercial alloys, which are in the vicinity of the phase fields. These are the α/α+α2

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interface in alloys rich in aluminum and the α/α+TixMy interface in alloys of titanium with eutectoid-forming elements. Most commercial pseudo-α-Ti alloys use eutectoid-forming elements as a β-stabilizing element and aluminum as an α-stabilizing element. Alloy OT4 containing 4.2% Al and 1.8% Mn (plate specimens, 2.5 mm thick) was heated at a temperature of 400°C for 30 000 h without stress and under stress. As the duration of heating was increased, the β-phase was stabilized, its quantity decreased and it enriched with manganese. No intermetallic compound was found. Further increase in the duration of heating led to a minor increase of strength and a decrease of ductility of the alloy. Application of stress did not result in any significant changes of the properties and structure of the alloy. An experimental alloy containing 7.3% Al and 1.2% Mn (plate specimens, 3 mm thick) was heated at a temperature of 500°C for up to 10 000 h without stress and under stress. The structure and properties of the alloy after these treatments are given in Table 9. Table 9 Mechanical properties of experimental alloy Ti–7.3Al–1.2 Mn. State

Steady-state phase composition

Mechanical properties σB, MPa

σ0.2, MPa

δ5, %

a, kgf/cm2

α+β

1040

960

18

4.2

α + (β) ----------------α + (β)

1080 -----------1110

990 -----------1030

18 -----15

4.0 ------3.0

500°C, 1000 h

α + (β) ------------------------------------------α + ( ( β ) ) + ( ( α2 ) )

1100 -----------1170

1030 -----------1120

16 -----7

3.2 ------1.8

500°C, 10000 h

α +((β))

1150

1070

12

2.6

Initial (annealed) 500°C, 100 h

Note: ( ) very small, (( )) traces; numerator, no stress; denominator, stress of 300 MPa.

An increase in the duration of heating led to the stabilization of the β-phase, its amount decreased. The intermetallic compound Ti–Mn was not observed. Despite a considerable decrease of ductility and an increase of strength after a prolonged heating, the presence of the α2-phase was not observed, either, but its effect on the mechanical properties was rather significant. The mechanical properties and phase composition of the annealed test alloy containing 7.3% Al and 1.2% Mn (plate specimens, 3 mm thick) after a prolonged heating without stress and under stress are given in Table 9. The α2-phase in the Ti–7.3% Al–1.2% Mn test alloy was found by the X-ray diffraction analysis only at an Al content more than 7.8% or stress more than 300 MPa applied for no less than 10 000 h. The character of the stability of solid α-solution can be more vividly illustrated by example of test high-temperature titanium alloys VT18. The alloys have the following chemical composition:

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Alloy

Ti

Al

Mo

Zr

Nb

Si

VT18

basis

7.8–8.3

0.4–0.8

7.8–8.5

0.8–1.2

0.16–0.30

Heat treatment of the alloys consists of annealing at 900–980°C for 1–4 h and cooling in air. After this annealing, one more heating at 600°C for 6 h with subsequent cooling in air is admissible. This heat treatment can stabilize the structure to some extent. However, the subsequent prolonged heating at working temperatures leads to a sharp decrease of ductility (Table 10). Table 10 Mechanical properties and phase composition of alloy VT18 after heating at 600°C for various times. Duration of heating, h

Phase composition

Initial state 100 200 500 1000

α +((β)) α +((β)) + ((α2)) α +((α2)) α +((α2)) α +((α2))

Mechanical properties σB, kgf/mm2

δ, %

ψ, %

108 110 110 113 115

15 4 4 3 3

28 10 9 9 8

Note: 1. Test specimens from a rod 30-mm dia. 2. (( )), traces.

As it follows from these data, the amount of aluminum in alloy VT18 is more than its solubility in α-titanium. Stabilization of the structure at 600°C leads to the formation of the α-phase, which results in the decrease of ductility. A prolonged heating under stress can, probably, also cause the formation of a chemical compound, which contributes to the decrease of ductility. The oxidation of the surface at a prolonged heating in air can also affect the ductility of the metal. It should be noted that transformations in solid α-solution at a prolonged stabilizing annealing and under stress, related to the formation of the α2-phase, is also observed in the known alloy Ti–8% Al–1% V–1% Mo. The thermal instability of solid β-solution can be illustrated by example of pseudo-β-titanium alloy VT15 (Ti–3% Al–7% Mo–11% Cr), which is a structural analog of the known alloy V120-VCA, containing 15% V instead of molybdenum. Despite the high content of an eutectoid element (Cr), the β-phase in this group of alloys is rather stable and undergoes a eutectoid transformation only under certain conditions. Table 11 gives the structure and mechanical properties of alloy VT15 after heat treatment in the following sequence: 750°C for 30 min, cooling in water, aging at 480°C for 20 h; then 560°C for 15 min, cooling in air, after heating without stress and under stress of σB = 600 MPa, which causes residual deformation 0.2%. Heating of the alloy at temperatures up to 350°C for up to 1000 h is accompanied by a

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decrease of ductility. Application of stress in the first 100 h leads to the embrittlement of the specimen (Table 11). Table 11 Phase composition and mechanical properties of alloy VT15 (plate specimens, 2.5 mm thick) in thermally hardened state after long-term heating without stress and under stress. State

Initial 300°C, 100 h 300°C, 1000 h 300°C, 100 h, 600 MPa 300°C, 1000 h, 600 MPa

Established phase composition

β β β+α α+β

Mechanical properties σB, MPa

σ0.2, MPa

δ5, %

astandard, kgf/cm2

1380 1310 7 0.8 1370 1320 6 0.7 1420 1340 4 0.4 Embrittlement without signs of ductility Embrittlement without signs of ductility

The above considerations indicate the need for strict regulation of the amount of alloying elements and impurities in alloys in the cases when their contents are within the limits close to the solubility of solid α- and β-solutions. On the other hand, real conditions in which alloys are to be operated should also be taken into account.

1.6 PHASE TRANSFORMATIONS IN TITANIUM ALLOYS Phase transformations in α+β titanium alloys are characterized by a great diversity and complexity. This concerns, first of all, solid-solution alloys with transition elements used most often for alloying in commercial titanium alloys. The metastable structural state in titanium alloys occurs during operations associated with heating and cooling of the metal – hot deformation, heat treatment, welding, etc. Metastable structures formed have a significant effect on the physical and mechanical properties and this is to be taken into account when processing titanium alloys. Structural transformations occurring during the sharp cooling of titanium alloys with various contents of β-stabilizing elements can be followed using a generalized “titanium–β-stabilizing element” phase diagram (Fig. 13). The diagram is valid for isomorphic β-stabilizing elements, i.e., for elements forming no chemical compounds with titanium. To an approximation, this diagram can be also used for β-stabilizing elements forming eutectoid or peritectoid systems with titanium. The generalized diagram consists of two curves originating from one common point, which corresponds to the titanium allotropic transformation temperature. The lower curve restricts the region of the existence of solid α-solution, and the crosssection of this curve with the abscissa axis in point Cα corresponds to the ultimate concentration of solid solution at room temperature. The upper curve determines the border between the (α+β)- and β-phases, and the point of its cross-section with the

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σB, kgf/mm2

100

Fe Mn

Al

Mo

80

V

Sn

60

Cr Co

Ta Zr

40

Nb

40

Zr

Nb

Ta

Mn Cr

δ5 , %

30 Sn

20 Al

V

Mo

Co Fe

10 0 0

2

4

6

8

Al, Zr, Sn, %

10

0

2

4

6

8

Ta, Nb, V, Mo, %

10

0

2

4

6

8

10

Fe, Cr, Mo, Co, %

Figure 13 Change of phase composition for titanium–β-stabilizing element alloys as a function of the quenching temperature.

abscissa axis, Cβ, corresponds to the minimal concentration of the β-stabilizing element required to form solid β-solution stable within the entire temperature range. This diagram is valid both for binary titanium alloys with β-stabilizing elements and for alloys with several alloying elements, which stabilize the β-phase in titanium. Under equilibrium conditions, the diagram consists of three regions of solid solutions: α, (α+β) and β. As the concentration of the β-stabilizing element is gradually increased, the first process observed during quenching from the β-region is martensite transformation, i.e., the β-phase is not fixed in quenching. As the concentration is increased further, at some point no martensite transformation will occur and a 100% structure will be observed. The concentration of the β-stabilizing element, corresponding to this point, is called the critical concentration (Ccr). If a vertical line is drawn through point Ccr, it will cross the border of the β-region at the point corresponding to the critical temperature, Tcr. As martensite transformation occurs in a temperature range, the dashed lines show the onset (Mo) and end (Me) of martensite transformation. A metastable ω-phase is formed at a concentration of the alloying element within the range from Ccr to C2 at a sharp cooling from the β-region. As this transformation is never complete, alloys have the β+ω phase composition. At a concentration exceeding the value of C2, only the high-temperature β-phase is registered as the result of quenching. A change of the quenching temperature in the (α+β)-region is accompanied by a change in the phase composition of the alloy of a given concentration. Four groups of alloys are to be distinguished by the character of the change of the phase composition at a sharp cooling from various temperatures. The first group includes alloys with the concentration of β-stabilizing elements of up to C1, i.e., alloys which in quenching from the β-region have an exceptionally

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α′(α″) structure. After quenching of these alloys from temperatures of the (α+β)-region, in the range from the temperature of total polymorphic transformation up to T1, their structure is a combination of the α′+α+β-phases. And after quenching from temperatures below Tcr, they have the (α+β) structure. The second group are alloys with the concentration of alloying elements from C1 to Ccr. They have the α′(α″)+β structure and in quenching from the β-region, martensite transformation in them does not complete. Alloys of this group after quenching from temperatures of polymorphic transformation up to Tcr have the (α′+α″+β) structure, and from temperatures below Tcr the (α+β) structure. For alloys of the third group with the concentration of the β-stabilizing elements of Ccr to C2, quenching from temperatures of the β-region or from the polymorphic transformation temperature up to T2 is accompanied by the transformation of part of the β-phase to the ω-phase. Alloys of this type have the (β+ω) structure after quenching. Titanium alloys of a precritical composition, which have the β-phase at the quenching temperature, with the concentration of β-alloying elements from Ccr to C2, may form a small amount of ω-phase during quenching. The effect of this phase on the properties is insignificant. Alloys of this group have an (α+β) structure after quenching from temperatures below T2. Alloys of the fourth group have an exceptionally β structure after quenching from temperatures above total polymorphic transformation; and from temperatures below polymorphic transformation, a (β+α) structure. The above scheme gives a general idea of the transformations occurring in solid-solution titanium alloys at their cooling from various temperatures during heat treatment, thermomechanical treatment, hot deformation, welding and other treatments. The presence of metastable structures in titanium alloys has a significant effect on their physical, mechanical and operating properties. In a manufactured item, titanium alloys are, with rare exceptions, in a stable state, i.e., after annealing or hardening heat treatment. They contain no metastable structural components. However, metastable structural states often occur in various technological operations and in repair work. Some phase components have little effect on the properties, but the effect of others, for instance, the ω-phase, is rather considerable. In alloys with α- and β-stabilizing elements, when their concentration is close to the solubility in α-titanium, a martensite transformation of the β-phase into the α′-phase occurs during accelerated cooling. The α′-phase can be considerably oversaturated by alloying elements. It has a hexagonal structure, the same as α-titanium. The X-ray of the α′-phase features blurred interference lines characteristic of hexagonal titanium, which is due to the generation of internal stress in the lattice. Under an optical or electron microscope, the α′-phase has a typical needle-like structure. The martensite α′-phase has no high hardness and strength, as martensite of steel has. However, formation of the α′-phase in alloys with oversaturation of solid α-solution by alloying elements leads to a noticeable increase of hardness and strength. Another metastable structural component formed in titanium alloys is the α″-phase. It is an even more oversaturated solid solution based on α-titanium and

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occurs in alloys containing elements which are isomorphic to α-titanium – Mo, V, Nb and Ta. An X-ray of alloys with the α″-phase structure is characterized by cleavage of some interference lines peculiar to a hexagonal structure. The cleavage grows as the concentration of the β-alloying element increases. There is a similarity between the α″- and α′-phase, but the α″-phase is distinguished by a lower symmetry – rhombic instead of hexagonal. The rhombic α″-phase can be considered to be an intermediate stage between the body-centered and hexagonal structures. Under an optical or electron microscope, the α″-phase has a needle-like martensite structure. The mechanical properties of the α″-phase are close to the properties of a stable mixture of solid α- and β-solutions of a similar chemical composition. An exceptionally metastable β-solution is observed under an optical microscope in titanium alloys with a high concentration of β-stabilizing elements during sharp cooling. The concentration of β-stabilizing elements at which a solid solution is observed is called a critical concentration and corresponds to an electron concentration equal to 4.18–4.21 el/atom. Earlier, mainly based on metallographic studies, it was believed that quenched alloys, in which α′- or α″-phase are not formed, are homogeneous, i.e., have a completely high-temperature solid β-solution. However, a homogeneous solid β-solution is usually characterized by a good ductility and low hardness. Detailed studies of a solid β-solution, close by its composition to a critical solution, showed for some systems of alloys that microstructurally a homogeneous quenched β-solution has an anomally high hardness and, therefore, is very brittle. A higher hardness and a brittleness of such alloys is explained by the presence of an ω-phase which can be formed under certain conditions during the decomposition of solid β-solution in quenching or aging only in titanium alloys with transition elements. Some investigators believe that the ω-phase should not be singled out into an independent phase, because it is coherent with respect to the matrix (β-phase). It should be considered to be a special state of the solid β-solution. Nevertheless, a serious effect of the ω-phase on the physical and mechanical properties of titanium alloys should be taken into account. Metastable solid solutions – α′, α″, ω and β – formed in accelerated cooling, under the action of temperature and stress can undergo transformations, which have a significant effect on the physical, mechanical and technological properties of titanium alloys. Decomposition of metastable α′-, α″-, ω- and β-phases during heating in the temperature range of 450–650°C makes it possible to obtain a considerable increase of strength at satisfactory ductility by disperse hardening. In conclusion, it should be noted that there is information on the use of the metastable state of titanium alloys. Thus, for instance, heat treatment in metastable solid α″- or β-solution makes it possible to significantly increase the deforming properties of (α+β)-titanium alloys and to slightly increase the crack resistance. Heat treatment of martensite-type (α+β)-titanium alloys in α″-phase enables a minor increase of strength with sufficient ductility preserved.

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1.7 ENHANCEMENT OF MECHANICAL STRENGTH OF TITANIUM ALLOYS High mechanical strength and high-temperature strength of titanium alloys, which are solid α- and β-solutions, can be achieved by alloying, heat treatment, mechanical hardening or a combination of these treatments. Using a sufficiently high strain hardening (50%), the strength of technical-grade titanium can be increased from 350 to 800 MPa. In strain hardening of titanium alloys based on the β-structure, the effect is even more significant: sheets from β-alloy (VT16) can be strengthened up to 1500 MPa by cold deformation by 50–70%. The strength characteristics of titanium alloys can be noticeably increased by thermomechanical treatment, i.e., by choosing an optimal combination of plastic deformation and heat treatment. Alloying of titanium with various elements makes it possible to obtain titanium alloys whose ultimate strength in annealed state reaches 1200–1300 MPa, and the application of a hardening heat treatment (quenching and aging) increases the strength even more. Properties and structure of binary and more complex titanium alloys with stabilized solid α- and β-solutions have been studied sufficiently well. If one considers alloying additives from the group of β-stabilizers, iron and manganese prove to be the strongest strengtheners, then come chromium, tungsten, molybdenum, vanadium, etc. One can be guided by the following average data of the hardening action of alloying elements, used in practical work with titanium alloys (Table 12). In the case of multicomponent alloys, the hardening effect would, probably, be the sum of hardening effects of all elements. Alloying elements, being dissolved in α- and β-phases, act as strengtheners, but also change the ratio of the phases in the structure of the alloys. Thus, the strength characteristics of solid-solution (α+β)-titanium alloys are determined by the strength of α and β components. Another factor determining the strength of these alloys is the heterogeneity of the structure, i.e., the dispersity of the mixture of α- and β-phases. An increase of the extent of interphase boundaries is accompanied by a significant increase of strength and reaches a maximum in alloys, which are a mixture of an approximately the same amounts of α- and β-phases. Table 12 Increase of ultimate strength of titanium in MPa per percent of alloying additive. Element Aluminum Tin Zirconium Molybdenum Vanadium Tungsten

∆σB, MPa

Element

∆σB, MPa

50.0 25.0 20.0 50.0 35.0 35.0

Manganese Chromium Iron Cobalt Silicon Niobium

75.0 65.0 75.0 55.0 12.0 15.0

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β

α+β

α 50% α

50% β

t, °C

σB, Post annealing

VALENTIN N. MOISEYEV

α+β α

50% α

β 50% β

Figure 14 Change of the ultimate strength of annealed binary titanium alloys within a broad range of β-stabilizing elements.

This tendency can be illustrated by example of annealed Ti–Mo and Ti–V alloys (Fig. 14) and also by a generalized diagram (Fig. 15). Strength of titanium alloys in annealed and aged state can be significantly increased. The metastable α′-, α″-, ω- and β-phases formed during a sharp cooling of (α+β)-titanium alloys, depending on the content of β-stabilizing elements and → α transformation. Isothermal quenching temperature, are intermediate phases of β ← heating at temperatures over 350–400°C leads to the decomposition of the metastable phases to form an equilibrium (α+β) structure. At the first stage of decomposition at temperatures of 450–600°C, disperse particles of α- and β-phases are formed to lead to a significant hardening of alloys. A high strength at a satisfactory ductility can be achieved in various alloys with (α+β) structures by selecting quenching and aging modes. Depending on the type of β-stabilizing element, its content in the alloy, quenching temperature and aging mode the extent of hardening can differ. Figure 16 demonstrates a change of strength of heat treated alloys of titanium with various β-stabilizing elements within a wide range of compositions (covering α-, (α+β) and β-regions) by example of Ti–Mo and Ti–V alloys, which is typical of other alloys. The alloys were quenched and aged for maximum strength on condition of preserving a certain ductility ( δ 5 ≥ 3% , ( ψ ≥ 6%). As the quenching temperature was increased up to the α+β → β transition boundary and the aging temperature was decreased down to 450°C, the strength increased and ductility decreased in all (α+β) alloys. The data of the figure show that the strength, which can be obtained by

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σB, kgf/mm2

2 σB, kgf/mm2 σB, kgf/mm σB, kgf/mm2

σB, kgf/mm2

40 140

4

9

120

50

20

4

9

1 1.5 3

7 11 13 16 Mn, % 2 2 1

2

2 4

1.5 3 7 11 13 16 9 15 Mn, %20 V, % 1

2 4

9

20

15

V, % 2 1 2 1

2 4 20

9

152

V, % Ta, %

20

50

50

20

Nb, %

1

Mn, %

1

2 σB, kgf/mm 2 σB, kgf/mm

Fe, % Cr, % 160 60 160 1 140 40 1 140 0.5 2 4 9 12 0.5 2 4 1 9 12 15 120 Fe, % Cr, % 2 120 160 100 1 2 100 2 140 80 2 80 120 60 60 100 40 0.5 2 4 9 12 0.5 2 4 9 12 15 40 80 1.5 3Fe, 7 9 11 13 16 20 5 % 30 Cr, % Mo, % 160 60 1 160 140 40 1401.5 3 5 7 9 11 13 16 20 30 2 120 Mo, % 120 160 1 100 100 140 2 80 80 120 60 1 60 100 40 401.5 3 5 7 9 11 132 16 20 30 80 50 4 9 4 9 20 Mo, % Nb, % 160 60

31

Ta, % 1

100 2

2 80 60 40 4

9

50

20 Nb, %

4

9

50

20 Ta, %

Figure 15 Ultimate strength of annealed and thermally hardened (quenched and aged) titanium alloys vs the content of β-stabilizing elements.

quenching and aging, increases with the increase of the β-stabilizing element up to the critical concentration. Critical-composition alloys, as a rule, can be heat treated up to maximum strength (Fig. 15). An increase of the amount of the β-alloying element in super-

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βinstab → α + β

Ms

t, °

βinstab → α + β α'(α'') → α + β

α

α+β βinstab,α'(α'')

β

βinstab

Figure 16 Change of the ultimate strength of thermally hardened (quenched and aged) binary titanium alloys within the broad range of β-stabilizing elements.

critical-composition alloys contributes to the stabilization of the β-phase and the hardening effect during aging decreases. This change of the mechanical properties in heat-strengthened state is due to the rise of the heterogeneity of the structure and the increase of disperse formations of the α- and β-phases in aging with the concentration of the β-stabilizing element increased up to the critical concentration. It can be said that the maximal amount of β-phase undergoes a dispersive decomposition in alloys of critical composition. If we are to consider the volume of β-phase decomposed in aging as the difference between the volume fixed by quenching and the volume obtained after aging, then it is evident that critical-composition alloys are optimal for a given system of titanium with β-stabilizing element. Figure 16 shows the change of the phase composition of Ti–Mo alloys depending on the composition after quenching from the temperatures of the (α+β)region near polymorphic transformation. Quenching from these temperatures makes it possible to obtain the highest ultimate strength at a satisfactory ductility. The first dashed line shows the quenching temperature per maximal amount of metastable α′(α″)-phase or α′(α″)+β-phase. The second dashed line indicates the quenching temperature per maximal amount of metastable β-phase. Table 13 shows the change of the effect of hardening after quenching and aging per maximal strength, depending on the content of molybdenum in a Ti–Mo alloy. In the table, the amount of decomposed β-phase is a value expressed by the ratio V βquench – V βquench+age -------------------------------------------------------- × 100% , where Vβquench is the volume of β-phase after V βquench+age quenching; Vβquench+age is the volume of β-phase after quenching and aging.

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Table 13 Change of the effect of hardening after heat treatment of an alloy depending on the content of molybdenum. Content of molybdenum in the alloy, %

Amount of decomposed β-phase, %

0 2.5 5.0 7.5 10.0 15.0 20.0 25.0 30.0

0 17 35 50 66 50 33 18 0

Effect of hardening after quenching and aging, % after quenching to α′(α″) or α′(α″)+β-phases

after quenching to β-phase

0 65 70 75 – – – – –

0 25 40 60 80 40 32 28 10

The effect of hardening as the result of quenching and aging is presented as a σ B quench+age – σ B anneal ratio ---------------------------------------------------------- × 100% , where σB quench+age is the tensile strength of σ B anneal the quenched and aged alloy; σB anneal is the ultimate strength of the annealed alloy. Data of Fig. 16 and Table 14 confirm that the greatest effect of hardening is observed in alloys with the maximally decomposed amount of β-phase, i.e., in alloys of critical composition. Table 14 Effect of hardening during quenching and aging of binary titanium alloys of critical composition with various alloying elements. Alloy

Content of alloying element, %

Amount of decomposed β-phase, %

Effect of hardening after quenching and aging, %

Ti–Fe Ti–Mn Ti–Cr Ti–Mo Ti–V Ti–Nb Ti–Ta

4 7 7 10 15 35 50

73 69 59 66 55 37 28

85 66 63 80 38 28 18

Thus, by the effect of hardening the binary alloys of titanium with investigated β-stabilizing elements can be arranged in the following sequence (with respect to the increasing capability of these elements to stabilize the β-phase in titanium): Ti–Ta, Ti–Nb, Ti–V, Ti–Cr, Ti–Mo, Ti–Mn and Ti–Fe. Any further hardening of solid α- and β-solutions in thermally hardened, as in annealed, alloys, can be achieved by using additives of an α-stabilizing element, for

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σB, GPa

1.4

VT15 VT14

1.2 1.0

VT1-0

σ0.2, GPa

0.4

VT15

VT14

1.3 1.1 0.9 0.7

OT4 VT16

0.5 0.3 δ 5, %

VT16

OT4

0.8 0.6

VT1-0 VT1-0

30 20 10

VT15 0

OT4

VT16 VT14

10 20 30 40 50 60 70 Cold deformation, %

Figure 17 The mechanical properties of various titanium alloys vs the extent of cold deformation (sheet, 1.5 mm).

instance, aluminum, or neutral strengtheners dissolving in both α- and β-titanium – tin and zirconium. Increase of strength of titanium alloys by cold deformation (cold work hardening) is used restrictedly. Cold deformation is not widely used for increasing the strength of titanium alloys. Nevertheless, one comes across this technique when dealing with titanium alloys. At present, fastening items (bolts, screws, etc.) from titanium alloy VT16 are manufactured using deformation hardening. This method is rather efficient from the point of view of obtaining a required strength of alloy and labor intensity of the process. Sheet semifinished items from titanium alloys after annealing are planished in rolls with a deformation of 2–5% at temperatures considerably lower than the recrystallization temperatures. A certain hardening should be taken into account in further process treatment and further application of such semifinished items. Hardening due to cold work is determined by an increase of the density of dislocations in the metal. In most perfect monocrystals of pure titanium, it varies from 103 to 105 cm–2. In polycrystalline titanium, the density of dislocations is 107– 109 cm–2, and in deformed titanium it reaches 1010 –1011 cm–2. At the same time, a deformation increase is not always accompanied by a respective increase of the density of dislocations. It rises the most intensively at small deformations. The effect of hardening in titanium alloys with stable α, (α+β) and β structures depending on the extent of cold deformation was assessed in studies of commercial titanium alloys with various structures, which allow a significant plastic deformation – VT1-0 (α structure), OT4 (α + 5% β structure), VT14 (α + 12% β structure), VT16 (α + 25% β structure) and VT15 (β structure) in a stable annealed state. The

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60 50 40 30 20 10

0

0.4

0.8

1.2 1.6 Kβ →

2.0

Figure 18 The effect of cold hardening of titanium alloys vs phase composition.

β structure in alloy VT15 (Ti–3% Al–7% Mo–11% Cr) was obtained by heating in the β-region (800°C) followed by cooling in air. The other alloys after annealing were cooled at a rate of 3°C/min. Alloys as sheet specimens were deformed by rolling with various degrees of deformations to obtain sheets 1.5-mm thick to the emergence of visible cracks on the margins. The mechanical properties of cold work-hardened sheets were determined in specimens cut along the rolling direction. Figure 17 presents the changes of ultimate strength, yield strength and relative elongation of the sheets depending on the extent of deformation. The greatest effect of hardening was observed in single-phase titanium alloys with α (VT1-0) or β (VT15) structures. As the content of the β-phase in the series OT4, VT14, VT16 increases, the hardening effect of cold-work hardening decays and is observed only at the initial stage of deformation (Fig. 18). Phase transformations in the process of deformation also affect the hardening of the alloys. Alloy VT16, which is distinguished by the lowest effect of hardening as → β phase transformation. Alloys the result of plastic deformation, features an α ← VT1-0, OT4 and AT15, having the greatest effect of cold-work hardening, featured no phase transformations in deformation.

1.8 ENHANCEMENT OF HIGH-TEMPERATURE STRENGTH OF TITANIUM ALLOYS The general ideas of the theory of metal high-temperature strength extend to titanium alloys. It is generally recognized that the main factors determining the high-temperature

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strength of metals are melting temperature, strength of interatomic bonds, diffusion processes, structure and some other factors. However, the specific properties of titanium are a reason for some peculiarities of its behaviour at elevated temperatures. Thus, having the melting temperature (1668°C) higher than that of, for instance, nickel (1455°C), titanium has a lower high-temperature strength. Nickel hightemperature alloys operate at temperatures of up to 900–1100°C, whereas for hightemperature titanium alloys the range of working temperatures is limited by 450–550°C. Table 15 presents the data on the creep resistance during compression for some metals with high melting temperature. The tests were carried out in vacuum (to avoid oxidation) up to 1% residual deformation for 24 h at a temperature of 1000°C. As seen in the table, the creep resistance of titanium, whose melting temperature is higher than that of nickel, is more than half as less. Table 15 Creep resistance of metals at compression. Metal

Melting temperature, °C

Tungsten Iridium Molybdenum Tantalum Chromium Niobium Rhodium Cobalt Iron Nickel Vanadium Titanium Zirconium

3410 2494 2625 3000 1800 2000 1966 1500 1539 1450 1750 1665 1750

Creep strength, kgf/mm2 9.5 9.5 4.7–6.0 4.7–6.0 3.15–4.7 4.3 4.7 1.05 0.7 0.35 0.35 0.14 0.14

Density, g/cm3 19.2 22.4 10.2 16.2 7.2 8.5 12.3 8.9 7.8 8.9 6.0 4.5 6.37

Creep strength to density ratio 0.5 0.43 0.45–0.6 0.3–0.45 0.46 0.5 0.4 0.12 0.09 0.04 0.06 0.03 0.02

Thus, the existing views of the close relation of high-temperature strength to only melting temperature of metals is not absolute. At polymorphic transformation temperature, metals which undergo polymorphic transformation, including titanium, are characterized by a significant weakening of interatomic forces and, as a rule, a sharp decrease of high-temperature strength. Taking into account that in titanium alloys the polymorphic transformation occurs in a temperature range, one should expect a sharp decrease of hightemperature strength within this range. Table 16 compares the values of temperatures of (α+β→β) polymorphic transformation and recommended working temperatures for solid-solution commercial titanium alloys. The column “Recommended working temperature” gives the ultimate recommended working temperature for alloys used in long-life aircraft items. It is

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determined based on the analysis of the prolonged high-temperature strength characteristics and on the experience of operation of items from titanium alloys. Table 16 Temperature of (α+β→β) polymorphic transformation of commercial titanium alloys and recommended temperatures for their use in long-life items (in aircraft building and engine building). Alloy

VT20 VT5-1 VT6 OT4 VT14 OT4-1 OT4-0 VT22 VT16 VT35 VT32

Τemperature of α+β→β transformation, °C 980–1020 980–1030 980–1000 960–1000 920–960 910–950 860–920 860–890 840–880 770–790 750–780

Kβ

<2.5 <2.5 <2.5 <2.5 0.3 <2.5 <2.5 1.1 0.7 1.5 1.8

Recommended working temperature, °C (no more than) Annealed

Quenched and aged

500 450 400 350–400 350–400 350 300 350 350 350 400

8 – 400 – 350 – – 350 300 300 400

As it follows from the data presented, there is a correlation between the temperature of (α+β→β) transformation and the rate of high-temperature strength decrease for commercial titanium alloys based on solid α- and β-solutions. The absolute value of high-temperature strength of an alloy depends to a great extent on the properties of solid α- and β-solutions. One more feature of titanium alloys is their slightly increased tendency to creep and to a long-term strength decrease depending on the temperature and duration of heating. This tendency takes place not only at elevated temperatures but also at room temperature. Figure 19 presents a change of the long-term strength of two titanium alloys VT3-1 and VT8 in annealed state depending on the duration and temperature of the test. When selecting high-temperature titanium alloys, of great importance if the “heat stability,” i.e., the ability to preserve invariable physical and mechanical properties during the operation at increased temperature, duration and stresses occurring in the metal. It should be taken into account that these factors can contribute to structural transformations in the metal and affect the high-temperature strength of the alloys. Numerous studies and experience of work with titanium alloys indicate that high-temperature strength can be increased by the following: • cold-work hardening by deformation at temperatures lower than recrystallization temperature;

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σ, kgf/mm2

VALENTIN N. MOISEYEV

110 100 90 80 70 60 50 110 100 90 80 70 60 50 40

20°C

(a)

100 300 400 450

20°C

(b)

100 300 450 500

102 103 104 Time before breakdown, h

105

Figure 19 Change of the long-term strength of as-annealed high-temperature alloys VT3-1 and VT8 vs temperature and time of test.

• fusion with components which form solid solutions with the basic metal; • formation of a highly dispersed mixture of phases of solid solutions by heat treatment or thermomechanical treatment, quenching and aging; • fusion with elements producing disperse formations of chemical compounds in crystallization of liquid metal or in heat treatment; • fusion with components forming a new high-temperature phase in the shape of a net or skeleton-like core in the solid-solution matrix (natural composite) during crystallization or heat treatment; • development of high-temperature alloys with an ordered structure, which represents chemical compounds of the type of Ti3Cl, TiAl – absolutely different material as compared with titanium. Combinations of these methods are possible. We should note that these methods of increasing high-temperature strength are not always easily realizable or have a significant effect. Cold-work hardening is not widely used in practice due to its low efficiency and limited temperature range of applications. Its effect is preserved at a prolonged heating at temperatures of not higher than 300-400°C. The main method of producing modern commercial alloys, including hightemperature alloys, is solid-solution alloying. The additive used for this purpose most often is aluminum, which dissolves in α-titanium within 5–8%, thus increasing the polymorphic transformation temperature. At a higher Al content, an α2-phase is formed – ordered solid solution of Ti3Al, which sharply decreases ductility. Aluminum is used in virtually all commercial alloys, i.e., it is the most efficient strengthener increasing high-temperature strength. Other metals used to stabilize the α-modification of titanium and of interest as additives to high-temperature alloys are gallium, indium, antimony, and bismuth.

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Of special interest is gallium, due to its high solubility in α-titanium. Neutral strengtheners – zirconium and tin – are widely used as alloying elements. They have minor effect on the polymorphic transformation temperature but significantly increase the high-temperature strength of solid solutions. The advantages of titanium alloys with α structure are high thermal stability, good weldability, but these alloys are distinguished by a decreased ductility. β-Stabilizing elements are also used in high-temperature alloys, together with the elements stabilizing the α-phase in titanium. Introduction of β-stabilizing elements, even in minor amounts, is accompanied by heterogenization of the structure, which increases the strength and high-temperature strength of the alloys. The advantages of heterogeneous solid (α+β)-solution as compared with solid α-solution consists in a higher ductility and, mainly, in a possibility to perform a hardening heat treatment by quenching and aging. Elements isomorphic to β-titanium – molybdenum, vanadium, more rarely niobium – are used as β-stabilizing elements in high-temperature alloys. Eutectoid elements – chromium, iron, tungsten and other elements – are used within the limits providing the existence of stable solid β-solution. Solid-solution titanium alloys can be operated long-term as a high-temperature material at temperatures of up to 500–550°C. The use of a hardening heat treatment (quenching and aging) to produce a disperse mixture of α- and β-phases and an increase of strength and hightemperature strength gives an effect only if the alloys are operated up to temperatures of 400–450°C. At higher temperatures, particles of the mixture of solid solutions coagulate and the strength and high-temperature strength rapidly decrease. The most efficient method of increasing high-temperature strength is fusion with components which produce disperse formations of chemical compounds in crystallization or subsequent heat treatment. The basis of these alloys is the solidsolution matrix, and little-soluble elements – silicon, carbon, boron and some others – are used as components forming chemical compounds. Many commercial high-temperature titanium alloys of this type contain excess amounts of aluminum (more than the solubility limit), which makes it possible to increase their high-temperature strength due to the forming α2-phase without any significant decrease of ductility. The predominant majority of modern commercial high-temperature titanium alloys, intended for engine blades and disks, are based on this principle of alloying. These alloys can be operated long-term at temperatures of up to 550–600°C. A serious problem in producing heterogeneous intermetallic dispersion structures is to control their formation. Chemical compounds formed in the melting of an ingot are of nonregulated size, are arranged irregularly in the body of the ingot and border of the grain, are rather stable and do not yield to transformation in heat treatment. Therefore, in commercial titanium alloys the excess of the chemical compound and the process of its formation are restricted by the initial stage of rearrangement of the lattice. The effect of high-temperature strength increase in alloys is the greatest just at the initial stage of lattice rearrangement. The most acceptable method of producing optimal-size disperse particles of

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chemical compounds in titanium alloys is superfast crystallization to obtain oversaturated solid solutions and then aging at various temperatures. The kinetics of formation of chemical compounds in titanium alloys has been studied insufficiently yet, though it is of great interest for developing novel more high-temperature alloys based on solid solutions and dispersion intermetallic hardening. Table 17 gives the high-temperature strength characteristics of titanium alloy VT22 (Ti–5% Al–5% Mo–5% V–1% Fe–1% Cr) and alloy VT22PT, additionally alloyed with 0.25% C and 0.20% B. Table 17 High-temperature strength characteristics of titanium alloys VT22PT and VT22. Alloy

σ 450 B

σ 500 B

450 σ 100

450 σ 0.2/100

MPa VT22PT VT22

1070 880

960 760

650 520

230 120

For dispersion and even distribution of chemical compounds over the body of the grain, the technology of superfast crystallization of alloy granules was used, followed by compactization and heat treatment of the alloy. High-temperature strength increase achieved on a model titanium alloy due to the controlled dispersion hardening by chemical compounds shows the efficiency of the method and raises hopes that it could be widely used to increase the hightemperature strength of titanium alloys. One more way to increase the high-temperature strength of titanium alloys is a mechanism of developing the intermetallic core in the plastic solid-solution matrix (natural composite). This aim can be achieved by fusing titanium with elements forming the eutectoid – iron, chromium, manganese, tungsten, cobalt, etc. As eutectoid transformation is weak in most cases, there is the problem of developing commercial alloys with eutectoid type of hardening. Table 18 and Fig. 20 show the effect of increasing high-temperature strength of titanium alloys by developing the intermetallic core in the plastic solid-solution matrix (natural composite) by example of Ti–4% Fe alloy. Heat treatment for eutectoid (α+TiFe) at good ductility provides a high hightemperature strength which considerably exceeds the high-temperature strength of this alloy in solid-solution (α+β) state. If the volume of the eutectoid component could be increased, the high-temperature strength of the alloy could be made even greater. This aim could also be achieved by selecting a more high-temperature eutectoid and solid-solution matrix. In the recent years, great attention was paid to high-temperature titanium alloys, which are chemical compounds of titanium with aluminum Ti3Al (α2-phase) and TiAl (γ-phase). Alloys of this type, especially aluminide TiAl, are heat-resistant up to temperatures of 800–850°C. In the recent years, great attention was paid to high-temperature titanium alloys, ooooo

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Table 18 Effect of increasing the high-temperature value of alloy Ti–4 Fe by heat treatment to produce eutectoid. Heat treatment

Phase composition

σB,

δ5

ψ

σ 500 B

% Annealing Treatment to produce eutectoid

α+β (α+β)+eutect.

750 520

20 23

550 σ 100

MPa 45 42

300 450

80 250

which are chemical compounds of titanium with aluminum Ti3Al (α2-phase) and TiAl (γ-phase). Alloys of this type, especially aluminide TiAl, are heat-resistant up to temperatures of 800–850°C. The cause of high-temperature strength of aluminides is the limited number of slip planes and the blocking action of impurity atoms. Besides, in the case of the chemical compound TiAl, there is no allotropic transformation and its crystal lattice remains invariable up to the melting temperature of 1460°C. → β phase transIn the case of the chemical compound Ti3Al (α2-phase), the α ← formation occurs at a high temperature (over 1000°C), which determines the elevated high-temperature strength of this intermetallic. A drawback of titanium aluminides is their low ductility. To increase it, alloys of this type are additionally alloyed by a minor amount of β-stabilizing elements (Cr, Mo, Nb and others). A good quality of alloys based on titanium aluminides is their low density, which is of primary importance in some fields of machine building. Alloys of greatest interest from the point of view of high-temperature strength, based on the chemical compound of TiAl type (Ti–48% Al–2% Ni–2% Cr), have not yet found commercial use due to their low ductility and crack resistance. One more trend for developing high-temperature titanium alloys based on aluminides is to use the chemical compound Ti3Al (α2-phase) in combination with a sufficiently large volume of β-phase, which makes it possible to increase the ductility

(a)

Figure 20 Structure of Ti–4% Fe alloy heat treated for eutectoid.

(b)

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of the alloy. This applies to alloys of the type of Ti–15% Al–22% Nb–1.5% Zr–1.5% Mo. Another direction is the development of titanium alloys based on β-structure with an intermetallic type of hardening (dispersive hardening or core hardening or both simultaneously). Here a decreased diffusion mobility in the β-phase as compared with the diffusion mobility in the α-phase, as well as the effect of intermetallic hardening are used. Such alloys make it possible to change the structure and properties of alloys within a wide range by heat treatment and thermomechanical treatment and to control the characteristics of heat treatment and ductility. These are alloys of the type of Ti–6% Al–5% Mo–25% Nb and Ti–6% Al–4% Fe–20% Nb. Both alloys contain minor amounts of silicon, carbon and boron. The typical properties of an alloy based on the β-structure are given in Table 19. Table 19 Properties of alloy Ti–6 Al–4 Fe– 20 Nb at various temperatures. Properties

Test temperature, °C 20

σB, MPa σ0.2, MPa δ, % ψ, % σ100, MPa

1050–1150 1000–1050 ≥4 ≥6 –

600 ≥1000 ≥950 ≥6 ≥8 ≥550

700 ≥700 ≥870 ≥8 ≥10 ≥350

A drawback of high-temperature β-alloys is an increased density as compared with titanium alloys based on aluminides.

References 1. Moiseyev, V.N. (2001) Titanium and titanium alloys. In Nonferrous Metals and Alloys (Encyclopedia). Moscow: Mashinostroenie, vol. II-3, pp. 272–353 (in Russian). 2. Moiseyev, V.N. (2001) Half a century of Russian titanium. Natsionaln. Metallurgiya 3: 25–30 (in Russian). 3. Glazunov, S.G. and Moiseyev, V.N. (1974) Structural titanium alloys. Moscow: Metallurgiya (in Russian). 4. Solonina, O.P. and Glazunov, S.G. (1976) Heat-resistant titanium alloys. Moscow: Metallurgiya (in Russian). 5. Borisova, E.A. et al. (1980) Metallography of titanium alloys. Moscow: Metallurgiya (in Russian). 6. Belov, S.P. et al. (1992) Physical metallurgy of titanium and its alloys. Moscow: Metallurgiya (in Russian). 7. Moiseyev, V.N. and Luzhnikov, L.P. (1960) Diffusion of gases in air-heated titanium. Titanium and its alloys. Moscow: USSR Acad. Sci., issue 3, pp. 17–22 (in Russian). 8. Khorev, A.I. et al. (1963) Effect of oxygen on the properties and heat treatment of titanium alloy VT14. Tsvetn. Metallurgiya 2: 69–74 (in Russian).

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9. Moiseyev, V.N. et al. (1980) Effect of oxygen on the structure and properties of alloy VT22. Tekhnol. Lyogkikh Splavov 8: 28–32 (in Russian). 10. Maksimovich, G.P. et al. (1982) Effect of a long-term high-temperature vacuum annealing on the structure and properties of titanium alloys. Metalloved. Term. Obrabotka Metallov 7: 11–14 (in Russian). 11. Glazunov, S.G. and Moiseyev, V.N. (1974) Structural titanium alloys. Moscow: Metallurgiya, pp. 18–26 (in Russian). 12. Glazunov, S.G. and Kolachev, B.A. (eds.) (1992) Physical metallurgy of titanium and its alloys. Moscow: Metallurgiya, pp. 65–84 (in Russian). 13. Glazunov, S.G. and Yakimova, A.M. (1960) Effect of hydrogen and oxygen on the structure and properties of alloy VT8 of the Ti–Al–Mo system. In Titanium and its Alloys. Moscow: Oborongiz, pp. 21–29 (in Russian). 14. Borisova, E.A. and Glazunov, S.G. (1960) Effect of oxygen and hydrogen on the mechanical properties of technical-grade titanium at temperature of –196°C up to 350°C. In Titanium and its Alloys. Moscow: Oborongiz, pp. 114–120 (in Russian). 15. Yakimova, A.M. (1961) Mechanical properties and structure of alloy VT9 of the Ti–Al–Mo–Sn–Si system. In Titanium in Industries. Moscow: Oborongiz, pp. 203–215 (in Russian). 16. Moiseyev, V.N. (1960) Heat treatment and mechanical properties of titanium alloys with 5–13% aluminum. Metalloved. Term. Obrabotka Metallov 6: 30–39 (in Russian). 17. Luzhnikov, L.P. and Moiseyev, V.N. (1961) Alloys of the Ti–Mn and Ti–Mn–Al systems. Metalloved. Term. Obrabotka Metallov 7: 25–34 (in Russian). 18. Moiseyev, V.N. (1962) Heat treatment and mechanical properties of the Ti–Mo–Al system. Metalloved. Term. Obrabotka Metallov 4: 14–26 (in Russian). 19. Khorev, A.I. and Moiseyev, V.N. (1965) Studies of Ti–Al–Mo–Fe alloys. Tsvetn. Metally 1: 84–89 (in Russian). 20. Moiseyev, V.N. and Sholokhova, L.V. (1972) Comparison of the effects of iron, chromium, manganese, molybdenum and vanadium on the properties of low-alloyed titanium alloys. Moscow: Department of Scientific and Technical Information, All-Russian Institute of Aircraft Materials, pp. 160–168 (in Russian). 21. Dolzhansky, Yu.M. and Moiseyev, V.N. (1973) Studies of the statistical regularities of the effect of alloying elements on the mechanical properties of Ti–Al–Mo–V alloys. Izv. Vuzov, Tsvetn. Metallurgiya 4: 42–46 (in Russian). 22. Moiseyev, V.N. (1967) Properties and heat treatment of Ti–Mn and Ti–Mn–Al alloys. Metalloved. Term. Obrabotka Metallov 2: 132–137 (in Russian). 23. Moiseyev, V.N. and Sholokhova, L.V. (1968) Effect of chromium, manganese, molybdenum, and vanadium on the properties of Ti–Al–Sn alloys. In Applications of Titanium Alloys, Part 2. Moscow: Department of Scientific and Technical Information, All-Russian Institute of Aircraft Materials, pp. 72–80 (in Russian). 24. Moiseyev, V.N. (1969) Properties and heat treatment of Ti–Fe and Ti–Fe–Al alloys. Metalloved. Term. Obrabotka Metallov 5: 2–7 (in Russian). 25. Moiseyev, V.N. (1971) Properties and heat treatment of Ti–V and Ti–V–Al alloys. Metalloved. Term. Obrabotka Metallov 3: 24–28 (in Russian). 26. Moiseyev, V.N. (1971) Properties and heat treatment of Ti–Nb and Ti–Nb–Al alloys. Metalloved. Term. Obrabotka Metallov 9: 35–38 (in Russian). 27. Moiseyev, V.N. (1975) Properties and heat treatment of Ti–Co and Ti–Co–Al alloys. Metalloved. Term. Obrabotka Metallov 4: 30–33 (in Russian). 28. Moiseyev, V.N., Dolzhansky, Yu.M., Zakharov, Yu.I. et al. (1979) Studies of the structure and properties of Ti–Al–Mo–V–Fe–Cr–Zr–Sn alloys. Metalloved. Term. Obrabotka Metallov 11: 59–61 (in Russian). 29. Moiseyev, V.N. (1964) Major prerequisites for the development of high-strength titanium alloys with α+β structure by alloying and heat treatment. Physical metallurgy of titanium. Moscow: Nauka, pp. 177–183 (in Russian). 30. Moiseyev, V.N. (1965) Changes of the structure and properties of titanium alloys depending on heat treatment. Metalloved. Term. Obrabotka Metallov 5: 3–9 (in Russian).

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31. Moiseyev, V.N. (1963) High-strength titanium alloys with α+β structure. Ph.D. thesis. Moscow: All-Russian Institute of Aircraft Materials, pp. 22–45 (in Russian). 32. Kolachev, B.A., Moiseyev, V.N., and Belov, S.P. (1992) On the relation of the properties of titanium alloys to their chemical and phase composition. Physical metallurgy of titanium and its alloys. Moscow: Metallurgiya, pp. 215–234, 244–251 (in Russian). 33. Moiseyev, V.N. (1992) Effect of cold deformation on mechanical properties. Physical metallurgy of titanium and its alloys. Moscow: Metallurgiya, pp. 97–99 (in Russian). 34. Moiseyev, V.N. (1992) Chemical compounds in titanium alloys. Physical metallurgy of titanium and its alloys. Moscow: Metallurgiya, pp. 179–192 (in Russian). 35. Moiseyev, V.N. et al. (1980) Effect of carbon on the structure and properties of alloy VT22. Tekhnol. Lyogkikh Splavov 8: 32–37 (in Russian). 36. Moiseyev, V.N., Sysoyeva, N.V., and Karpova, I.G. (1988) Effect of carbon on the structure and phase composition of alloy VT22 hardened from liquid state. Metalloved. Term. Obrabotka Metallov 12: 13–15 (in Russian). 37. Sysoyeva, N.V., Moiseyev, V.N., and Polyakova, I.G. (1995) Carbides and borides in titanium alloys obtained by rapid hardening from a melt. Tekhnol. Lyogkikh Splavov 6: 60–66 (in Russian). 38. Moiseyev, V.N. (1992) Processes of structure ordering. Physical metallurgy of titanium and its alloys. Moscow: Metallurgiya, pp. 187–192 (in Russian). 39. Moiseyev, V.N. (1992) Eutectoid transformation. Physical metallurgy of titanium and its alloys. Moscow: Metallurgiya, pp. 182–187 (in Russian). 40. Goncharova, V.N. (1962) Heat treatment and stability of alloy VT8 in long-term tests. In Applications of Titanium Alloys. Moscow: Department of Scientific and Technical Information, All-Russian Institute of Aircraft Materials, pp. 59–67 (in Russian). 41. Glazunov, S.G., Tarasenko, G.N., and Polkin, I.S. (1963) Stability of the β-phase in alloy VT15. In Applications of Titanium Alloys. Moscow: Department of Scientific and Technical Information, All-Russian Institute of Aircraft Materials, pp. 38–45 (in Russian). 42. Potapenko, Yu.I. and Efimova, M.V. (1963) Studies of the stability of the β-phase in VT14 and VT15 alloys. In Applications of Titanium Alloys. Moscow: Department of Scientific and Technical Information, All-Russian Institute of Aircraft Materials, pp. 46–58 (in Russian). 43. Moiseyev, V.N. and Shokholova, L.V. (1972) Thermal stability of structural titanium alloys OT4 and OT4-1 in long-term operation at increased temperatures. In Titanium Alloys and their Applications. Moscow: Department of Scientific and Technical Information, All-Russian Institute of Aircraft Materials, p. 14 (in Russian). 44. Moiseyev, V.N. and Znamenskaya, E.V. (1977) Effect of heating in the α+β-region on the properties and structure of titanium alloys. In Titanium Alloys. Moscow: Department of Scientific and Technical Information, All-Russian Institute of Aircraft Materials, pp. 73–75 (in Russian). 45. Moiseyev, V.N. et al. (1981) Studies of the metastable transformations in titanium alloys by the method of acoustic characteristics measurements. Metalloved. Term. Obrabotka Metallov 12: 39–41 (in Russian). 46. Moiseyev, V.N. and Antipov, A.I. (1995) Effect of aluminum on the stability of β-phase in β-titanium alloys. Metalloved. Term. Obrabotka Metallov 9: 30–35 (in Russian). 47. Antipov, A.I. and Moiseyev, V.N. (1997) On the β-stabilization coefficient of titanium alloys. Metalloved. Term. Obrabotka Metallov 12: 2–5 (in Russian). 48. Vinogradova, E.A., Lashko, N.F., and Moiseyev, V.N. (1963) Metastable structural transformations and their effect on the properties of α+β-titanium alloys. In Titanium and its Alloys. Moscow: USSR Acad. Sci., Issue X, pp. 293–299 (in Russian). 49. Fedotov, S.G. (1964) On the metastable phases in titanium alloys and conditions of their formation. Physical metallurgy of titanium. Moscow: Nauka, pp. 308–314 (in Russian). 50. Kishkin, S.T. et al. (1965) Structural transformations in titanium alloys. In New Studies of Titanium Alloys. Moscow: Nauka, pp. 82–88 (in Russian). 51. Moiseyev, V.N. (1965) Change of the structure and properties of α+β-titanium alloys

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depending on heat treatment. In New Studies of Titanium Alloys. Moscow: Nauka, pp. 198–205 (in Russian). 52. Kolachev, B.A. and Lyasotsky, V.S. (1966) Metastable phase diagram of the titanium–molybdenum system. In Production of Titanium Alloys. Moscow: Department of Scientific and Technical Information, All-Russian Institute of Light Alloys, pp. 4–14 (in Russian). 53. Nosova, G.I. (1968) Phase transformations in titanium alloys. Moscow: Metallurgiya (in Russian). 54. Ermolova, M.I., Znamenskaya, E.V. et al. (1969) Studies of phase transformations in heat treatment of high-strength titanium alloy VT22. In Production of Titanium Alloys. Moscow: All-Russian Institute of Light Alloys, No 5, pp. 94–101 (in Russian). 55. Moiseyev, V.N. (1972) Martensite transformations in titanium alloys with metastable β-phase during deformation. Metalloved. Term. Obrabotka Metallov 5: 18–23 (in Russian). 56. Moiseyev, V.N. et al. (1981) Studies of the metastable transformations in titanium alloys by the method of acoustic characteristics measurements. Metalloved. Term. Obrabotka Metallov 12: 39–41 (in Russian). 57. Moiseyev, V.N. (1992) Phase transformations occurring in the metastable β-phase under the action of plastic deformation. Physical metallurgy of titanium and its alloys. Moscow: Metallurgiya, pp. 187–192 (in Russian). 58. Moiseyev, V.N. (1964) Major prerequisites for the development of high-strength titanium alloys with α+β structure by alloying and heat treatment. Physical metallurgy of titanium. Moscow: Nauka, pp. 177–183 (in Russian). 59. Moiseyev, V.N. (1968) New commercial high-strength titanium alloys. Moscow: State Institute of Sci. & Techn. Information, pp. 1–29 (in Russian). 60. Moiseyev, V.N. (1977) Prospects of developing the strengthening heat treatment of titanium alloys. Metalloved. Term. Obrabotka Metallov 10: 63–68 (in Russian). 61. Moiseyev, V.N., Znamenskaya, E. V., and Tarasenko, G.N. (1977) Effect of structure and heat treatment on the properties of high-strength titanium alloys. Metalloved. Term. Obrabotka Metallov 5: 38–42 (in Russian). 62. Moiseyev, V.N. (1980) Modern structural titanium alloys. Metalloved. Term. Obrabotka Metallov 7: 29–34 (in Russian). 63. Moiseyev, V.N. (1983) High-strength titanium alloys and prospects of their development. In Aircraft Materials. Moscow: Department of Scientific and Technical Information, All-Russian Institute of Aircraft Materials, pp. 101–112 (in Russian). 64. Moiseyev, V.N. (2002) High-strength titanium alloys for aerospace engineering. In Aircraft Materials. Moscow: MISIS–VIAM, pp. 115–126 (in Russian). 65. Moiseyev, V.N., Sysoyeva, N.V., and Ishunkina, T.V. (1995) Granular metallurgy of high-strength titanium alloys. Metalloved. Term. Obrabotka Metallov 6: 28–30 (in Russian). 66. Grigorovich, V.K. (1969) Heat resistance and phase diagrams. Moscow: Metallurgiya, p. 324 (in Russian). 67. Rozenberg, V.M. (1973) Basics of the heat resistance of metal materials. Moscow: Metallurgiya, p. 328 (in Russian). 68. Kornilov, I.I. (1964) Prospects of the studies of heat resistance of titanium alloys. In Physical Metallurgy of Titanium Alloys. Moscow: Nauka, pp. 7–13 (in Russian). 69. Glazunov, S.G., et al. (1978) Ways to increase the heat resistance of titanium alloys. Titanium, vol. 3, Proc. 3rd Int. Conf. on Titanium. Moscow: All-Russian Institute of Light Alloys, p. 363 (in Russian). 70. Solonina, O.P. and Glazunov, S.G. (1976) Heat-resistant titanium alloys. Moscow: Metallurgiya (in Russian). 71. Solonina, O.P. (1980) Structure and properties of heat-resistant titanium alloys. Metallography of titanium alloys. Moscow: Metallurgiya, pp. 358–421 (in Russian). 72. Solonina, O.P. (1883) Modern heat-resistant titanium alloys. In Aircraft Materials. Moscow: Department of Scientific and Technical Information, All-Russian Institute of

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Aircraft Materials, pp. 116–122 (in Russian). 73. Glazunov, S.G., Moiseyev, V.N., and Danilov, Yu.S. (1965) Deformed heat-resistant titanium alloys. In Structural Materials (encyclopedia). Moscow: Sov. Entsyklopediya, pp. 328–330 (in Russian). 74. Shalin, R.E., and Ilyenko, V.M. (1995) Titanium alloys for aircraft gasturbine engines. Titan 1–2: 23–29 (in Russian). 75. Moiseyev, V.N. (2001) Heat-resistant titanium alloys. In Materials in Machine Building (encyclopedia). Moscow: Mashinostroenie, Vol. II-3, pp. 313–330 (in Russian). 76. Antashev, V.G. et al. (2002) Heat-resistant titanium alloys. In Aviation Materials. Moscow: MISIS–VIAM, pp. 111–115 (in Russian). 77. Neugodova, V.N. (1960) Development of heat-resistant titanium alloys for operation at temperatures above 400°C. In Titanium and its Alloys. Moscow: Oborongiz, pp. 9–20 (in Russian). 78. Glazunov, S.G., Solonina, O.P., and Kokhova, G.M. (1960) Heat-resistant titanium alloys VT3 and VT3-1. In Titanium and its Alloys. Moscow: Oborongiz, pp. 30–42 (in Russian). 79. Neugodova, V.N. and Neugodova, Z.N. (1961) Structure and properties of alloy VT8. In Titanium in Industries. Moscow: Oborongiz, pp. 176–184 (in Russian). 80. Neugodova, V.N. (1961) Development of a heat-resistant titanium alloy VT9. In Titanium in Industries. Moscow: Oborongiz, pp. 185–202 (in Russian). 81. Glazunov, S.G. and Kurayeva, V.P. (1961) Titanium alloy VT10 with increased creep limit. In Titanium in Industries. Moscow: Oborongiz, pp. 216–226 (in Russian). 82. Solonina, O.P. and Kurayeva, V.P. (1964) Heat-resistant titanium alloys for operation at 600°C. In Applications of Titanium Alloys. Moscow: Department of Scientific and Technical Information, All-Russian Institute of Aircraft Materials, pp. 70–75 (in Russian). 83. Kurayeva, V.P. and Solonina, O.P. (1964) α-Structure-based titanium alloys for operation at a temperature of 600°C. In Production of Titanium Alloys. Moscow: Metallurgiya, pp. 12–16 (in Russian). 84. Boyarskaya, E.A., Elagina, L.A., and Brun, M.Ya. (1966) Effect of the deformation conditions on the structure and mechanical properties of disks from titanium alloys VT8 and VT9. In Production of Titanium Alloys. Moscow: Department of Scientific and Technical Information, All-Russian Institute of Light Alloys, pp. 62–72 (in Russian). 85. Elagina, L.A. and Borzevovskaya, K.M. (1969) Effect of the structure on the properties of semiproducts from alloy VT18. In Production of Titanium Alloys. Moscow: Department of Scientific and Technical Information, All-Russian Institute of Light Alloys, pp. 10–17 (in Russian).

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Structural Titanium Alloys

2.1 GENERAL CHARACTERISTICS Solid-solution α-, (α+β)-, and β-titanium alloys have found wide use as structural material in mechanical engineering due to their high strength/ductility ratio, good machinability, and weldability by all types of welding applicable for structural metals. These alloys preserve good characteristics of mechanical and hightemperature strength at temperatures up to 350–400°C. Two-phase (α+β)-titanium alloys yield to hardening heat treatment due to the breakdown of the metastable β-phases preserved in quenching and precipitation of disperse particles of the α- and β-phases formed in aging. In structural titanium alloys, the alloying elements are used within the limits of their solubility in α- and β-titanium. The major alloying elements are molybdenum, vanadium, zirconium; more rarely, niobium and tantalum that form a continuous series of solid solutions in α- and β-titanium. Chromium and iron are often used; more rarely, copper and tungsten that are little soluble in α-titanium. The amount of these elements is limited by the feasibility of eutectoid transformation of solid solution. Some elements are inevitable impurities in titanium and its alloys. These are oxygen, nitrogen, hydrogen, carbon, silicon, and some other elements that affect the physical and mechanical properties of titanium alloys. Some of them are sometimes used as alloying elements. By their structure in stable condition, solid-solution titanium alloys are divided into three types: α-, (α+β)-, and β-alloys. β-Alloys are often considered to include all titanium alloys in which 100% of the metastable β-phase found by visual methods could be quenched. Taking into account the metastable “titanium–β-stabilizing element” phase diagram, solid-solution titanium alloys are divided into subgroups that strongly differ by their physical and mechanical properties: α-alloys, pseudo-α-alloys, martensite (α+β)-alloys, transition alloys, pseudo-β-alloys, and β-alloys. Alloys with α-structure represent stable α-solid solutions containing β-stabilizing elements within the limits of solubility in the α-phase. Pseudo-α-alloys are those based on α-phase with some amount (2–6%) of β-phase in stable state. These alloys are close to α-alloys, but have better strength/

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ductility ratios, better hot and cold deformability. A distinguishing feature of these alloys is “soft martensite” with strength close to the α-phase. Martensite (α+β)-alloys can be heat treated by quenching and aging. The capability of thermal hardening increases as the content of β-stabilizing elements in an alloy goes up. After hot plastic deformation, alloys of this type should be heat treated to stabilize the structure. Transition alloys are quenched up to β-phase; however, their structure can contain some amount of metastable W-phase. These alloys have a good strength/ ductility ratio, are efficiently strengthened by quenching and aging, have a high hardenability. After hot deformation and welding, these alloys need to be heat treated. Pseudo-β-alloys are distinguished by increased cold ductility, are efficiently strengthened by quenching and aging. They are intended mainly for sheet semiproducts subjected to intensive plastic deformation. β-Alloys have a stable β-phase not liable to transformations during heat treatment. These are titanium alloys highly alloyed with β-stabilizing elements, of Table 20 Classification of commercial solid-solution structural titanium alloys by the type of structure. Kβ

Grade

– – – – –

VT1–00 VT1-0 VT5 VT5-1 PT7M

Nonalloyed titanium Nonalloyed titanium 5Al 5Al–2.5Sn 2.2Al–2.0Zr

0.25

OT4-0 OT4-1 OT4-1V OT4 VT20

0.8Al–0.8Mn 1.5Al–1.0Mn 3.0Al–2.5V 3.5Al–1.5Mn 6.0Al–2.0Zr–1.0Mo–1.0V

Martensite (α + β)-alloys 0.3–0.9

VT6S VT6 VT14 VT16 VT23

5.0Al–4.0V 6.0Al–4.5V 4.5Al–3.0Mo–1.0V 2.5Al–5.0Mo–5.0V 5.5Al–2.0Mo–4.5V–1.0Cr–0.7Fe

Transition (α + β)-alloys

1.0–1.4

VT22 VT22I VT30 VT37

5.0Al–5.0Mo–5.0V–1.0Fe–1.0Cr 3.0Al–5.0Mo–5.0V–1.0Fe–1.0Cr 11Mo–6.0Sn–4.0Zr 5Al–5Mo–5V–1Fe–1Cr–2.5Cr–1.7Sn

Pseudo-β-alloys

1.5–2.4

VT35 VT32 VT15

3Al–1.5Mo–15V–3Sn–3Cr 2.5Al–8.5Mo–8.5V–1.2Fe–1.2Cr 3.0Al–7.0Mo–11Cr

β-Alloys

2.5–3.0

4201

33.0Mo

Groups α-Alloys

Pseudo-α-alloys

Average chemical composition (wt. %)

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commercial interest as corrosion-resistant in certain aggressive media (Table 20). There is information that titanium alloys with stable β-phase can be used for shortduration work at temperatures up to 1000°C. This classification is based on the so-called β-phase stability coefficient (Kβ) that shows the ratio of the content of the β-stabilizing element in a binary alloy to its content in a critical composition alloy: C-, K β = ---------C cr where C is the content of the β-stabilizing element in the alloy, wt. %; Ccr is the content of the β-stabilizing element in the alloy; β is the stabilizing element of critical composition, wt. %. For instance, if 10% Mo is taken to be the critical concentration for binary titanium alloys with molybdenum, then a Ti–Mo alloy with 4% Mo would have Kβ = 4/10 = 0.4; for an alloy with 16% Mo, Kβ = 16/10 = 1.6. Thus, Kβ of critical-composition titanium alloys is unity; of precritical composition, less than unity; and of hypercritical composition, more than unity. The β-phase stability coefficient in alloys with several alloying elements stabilizing the β-phase in titanium is the sum of Kβ of each particular element: C2 Cn C1 K β = --------- + --------- + … + ---------. C cr1 C cr2 C crn For instance, for titanium alloy VT22 containing 5%Al, 5%Mo, 5%V, 1%Fe, 1%Cr, C Mo C V C Fe C Cr 5 1 1 5 K β = ---------+ ------- + --------- + --------- = ------ + ------ + --- + --- = 1.20 . 10 16 4 7 C cr C cr C cr C cr The content of aluminum which is an α-stabilizing element, has no effect on the value of Kβ. As of today, there is no grounds, either, to take into consideration such alloying elements as tin and zirconium, as they have no noticeable effect on the polymorphic transformation temperature in titanium alloys. The content of impurities can be disregarded in the calculation of Kβ, too, as their amounts are minor. One should take into account the conditional character of Kβ, as the critical concentrations for particular β-stabilizing elements vary within 10% according to various sources. Calculations of Kβ do not take into account the solubility of the β-stabilizing element in α-titanium, which for most elements varies within the limits of 0.2–0.8%. (Exceptions are vanadium, niobium, and tantalum, where the solubility in α-titanium reaches several percent.)

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The stability of β-phase is assumed to change linearly with the increase in the β-stabilizing element content, and the action of the alloying elements on β-phase stability is equal to the sum of action of each element stabilizing the β-phase. Nevertheless, the classification of the alloys with account for the metastable structural state determined by the β-phase stabilization coefficient gives much information on their behavior at various heat treatments: hot deformation, thermal hardening, welding, etc. In practical work with structural titanium alloys, it is preferable to consider them by the level of their strength at room temperature. With respect to this characteristic, structural alloys are divided into low-strength highly ductile, medium-strength, highstrength thermally hardened, and special purpose alloys. We will consider structural titanium alloys in this order (Table 21). The physical-mechanical and operational characteristics of the alloys were determined in specimens from sheets or forged rods. In all cases, the sheet specimens were cut transversally, as per the specifications for these semiproducts. Table 21 Classification of commercial structural titanium alloys by strength. Type of alloy

Grade of alloy

State

Low-strength, highly ductile

VT1-00 VT1-0 OT4-0 PT-7M OT4-1 OT4-1V

Mediumstrength

High-strength, thermally hardened

Mechanical properties σB, MPa

σ0.2, MPa

δ5, %

annealed annealed annealed annealed annealed annealed

300–450 400–550 500–650 500–650 600–750 600–750

250–400 360–490 440–580 450–580 550–690 540–680

25 20 20 20 15 15

PT3-V OT-4 VT5-1 VT6S VT6 VT20 TS5

annealed annealed annealed annealed annealed annealed annealed

680–900 700–900 800–1000 850–1000 920–1070 950–1150 950–1100

620–820 640–830 730–900 790–920 850–820 860–1040 850–1020

12 12 10 10 10 10 10

VT6 VT14

quenched and aged

1050–1200 1100–1250

970–1090 1020–1130

6 6

1100–1250 1100–1250 1150–1300 1100–1250 1100–12500 1100–125000 1150–1300 1150–1300

1000–1100 1020–1150 1060–1140 1000–1120 1040–1170 1040–1170 1050–1160 1050–1180

10 8 7 8 5 5 6 6

VT16 VT23 VT22 VT22I VT15 TS6 VT32 VT35

quenched and aged quenched and aged quenched and aged quenched and aged quenched and aged quenched and aged quenched and aged quenched and aged quenched and aged

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Tension and impact strength tests were performed according to the usual method. The elasticity modulus was determined under static tension and also by the dynamic method. Mention was made in the text and in the plots when it was done by the dynamic method. The long-term strength test consisted in loading a specimen with a given load in cold state. Then the specimen was rapidly heated by using its electrical resistance or by the induction method. The voltage applied was adjusted so as to provide the life of the specimen for several hundred seconds. Sometimes, this method was used to determine not only its ultimate strength but also yield strength at different residual deformations. The notch sensitivity in static tests was determined both for sheet and rod specimens. The notch radius was 0.1 mm. On rod specimens, the notch was circumferential; on sheet specimens, from two facets by the thickness. The notch sensitivity was assessed by the ratio of the ultimate strengths of the notched and smooth specimens (σBn /σB ). Crack sensitivity at bending impact was determined by measuring the unit rupture work for a specimen with a crack. As impact strength, unit rupture work was assessed in kgf m/cm2. Specimens were prepared from a sheet and rod. A specimen was first notched, and then processed in a resonance vibrator to develop a fatigue crack of a given depth. This book gives data on crack sensitivity obtained on specimens cracked transversal to the direction of the fiber. Creep tests were used to determine the general and residual deformations. Tables give the creep limit at a residual deformation of 0.2%. Mentions about this is made in the plots, too. Low-cycle endurance in static loads was determined on specimens with stress concentrators at a load (tension) frequency of 8–10 cycles per minute. On rod specimens, a concentrator was a circumferential notch 0.75 mm in radius; on sheet specimens, a round hole 5 mm in diameter to ensure a concentration of 2.6. Sheet specimens were fatigue tested by bending through an angle in one plane on a machine with the load frequency of about 3000 cycles per minute. Sheet specimens had no special concentrator. Rods for fatigue tests were smooth and notched (notch radius, 0.75 mm). The tests were conducted at room temperature (bending through an angle) and elevated temperature (rotational bending).

2.2 HIGH-DUCTILITY LOW-STRENGTH ALLOYS This group includes alloys with ultimate strength up to 700 MPa: OT4-0, OT4-1, OT4-1V; and also technical-grade titanium VT1-00 and VT1-0 that have relatively high ductility. These alloys permit considerable cold deformation. Alloys OT4-0, OT4-1, OT4-1V are low-aluminum-alloyed (no more than 3%) pseudo-α-alloys with minor amounts of manganese or vanadium that contain about 3–5% β-phase. Alloy PT-7M, alloyed with aluminum and zirconium, is exclusively an α-alloy.

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At the time of development, pseudo-α-alloys were a significant step forward in the history of structural titanium alloys owing to their higher processability as compared with the α-alloys that existed then, such as VT5, VT5-1, etc. A particular feature of pseudo-α-alloys is that, preserving all major advantages of single-phase α-alloys, they acquire a number of valuable properties important for structural materials. Pseudo-α-alloys are practically indifferent to hardening heat treatment. Martensite α-phase forming in quenching from hypercritical temperatures is close to the α-phase by its physical and mechanical properties. Dispersion hardening during the low-temperature breakdown of metastable β- and α′-phases is small and of no practical importance. The amount of β-phase in a relatively stable state is so small that its eutectoid breakdown, in the case of alloys with eutectoid-forming elements, has no noticeable effect on the physical and mechanical properties. A peculiar feature of alloys based on α-structure with minor amount of β-phase is good weldability and high thermal stability. Introduction of a minor amount of β-stabilizing element above its solubility in α-titanium significantly increases the mechanical and high-temperature strength at moderate temperatures without any considerable decrease of ductility. Technical-grade titanium VT1-00 This grade is produced from the most pure grades of titanium sponge. It contains a smaller amount of such impurities as carbon, iron, silicon, and oxygen. VT1-0 is distinguished by low strength and high ductility. It welds very well by fusion welding (argon arc, submerged, electroslag) and resistance welding (spot and seam). Welded joints made by fusion welding have strength and ductility close to the base metal. Titanium VT1-00 deforms well in both a hot state (as hot work) and cold state (as cold work). Forging, die forging, rough rolling, and pressing are performed as hot work; finish rolling, sheet stamping, drawing, etc. are done as cold work. VT1-00 is used to fabricate all kinds of semiproducts: foil, band, sheets, plates, forgings, shapes, tubes, etc. By the content of impurities, the chemical composition of titanium VT1-00 should not exceed the following rates (wt. %): C, 0.05; Fe, 0.20; Si, 0.08; O2, 0.20; N2, 0.04; H2, 0.008; other impurities, 0.10. The mechanical properties of sheets and rolled rods as per the current technical documentation are given in Table 22. The major physical properties of VT1-00 are as follows. • density at 20°C: 4.52 g/cm3 • thermal conductivity at various temperatures: Temperature, ºC λ, Cal/cm s deg

20 0.046

100 0.045

200 0.044

• heat capacity at various temperatures:

300 0.043

400 0.043

500 0.043

600 0.043

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Temperature, ºC C, Cal/(g deg)

100 0.120

200 0.131

300 0.135

400 0.140

53

500 0.151

600 0.160

• linear expansion coefficient within the temperature range: Temperature, ºC α×106, deg–1

20–100 100–200 200–300 300–400 400–500 500–600 600–700 8.8 8.9 9.3 9.8 10.2 10.4 10.5

• specific electrical resistance at 20°C: 45·10 –6 Ω cm. Technical-grade titanium and all commercial alloys are paramagnetic. Table 22 Mechanical properties of sheets and rolled rods from alloy VT1-00 according to technical specifications. Semiproduct State of tested specimens

Thickness, mm

Mechanical properties σB, MPa

δ, % ψ, % KCV, MJ/m2 no less than

Sheet

as received

0.3–1.8 1.8–6.0 6.0–10.5

300–450 300–450 300–450

30 25 20

– – –

– – –

Rod

annealed

10–60 65–100 100–150

300–450 300–450 270–450

25 25 24

55 55 42

1.2 1.2 0.6

One of the remarkable properties of titanium, grade VT1-00 including, is excellent corrosion resistance (Table 22). It is determined by a protective oxide film on its surface. The corrosion resistance of technical-grade titanium in air is preserved up to 600°C. Titanium ha a high corrosion resistance in water, seawater, and humid atmosphere, even when in contact with stainless steel and copper–nickel alloys. Recently, titanium was reported to be corroded by the combined action of seawater salts and elevated temperatures (350–500°C). In some inorganic acids, titanium is insufficiently corrosion-resistant. Hydrofluoric acid at any concentrations evokes violent corrosion and degradation of titanium. The rate of interaction of titanium with hydrochloric acid depends on concentration and temperature. In a 3% solution of hydrochloric acid, titanium resists corrosion well up to a temperature of 70°C. At the boiling temperature, the ultimate concentration of the acid decreases down to 1%. The resistance of titanium to corrosion in sulfuric acid also depends on concentration and temperature. The rate of corrosion at a concentration of the acid lower than 5% does not exceed 0.012 mm per year and at a concentration of 10% the corrosion rate is no more than 0.25 mm per year. However, even an insignificant temperature increase noticeably enhances corrosion. Nitric acid at concentrations up to 98% does not corrode titanium. At the

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σB, σ0.2, σfc, kgf/mm2

δ5 , %

50 40 E 30

σ0.2

δ

σB

20

10 0 −100 0

σfc

100 200 300 400 500

70

10000

60

8000

50

6000

40

4000

30

2000

Estat, kgf/mm2

01 Chapter 2

Temperature, °C

Figure 21 Change of the mechanical properties of titanium VT1-00 (sheet, 1 mm) at shortterm rupture vs test temperature.

boiling temperature, it is stable in nitric acid, if the concentration of the acid does not exceed 65%. Titanium has a good corrosion resistance in phosphoric and chromic acids and in aqua regia. Titanium shows an excellent corrosion resistance in solutions of inorganic acids, including chlorides, and also in most organic acids. Typical mechanical properties of titanium VT1-00 at room and elevated temperatures are given in Fig. 21 and Table 23. Brinell hardness of VT1-00 (HB 10/3000/30) varies within the range of 116–149 kg/mm2. The use of titanium VT1-00 as structural material is limited due to its low strength. The metal is mainly used when high ductility is required. It can be used in constructions operated long-term at temperatures up to 200–250°C. Table 23 Mechanical properties of alloy VT1-00. Properties

Impact strength, kgf/cm2 Long-term strength for 100 h, kgf/mm2 (no less than) Notch sensitivity at static load Creep limit for 100 h, kgf /mm2 (no less than) Endurance limit based on 107 cycles, kgf/mm2 (no less than) Resistance to repeated static loads at σ = 0.7σB, cycles Crack sensitivity at bending impact, kgf/cm2

Semiproduct

Test temperature, ºC 20

200

300

rod, 20 mm dia sheet, 1.5 mm

15 –

– 19

– 20

sheet, 1.5 mm sheet, 1.5 mm sheet, 1.5 mm

1.5 – 16

– 14 –

– – 11

sheet, 1.5 mm

22,800

–

–

sheet, 1.5 mm

13

–

–

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Technical-grade titanium VT1-0 This grade is produced from nonalloyed titanium sponge, that is slightly inferior to VT1-00 by its purity. Still, the content of impurities in VT1-0 is lower than in titanium alloys. VT1-0 is slightly less ductile as compared with VT1-00, but more strong and is widely used as structural material. The range of semiproducts fabricated is the same as for VT1-00. The chemical composition of titanium VT1-0 is as follows. The content of impurities should not exceed the following limits (%): C, 0.07; Fe, 0.30; Si, 0.10; O2, 0.20; N2, 0.04; H2, 0.010; the sum of other impurities, no more than 0.30. The mechanical properties of sheets and rolled rods according to the current technical documentation are given in Table 24. Table 24 Mechanical properties of sheets and rolled rods from alloy VT1-0 according to technical specifications. Semiproduct

State of tested specimens

Thickness, mm

Mechanical properties δ, % ψ, % KCV, MJ/m2

σB, MPa

no less than Sheet

as received

0.3–0.4 0.4–0.8 1.8–6.0 6.0–10.5

40–56 40–55 40–55 40–55

25 30 25 20

– – – –

– – – –

Rod

annealed

10–60 65–100 110–150

40–55 40–55 36–55

20 20 19

50 50 38

1.0 1.0 0.5

The physical properties of titanium VT1-0 are as follows: • density of VT1-0: 4.52 g/cm3 • thermal conductivity at various temperatures: Temperature, ºC λ, Cal/cm s deg

20 0.046

100 0.045

200 0.044

300 0.043

400 0.043

500 0.043

600 0.043

• heat capacity at various temperatures: Temperature, ºC C, Cal/(g deg)

100 0.120

200 0.131

300 0.135

400 0.140

500 0.151

600 0.160

• linear expansion coefficient within the temperature range: Temperature, ºC α×106, deg–1

20–100 100–200 200–300 300–400 400–500 500–600 600–700 8.8 8.9 9.3 9.8 10.2 10.4 10.5

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δ5 , %

60

60 11000

σB, σ0.2, σfc, kgf/mm2

50 E

40

δ

30

50 9000 40

7000

30

5000

20

3000

10

1000

σB 20

10 0 −100 0

σfc

Estat, kgf/mm2

01 Chapter 2

σ0.2

100 200 300 400 500 Temperature, °C

Figure 22 Change of the mechanical properties of titanium VT1-0 (sheet, 1 mm) at shortterm rupture vs test temperature.

• specific electrical resistance at 20°C is 47·10 –6 Ω cm. • normal spectral emissivity of etched sheets from air-heated VT1-0 (temperatures, 100 and 500°C) is, respectively, 0.22 and 0.37; of cold-rolled sheets at the same temperatures, 0.10 and 0.22, respectively. Typical mechanical properties of titanium VT1-0 at room and elevated temperatures are given in Fig. 22 and Table 25. Table 25 Mechanical properties of alloy VT1-0. Properties

Impact strength, kgf/cm2 Notch sensitivity at static load Long-term strength for 100 h, kgf/mm2 (no less than) Creep limit for 100 h, kgf/mm2 (no less than) Endurance limit based on 107 cycles, kgf/mm2 (no less than) Resistance to repeated static loads at σ = 0.7σB, cycles Crack sensitivity at bending impact, kgf/cm2

Semiproduct

Test temperature, ºC 20

200

300

400

rod, 20 mm dia sheet, 1 mm sheet, 1 mm

12 1.45 –

– – 24

– – 16

– – –

sheet, 1 mm

–

17

12

–

sheet, 1 mm

23

20

18

15

sheet, 1 mm

16,000

–

–

–

sheet, 1 mm

10

–

–

–

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20 σ0.2

57

20°C

35

σ, kgf/mm2

30 25 20 σfc

20

250 σfc

250 σ0.2 350 15 σ0.2

250 350

10 5 350 σfc

0

0.2

0.4

0.8 δ, %

0.6

Figure 23 Tension curves at yield strength for titanium VT1-00 (sheet, 1 mm) at room and elevated temperatures.

45

20 σ0.2 com

20°C

σ, kgf/mm2

40 35 30 20 25 σfc com

20

350 450

350com σ0.2

15

10 350com σfc 5 350 σfc

0

0.2

0.4

0.6

0.8 δ, %

Figure 24 Tension curves at yield strength for titanium VT1-0 (sheet, 1 mm) at room and elevated temperatures.

Brinell hardness of alloy VT1-0 (HB 10/3000/30) varied within the limits of 131–163 kgf/mm2. The tension/compression curves at yield strength at various temperatures are given in Fig. 23. The tests were conducted on 1-mm-thick sheets with yield strength of 50 kgf/mm2 at 20°C. Figure 24 shows the creep curves at temperatures of 20 and 350°C and residual deformation of 0.06, 0.08, 0.1, 0.2, and 0.5%. The creep was determined on annealed rods 20 mm in diameter, that had short-term

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ultimate strength of 55 kgf/mm2 at 20°C. Interestingly, at room temperature the technical-grade titanium, which has a comparatively high ultimate strength, develops a noticeable creep. The same phenomenon was also observed in some titanium alloys at high stresses. Alloy VT1-0 welds well by all types of welding applicable for titanium. The ductility of VT1-0 in forging, stamping, and hot rolling within a temperature range of 950–700°C is close to that of VT1-00. The major operations of sheet stamping can be done as cold work. This alloy can be used for pieces operated long-term at temperatures up to 250–300°C. Alloy OT4-0 This is a low-alloyed grade of the Ti–Al–Mn system. It is distinguished by high ductility, but has a relatively low strength. The alloy is used as annealed. The chemical composition of alloy OT4-0 is as follows (%): Al, 0.2–1.4; Mn, 0.2–1.3; impurities (%, no more than): C, 0.10; Fe, 0.30; Si, 0.15; Zr, 0.30; O2, 0.15; N2, 0.05; H2, 0.010; the sum of other impurities, 0.30. This is one of the most ductile titanium alloys in Russian practice. The alloy is used to fabricate the following semiproducts: sheets, band, foil, rods, shapes, wire, pipes, forgings, stampings, etc. The mechanical properties of sheets and rolled rods according to the current documentation are given in Table 26. Table 26 Mechanical properties of sheets and rolled rods from alloy OT4-0 according to technical specifications. Semiproduct State of tested specimens

Thickness, mm

Mechanical properties σB, MPa

δ, %

ψ, % KCV, MJ/m2 no less than

Sheet

as received

0.3– 0.4 0.4–0.8 1.8–6.0 6.0–10.5

500–650 500–650 500–650 500–650

25 30 25 20

– – – –

– – – –

Rod

annealed

10–60 65–100 110–150

500–650 500–650 450–650

20 20 20

45 40 32

0.7 0.7 0.5

The physical properties are as follows: • density of alloy OT4-0: 4.51 g/cm3 • thermal conductivity at various temperatures: Temperature, ºC λ, Cal/cm s deg

20 100 200 300 400 500 600 700 800 0.030 0.031 0.033 0.034 0.036 0.039 0.042 0.043 0.045

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δ5 , %

60 σB 50

70 11000 E

40

30

20

60 9000

σ0.2 σfc δ

10

−100 0

100 200 300 400 500 Temperature, °C

50

7000

40

5000

30

3000

20

1000

Estat, kgf/mm2

σB, σ0.2, σfc, kgf/mm2

59

Figure 25 Change of the mechanical properties of titanium alloy OT4-0 (sheet, 1 mm) at short-term rupture vs test temperature.

• heat capacity at various temperatures: Temperature, ºC C, Cal/(g deg)

100 200 300 400 500 600 700 800 900 0.130 0.140 0.151 0.160 0.170 0.181 0.201 0.210 0.220

• linear expansion coefficient within the temperature range: Temperature, ºC α×106, deg–1

20–100 100–200 200–300 300–400 400–500 500–600 600–700 8.0 8.5 9.0 9.5 9.9 10.1 10.2

Typical mechanical properties of alloy OT4-0 at room and elevated temperatures are given in Fig. 25 and Table 27. Table 27 Mechanical properties of alloy OT4-0. Properties

Impact strength, kgf/cm2 Notch sensitivity at static load Long-term strength for 500 h, kgf/mm2 (no less than) Creep limit for 100 h, kgf/mm2 (no less than)

Semiproduct

Test temperature, ºC 20

250

300

400

rod, 20 mm dia sheet, 1 mm sheet, 1 mm

8 1.4 –

– – 35

– – 32

– – 28

sheet, 1 mm

–

30

26

15

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(continued) properties of alloy OT4-0. Table 27 Mechanical Properties

Semiproduct

Endurance limit based on 107 cycles, kgf/mm2 (no less than) Resistance to repeated static loads at σ = 0.7σB, cycles Crack sensitivity at bending impact, kgf/cm2

Test temperature, ºC 20

250

300

400

sheet, 1 mm

30

28

25

22

sheet, 1 mm

9600

–

–

–

sheet, 1 mm

7

–

–

–

Brinell hardness of alloy OT4-0 (HB 10/3000/30) varies within the limits of 156–207 kgf/mm2. Alloy PT-7M The physical properties of this alloy are given in Table 28. Table 28 Physical properties of alloy PT-7M at various temperatures. Physical properties

Young’s modulus, kgf/cm2 Shear modulus, kgf/mm2 Specific resistance Specific heat Temperature coefficient of linear expansion, α·106 Thermal conductivity Poisson’s ratio Density Magnetic permeability

Test temperature, ºC 20

100

150

11200 4600 108 594 –

– – 115 – 8.8a

10300 4200

– 0.32 4.49 –1.004

9.3

200

300

350

400

127 625 8.9b

15 – 140 659 9.2c

8900 3700 – – –

– – 152 690 9.4d

10.5

11.9

–

13.3

a20–100ºC; b20–200ºC; c20–300ºC; d20–400ºC.

Typical mechanical properties of alloy PT-7M at room and elevated temperatures are given in Fig. 26 and Table 29. Alloy PT-7M has the following chemical composition (wt. %): Al, 2.0–2.5; Zn, 1.5–3.0. Impurities as in alloy OT4-0. The only heat treatment for this alloy is annealing. Full annealing is performed after cold or hot plastic deformation, in furnaces with protective atmosphere at temperatures of 650–700°C. Partial annealing for 1 h at temperatures of 500–550°C is recommended to eliminate residual stresses after welding.

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δ5 , %

60

70 11000

50 σB 40 σ0.2 30

E σfc

60 9000 50 7000 40 5000

20

Estat, kgf/mm2

σB, σ0.2, σfc, kgf/mm2

61

δ 30 3000

10

−100 0

20 1000 100 200 300 400 500 Temperature, °C

Figure 26 Change of the mechanical properties of titanium alloy PT-7M (sheet, 1 mm) at short-term rupture vs test temperature.

→ β transformation in alloy PT-7M is The temperature of polymorphic (α+β) ← 920±10°C. Overheating was observed to be accompanied with a sharp increase of the size of micrograin and deterioration of the mechanical properties. Table 29 Mechanical properties of alloy PT-7M. Properties

Semiproduct

Test temperature, ºC 20

Impact strength, kgf/cm2 Crack sensitivity at bending impact, kgf/cm2 Notch sensitivity at static load Endurance limit based on 107 cycles, kgf/mm2 (no less than) Creep limit for 100 h, kgf/mm2 (no less than) Long-term strength for 100 h, kgf/mm2 (no less than) Low-cycle fatigue at axial load of N = 2.105 cycles at KB = 2.6 cycles (no less than), at stress σ = 0.7σB

150

250

350

400

rod, 20 mm dia sheet, 1 mm

6–8

sheet, 1 mm sheet, 1 mm

1.25 26

sheet, 1 mm

–

–

28

24

18

sheet, 1 mm

–

–

32

29

23

sheet, 1 mm

16000

5–7

0.5σB

70000

0.4σB

300000

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The alloy welds well by all types of welding used for titanium alloys. The strength of the welded joint in fusion welding is 0.9–0.95 of that of the base metal depending on the welding method and grade of the filler metal. The recommended filler is titanium alloy VT1-00 or PT-7M. Alloy OT4-1 This is a low-alloyed titanium alloy possessing a good ductility and low strength. The alloy is based on the Ti–Al–Mn system, is intended mainly for sheet semiproducts (structural and sheathing sheeting material), but can be also used to fabricate forgings, stampings, rods, shapes, pipes, wire, and other semiproducts. The alloy has the following chemical composition (wt. %): Al, 1.0–2.5; Mn, 0.7–2.5. The content of impurities should be no more than (wt. %): C, 0.10; Fe, 0.30; Si, 0.15; Zr, 0.30; O2, 0.15; N2, 0.05, H2, 0.012; the sum of the other impurities, 0.30. The mechanical properties of sheets and rolled rods from alloy OT4-1 according to the current documentation are given in Table 30. Table 30 Mechanical properties of sheets and rolled rods from OT4-1 according to technical specifications. Semiproduct State of tested specimens

Thickness, mm

Mechanical properties σB, MPa

δ, %

ψ, % KCV, MJ/m2 no less than

Sheet

as received

0.3–0.7 0.7–1.8 1.8–6.0 6.0–10.5

60–75 60–75 60–75 60–75

25 20 15 13

– – – –

– – – –

Rod

annealed

10–60 65–100 110–150

60–75 60–75 55–75

15 15 13

35 35 24

0.45 0.45 0.40

The physical properties of alloy OT4-1 are as follows: • density: 4.55 g/cm3 • thermal conductivity at various temperatures: Temperature, ºC λ, Cal/cm s deg

20 0.023

100 0.025

200 0.027

300 0.029

400 0.032

500 0.035

600 0.039

• heat capacity at various temperatures: Temperature, ºC C, Cal/(g deg)

100 0.120

200 0.135

300 0.151

400 0.160

500 0.181

600 0.201

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70 δ5 , %

σB 60 σ0.2

60 11000 E

40

50 10000 σfc

30

40 9000

δ

20

30 8000

10

20 7000

0 −100

Estat, kgf/mm2

σB, σ0.2, σfc, kgf/mm2

50

10 6000 0 100 200 300 400 500 600 Temperature, °C

Figure 27 Change of the mechanical properties of titanium alloy OT4-1 (sheet, 1 mm) at short-term rupture vs test temperature.

• linear expansion coefficient within the temperature range: Temperature, ºC α×106, deg–1

20–100 8.0

100–200 8.6

200–300 9.1

300–400 9.6

400–500 9.4

Typical mechanical properties at room and elevated temperatures are given in Fig. 27 and Table 31. Table 31 Mechanical properties of alloy OT4-1. Properties

Impact strength, kgf/cm2 Notch sensitivity at static load Long-term strength for 100 h, kgf/mm2 (no less than) for 1000 h for 3000 h for 5000 h Creep limit for 100 h, kgf/mm2 (no less than) for 2000 h

Semiproduct

Test temperature, ºC 20

200

350

400

rod, 20 mm dia

7

–

–

–

sheet 1 mm sheet 1 mm

1.35 –

– 44

– 34

– 29

sheet 1 mm sheet 1 mm sheet 1 mm sheet 1 mm

– – – –

43 43 43 29

32 – – 26

27 – – 19

sheet 1 mm

–

25

22

16

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Table 31 Mechanical (continued) properties of alloy OT4-1. Properties

Semiproduct

107

Endurance limit based on cycles, kgf/mm2 (no less than) Resistance to repeated static loads at σ = 0.7σB, cycles Crack sensitivity at bending impact, kgf/cm2

Test temperature, ºC 20

200

350

400

sheet 1 mm

36

30

26

20

sheet 1 mm

9500

–

–

–

sheet 1 mm

6.5

–

–

–

Alloy OT4-1V This is a low-alloyed pseudo-α-titanium alloy with low strength and good ductility. By its physicomechanical properties, it is an analog of alloy OT4-1, in which an equivalent amount of vanadium is substituted for manganese. The alloy is produced by double remelting under vacuum, not in argon and in vacuum as alloys OT4-0 and OT4-1. Applications are mainly sheets, band, foil, thin-walled pipes, shapes, wire, but the alloy can be also used to fabricate forgings, stampings, plates, rods, and other semiproducts. The alloy has the following chemical composition (%): Al, 2.5–3.5; V, 1.5–2.5. The content of impurities is limited to (wt. %): C, 0.10; Fe, 0.25; Si, 0.12; Zr, 0.30; O2, 0.15; N2, 0.04; H2, 0.008; the sum of the other impurities, 0.30. The mechanical properties of sheets and rolled rods according to the current technical documentation are given in Table 32. Table 32 Mechanical properties of sheets and rolled rods from alloy OT4-1V according to technical specifications. Semiproduct

State of tested specimens

Thickness, mm

Mechanical properties σB, MPa

δ, %

ψ, %

KCV, MJ/m2

no less than Sheet

as received

0.3 0.7 0.7 1.8 1.8 6.0 6.0 10.5

60–75 60–75 60–75 60–75

25 20 15 13

– – – –

– – – –

Rod

annealed

10–60 65–100 110–150

60–75 60–75 55–75

15 15 13

35 35 24

0.45 0.45 0.40

The physical properties of alloy OT4-1V are as follows.

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• density: 4.48 g/cm3 • thermal conductivity coefficient: Temperature, ºC λ, Cal/cm s deg

100 9.0

200 10.0

300 11.0

400 12.2

500 13.5

600 15.2

700 17.2

800 19.9

100 200 300 400 500 600 700 0.538 0.572 0.598 0.622 0.651 0.692 0.750

800 0.83

• specific heat: Temperature, ºC C, Cal/(g deg)

• linear expansion coefficient: Temperature, ºC α×106, deg–1

20–100 87.0

Temperature, ºC α×106, deg–1

100–200 9.3

20–200 9.0

20–300 9.3

200–300 9.8

20–400 9.5

20–500 9.7

300–400 10.3

400–500 10.3

• specific electrical resistance: ρ·10 –6 Ω·cm = 127.5 Typical mechanical properties of alloy OT4-1V at room and elevated temperatures are given in Table 33 and Fig. 28. Table 33 Mechanical properties of alloy OT4-1V. Properties

Semiproduct

Test temperature, ºC 20

Notch sensitivity at static load, kgf/mm2 at Kt = 2.6 at Kt = 4.0 Crack sensitivity at bending impact, kgf/cm2 Critical plane-strain stress intensity factor, kg/mm2 Long-term strength for 100 h, kgf/mm2 (no less than) Creep limit for 100 h, kgf/mm2 (no less than) Endurance limit based on 107 cycles, kgf/mm2 (no less than) Low-cycle fatigue at axial load of N = 2.105 cycles at KB = 2.6 cycles (no less than), at stress σ from σB

300

350

400

sheet, 2 mm

sheet, 2 mm

1.06 1.30 5.0

sheet, 2 mm

390–430

sheet, 2 mm

33–47

33–46 29–37

sheet, 2 mm

30–44

27–38 26–32

sheet, 2 mm

32

sheet, 2 mm

114,760– 217,710

28

23

18

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OT4-1V

δ5 , %

70 60

40

σB

50 10000

E

30 σfc

60 11000

σ0.2

40 9000

20

30 8000

10

20 7000

0 −100 0

Edyn, kgf/mm2

σB, σ0.2, σfc, kgf/mm2

50

10 6000 100 200 300 400 500 600 Temperature, °C

Figure 28 Change of the mechanical properties of titanium alloy OT4-1V (sheet, 1 mm) at short-term rupture vs test temperature.

Brinell hardness (HB 10/3000/30) varies within the limits of 200–260 kgf/mm2. Alloy OT4-1V is used exclusively as annealed and welds well by all types of welding applicable for titanium. The strength and ductility of the welded joint are almost the same as those of the base metal. In calculations of the strength of a welded joint performed by fusion, the weld strength coefficient is taken to be 0.9–0.95. Alloy OT4-1V is used in constructions operated long-term at temperatures up to 400°C. Thus, high-ductility titanium alloys described above are alloys based on α-structure (technical-grade titanium and alloy PT-7M) or α-structure with minor amount of β-phase (OT4-0 and OT4-1). The strength of these alloys is improved as compared with pure titanium, owing to impurities in alloys VT1-00 and VT1-0 or minor alloying both with an α- and β-stabilizing element (OT4-0 and OT4-1), and also due to the occurrence of inevitable impurities. Despite the comparatively low strength, these alloys are widely used as structural metals due to their high ductility. Alloys VT1-0, OT4-0 and OT4-1, which have better strength, are of special interest. In the total volume of titanium alloys, these are about 20%, which shows their considerable popularity as structural material. Alloys of this group can be successfully used in components operated long-term both at low temperatures (from –253 up to –196°C) and high temperatures (up to 300–350°C).

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2.3

67

MEDIUM-STRENGTH ALLOYS

This category includes titanium alloys with a guarantied ultimate strength from 750 up to 1000 MPa. These are alloys OT4, PT3-V, VT5-1, VT6S, VT20, and TS5. The list can include alloys VT6, VT14, VT16 in annealed state, when their ultimate strength does not exceed 1000 MPa. It should be noted that some medium-strength alloys (OT4, VT6S) that contain an increased amount of β-stabilizing elements (greater solubility in α-titanium) can be hardened by quenching and aging. However, the hardening effect is insignificant and is of no commercial importance. An exception is alloy VT6S, which is sometimes used as thermally hardened. All medium-strength titanium alloys weld well and have a satisfactory thermal stability. These are alloys based on α-structure with a minor amount of β-phase (2–7% of β-phase in equilibrium state). They are suitable for producing different kinds of welded constructions and units. This category of alloys covers the predominant range of semiproducts used in mechanical engineering.

Alloy OT4 This alloy was developed based on the Ti–Al–Mn system. It is an alloy with α-structure and 2–4% β-phase. At a moderate strength, it has a satisfactory cold ductility in sheet forming. Alloy OT4 is used most often as sheet structural material. It is applicable for all kinds of semiproducts: sheets, band, rods, shapes, forgings, stampings, pipes, etc. The alloy is supplied as semiproducts with a guarantied ultimate strength of 700–750 MPa. The chemical composition of OT4 according to the current documentation is as follows (wt. %): Al, 3.5–5.0; Mn, 0.8–2.0. The content of impurities (wt. %), no more than: C, 0.10; Fe, 0.30; Si,0.15; Zr, 0.30; O2, 0.15; N2, 0.05; H2, 0.012; the sum of the other impurities, 0.30. The mechanical properties of sheets and rolled rods according to the current technical documentation are given in Table 34. The physical properties of alloy OT4 are as follows: • thermal conductivity as a function of temperature: Temperature, ºC λ, Cal/cm s deg

20 0.023

100 0.025

200 0.027

300 0.029

400 0.032

500 0.035

600 0.039

• heat capacity of alloys at various temperatures: Temperature, ºC C, Cal/(g deg)

100 0.120

200 0.135

300 0.151

400 0.160

500 0.181

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• linear expansion coefficient within the temperature range: Temperature, ºC α×106, deg–1

20–100 8.0

100–200 200–300 300–400 400–500 500–600 8.6 9.1 9.6 9.4 9.8

• density of the alloy: 4.55 g/cm3 Table 34 Mechanical properties of sheets and rolled rods from alloy OT4 according to technical specifications. Semiproduct

State of tested specimens

Thickness, mm

Mechanical properties σB, MPa

δ, %

ψ, % KCV, MJ/m2 no less than

Sheet

as received

0.5–1.0 1.0–1.8 1.8–10.5

700–850 700–900 700–900

20 15 12

– – –

– – –

Rod

annealed

10–60 65–100

700–900 700–900

11 10

30 30

0.40 0.40

110–150

650–900

9

21

0.35

Alloy OT4 has a high corrosion resistance in most aggressive media, similar to technical-grade titanium. Normal spectral emissivity of etched sheets from air-heated VT1-0 (temperatures, 100 and 500°C) is, respectively, 0.24 and 0.37. Typical mechanical properties of alloy OT4 are given in Table 35 and Fig. 28. Table 35 Mechanical properties of alloy OT4. Properties

Impact strength, kgf/cm2

Semiproduct

rod, 20 mm dia sheet Notch sensitivity at static load, 1.2 mm kgf/mm2 Long-term strength for 100 h, kgf/mm2 sheet (no less than) 1.2 mm sheet Creep limit for 100 h, kgf/mm2 1.2 mm (no less than) sheet Endurance limit based on 107 cycles, 1.2 mm kgf/mm2 (no less than) sheet Resistance to repeated static loads at 1.2 mm σ = 0.7σB, cycles sheet Crack sensitivity at bending impact, 1.2 mm kgf/cm2

Temperature, ºC 20

250

300

350

400

6

–

–

–

–

1.30

–

–

–

–

–

54

50

49

47

–

47

45

33

–

42

40

38

34

26

6500

–

–

–

–

6

–

–

–

–

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90

δ5, %

80 70 σ0.2

σB

60 12000 50 11000

50 40

40 10000

σfc

E

30 9000

30 δ 20

20 8000

10

10 7000

0 −100 0

0 200

400

Estat, kgf/mm2

σB, σ0.2, σfc, kgf/mm2

60

6000

600

Temperature, °C

Figure 29 Tension curves at yield strength for titanium alloy OT4 (sheet, 2 mm) at temperatures of 20, 250, 300, 350, and 400°C.

Brinell hardness (HB 10/3000/30) of alloy OT4 varies within the limits of 207–285 kgf/mm2. The tension curves at yield strength at temperatures of 20, 250, 300, 350, and 400°C for 2.0-mm sheets from alloy OT4 are given in Fig. 29.

Alloy OT4-V This alloy is based on the Ti–Al–V system and its physicochemical properties are analogous to alloy OT4 based on the Ti–Al–Mn system. Both are pseudo-α-alloys with approximately the same content of β-phase and aluminum. These alloys were developed in late 1950s, and preference at that time was given to manganese as a cheaper and more available element in Russia. At present, due to the world unification, it proved more preferable to produce and use titanium alloys closer by their chemical composition to those used in the world practice. Substitution of titanium alloy OT4-V for OT4 requires no additional research and can be done automatically. The chemical composition of alloy OT4-V according to the current technical documentation is as follows (wt. %): Al, 3.5–5.0; V, 1.2–2.5. The content of impurities is within the following limits (wt. %, no more than): C, 0.10; Fe, 0.30; Si,0.15; Zr, 0.30; O2, 0.15; N2, 0.05; H2, 0.012; the sum of the other impurities, 0.30. The mechanical properties of sheets and rolled rods from alloy OT4-V according to the current technical specifications are given in Table 36.

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Table 36 Mechanical properties of sheets and rolled rods from OT4-V according to technical specifications. Semiproduct

State of tested specimens

Thickness, mm

Mechanical properties σB, MPa

δ, %

ψ, % KCV, MJ/m2 no less than

Sheet

as received

0.5–1.0 1.0–1.8 1.8–10.5

700–850 700–900 700–900

20 15 12

– – –

– – –

Rod

annealed

10–60 65–100 110–150

700–900 700–900 650–900

11 10 9

30 30 21

0.40 0.40 0.35

Alloy OT4-V is moderately strong and has a satisfactory cold ductility in sheet forming. This enables its wide use as sheet semiproducts and as forgings, stampings, rods, pipes, etc. The alloy is used exclusively as annealed. It welds well by all types of welding applicable for titanium. The strength and ductility of the welded joint performed by fusion welding is close to those of the base metal. After welding of responsible aerospace constructions, it is recommended to anneal them to remove residual stresses in the welded joint. The alloy is well processed by cutting and other machining techniques. It has a high corrosion resistance under atmospheric conditions in seawater and in most aggressive media. Alloy OT4-V is intended for production of parts operated long-term at temperatures up to 400°C. The physical properties of alloy OT4-V in annealed state are as follows: • thermal conductivity at various temperatures: Temperature, ºC λ, Cal/cm s deg

20 0.023

100 0.025

200 0.027

300 0.029

400 0.032

500 0.035

600 0.039

• heat capacity at various temperatures: Temperature, ºC C, Cal/(g deg)

100 0.120

200 0.135

300 0.151

400 0.160

500 0.181

• linear expansion coefficient within the temperature range: Temperature, ºC α×106, deg–1

20–100 8.0

• density: 4.55 g/cm3.

100–200 200–300 300–400 400–500 500–600 8.6 9.1 9.6 9.6 9.8

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σ, kgf/mm2

80

20 σ0.2

71

20°C

70 60 50

350 σ0.2

20 σfc 250 σ0.2 300 σ0.2

400 40 σ0.2

250 300 350 400

250

30

σfc

300,350,400 σfc 20

10 0 0.2 0.5

1.0 δ, %

Figure 30 Change of the mechanical properties of titanium alloy OT4-1V (sheet, 1.2 mm) at short-term rupture vs test temperature.

Typical mechanical properties of alloy OT4-V are given in Fig. 30 and Table 37. Table 37 Mechanical properties of alloy OT4-V. Properties

Notch sensitivity at static load, kgf/mm2 Endurance limit based on 107 cycles, kgf/mm2 (no less than) Resistance to repeated static loads at σ = 0.7σB, cycles Long-term strength for 500 h, kgf/mm2 (no less than) Creep limit for 100 h, kgf/mm2 (no less than) Crack sensitivity at bending impact, kgf/cm2

Semiproduct

sheet, 1.2 mm sheet, 1.2 mm sheet, 1.2 mm sheet, 1.2 mm sheet, 1.2 mm sheet, 1.2 mm

Temperature, ºC 20

250

300

350

400

1.3

–

–

–

–

42

40

38

34

26

6500

–

–

–

–

–

54

50

49

47

–

47

45

33

–

6

–

–

–

–

Alloy PT-3V This is a titanium alloy based on the Ti–Al–V system. This is a typical pseudoα-alloy. Its applications are mainly sheet semiproducts, but it can be used to prepare rods, forgings, stampings, and other semiproducts.

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Alloy PT3M contains (wt. %): Al, 3.5–5.0; V, 1.5–2.5. The admissible content of impurities – as in alloy PT-7M. The mechanical properties of sheets and rolled rods according to the current documentation are given in Table 38. Table 38 Mechanical properties of sheets and rolled rods from PT3M according to technical specifications. Semiproduct

State of tested specimens

Thickness, mm

Mechanical properties σB, MPa

δ, %

ψ, % KCV, MJ/m2 no less than

Sheet Rod Forging

annealed annealed annealed

0.1–100 10–60 65–100

700–900 650–870 650–850

10 7 6

– 25 25

0.7 0.6 0.6

The physicomechanical properties of alloy PT3M are close to those of alloy OT4. • density: 4.40 g/cm3 • thermal conductivity at various temperatures: Temperature, ºC λ, Cal/cm s deg

20 0.021

100 0.023

200 0.025

300 0.027

400 0.029

500 0.034

600 0.037

700 0.040

• heat capacity at various temperatures: Temperature, ºC C, Cal/(g deg)

100 0.131

200 0.140

300 0.151

400 0.160

500 0.170

600 0.181

• linear expansion coefficient within the temperature range: Temperature, ºC α×106, deg–1

20–100 8.3

100–200 200–300 300–400 400–500 500–600 8.9 9.5 10.4 10.6 10.8

• specific electrical resistance at 20°C: 108·10 –6 Ω cm. Typical mechanical properties of PT3M are given in Fig. 31 and Table 39. Alloy PT3M welds well by all types of welding applicable for titanium. The strength and ductility of a joint made by fusion welding are close to those of the base metal. The alloy is intended for producing constructions operated long-term at temperatures up to 400°C. It has a high corrosion resistance close to technical-grade titanium. The alloy is widely used in shipbuilding.

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OT4-V 90

σB, σ0.2, σfc, kgf/mm2

70 60

σB

60

12000

50

σ0.2

50

11000

40

σfc

40

10000

30

9000

20

8000

10

7000

30 δ 20

Estat, kgf/mm2

δ5, %

80

E 10

0 0 −100 0 100 200 300 400 500 600 Temperature, °C

6000

Figure 31 Change of the mechanical properties of titanium alloy PT3M (sheet, 1.2 mm) at short-term rupture vs test temperature.

Table 39 Mechanical properties of a rod from alloy PT3M. Properties

Semiproduct

Temperature, ºC 20

300

400

500

Impact strength, kgf/cm2 Notch sensitivity at static load, kgf/mm2 Long-term strength for 100 h, kgf/mm2 (no less than)

rod, 20 mm dia rod, 20 mm dia rod, 20 mm dia

4.5 1.3 –

– – 48

– – 45

– – 26

Creep limit for 100 h, kgf/mm2 (no less than)

rod, 20 mm dia

–

–

40

12

Alloy VT5-1 This alloy is based on the Ti–Al–Sn system. This is a typical single-phase alloy with α-structure. Its ductility is decreased, but it is distinguished by good hightemperature strength. The alloy is used to prepare sheets, plates, forgings, stampings, rods, shapes, and other semiproducts. Alloy VT5-1 has the following chemical composition (wt. %): Al, 4.0–6.0; Sn, 2.0–3.0. The content of impurities should not exceed (wt. %): C, 0.10; Fe, 0.30; Si, 0.15; Zr, 0.30; O2, 0.15; N2, 0.05; H2, 0.015; the sum of the other impurities, 0.30.

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The mechanical properties of sheets and rolled rods according to the current documentation are given in Table 40. Table 40 Mechanical properties of sheets and rolled rods from alloy VT5-1 according to technical specifications. Semiproduct

State of tested specimens

Thickness, mm

Mechanical properties σB, MPa

δ, % ψ, % KCV, MJ/m2 no less than

Sheet

as received

0.8–1.2 1.2–1.8 1.8–6.0 6.0–10.5

750–950 750–950 750–950 750–950

15 12 10 8

– – – –

– – – –

Rod

annealed

10–60 65–100 110–150

800–1000 800–1000 760–1000

10 10 6

25 25 18

0.40 0.40 0.45

The alloys is satisfactorily hot deformed. Sheet forming of alloy VT5-1 should be done as hot work. The alloy easily welds by all types of welding applicable for titanium: manual and automatic argon arc welding, submerged welding and resistance welding. The strength and ductility of an argon arc weld is close to that of the base metal. Alloy VT5-1 is not hardenable by heat treatment. It is corrosion resistant in most aggressive media. The alloy is intended for producing forged-and-welded constructions operated long-term at temperatures up to 450°C and short-term up to 800–850°C. The physical properties of alloy VT5-1 are as follows: • density, 4.42 g/cm3 • thermal conductivity at various temperatures: Temperature, ºC λ, Cal/cm s deg

20 100 200 300 400 500 600 700 800 0.021 0.023 0.026 0.029 0.032 0.035 0.038 0.041 0.044

• heat capacity at various temperatures: Temperature, ºC C, Cal/(g deg)

100 200 300 400 500 600 700 800 0.120 0.131 0.135 0.140 0.151 0.160 0.170 0.190

• linear expansion coefficient within the temperature range: Temperature, ºC 20–100 100–200 200–300 300–400 400–500 500–600 600–700 α×106, deg–1 8.5 9.3 9.7 10.0 10.3 10.5 11.0

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Estat, kgf/mm2

90 80 σB, σ0.2, σfc, kgf/mm2, δ5, %

75

70 σB

12000

60 50

11000 σ0.2

40

10000

σfc

9000

30 δ

20

E

8000 7000

10 0 −100 0

6000 200

400

600

Temperature, °C

Figure 32 Change of the mechanical properties of titanium alloy VT5-1 (sheet, 1.5 mm) at short-term rupture vs test temperature.

Normal spectral emissivity of vacuum-heated VT5-1 (temperatures, 200 and 600°C) is, respectively, 0.110 and 0.285. Specific electrical resistance at 20°C: 138·10 –6 Ω cm. Typical mechanical properties of alloy VT5-1 at room and elevated temperatures are given in Fig. 32 and Table 41. Brinell hardness (HB 10/3000/30) of the alloy varies within the limits of 241–321 kgf/mm2. Table 41 Mechanical properties of alloy VT5-1 according to technical specifications. Properties

Semiproduct

Temperature, ºC 20

200

300

350 400

500

Impact strength, kgf/cm2 Notch sensitivity at static load, kgf/mm2 Long-term strength for 1000 h, kgf/mm2 (no less than)

rod, 20 mm dia rod, 20 mm dia

4.5 1.3

– –

– –

– –

– –

– –

sheet, 1.8 mm

–

59

56

54

39

17

Creep limit for 1000 h, kgf/mm2 (no less than) Endurance limit based on 107 cycles, kgf/mm2 (no less than) Crack sensitivity at bending impact, kgf/cm2

sheet, 1.8 mm

–

50

45

38

–

5

sheet, 1.8 mm

40

–

37

35

–

26

sheet, 1.8 mm

3.5

–

–

–

–

–

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δ5, %

Estat, kgf/mm2

VALENTIN N. MOISEYEV

30

12000

20

10000

10

8000

0

6000

100

σB, σ0.2, kgf/mm2

80 σB 60 σ0.2

E

40

δ 20 0 −100 0

200 400 Temperature, °C

500

Figure 33 Tension curves at yield strength for titanium alloy VT5-1 (sheet, 3 mm) at room and elevated temperatures.

80

20°C 20 σ0.2

σ, kgf/mm2

60

40

250 300 σ0.2 σ0.2

250° 300° 350° 400°

350 400 σ0.2 σ0.2

20

0

0.2

0.5

1.0

δ, %

Figure 34 Creep curves for titanium alloy VT5-1 (sheet, 3 mm) at temperatures of (a) 250°C, (b) 300°C, (c) 350°C.

The tension curves at yield strength of 3 mm-thick sheets from alloy VT5-1 are given in Fig. 33. The ultimate strength of the sheets at room temperature is 87 kgf/mm2. Figure 34 presents the creep curves by the total deformation of 3 mm-thick sheets at temperatures of 250, 300, and 350°C at different stresses. The ultimate strength of the sheets at room temperature is 87 kgf/mm2. Figure 35 shows a change of the creep limit at 0.1% residual deformation of rods from alloy VT5-1 as a function of test temperature. At room temperature, the rod has ultimate strength of 85 kgf/mm2.

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0.8

0.7

σB = 51 kgf/mm2

0.7 45

Total deformation, %

0.5

20

30

40 60 Time, h

(a) 0.3 80 100 0

20

(b)

40 60 Time, h

80 100

σB = 34 kgf/mm2

0.5

32 30

0.4 0.3

37

0.4

40 0

45

0.5

0.4

0.6

σB = 48 kgf/mm2

0.6

0.6

77

(c) 200

400 600 Time, h

800

1000

Figure 35 Creep limit (σ0.1) of alloy VT5-1 (rod, 20 mm in diameter) for 150 h at various temperatures.

As it follows from the above data, alloy VT5-1 has a relatively high creep resistance up to 450°C. This is one of the main positive characteristics of the alloy. Alloy VT6S This alloy is based on the Ti–Al–V system. As compared with VT6, it has a lower content of aluminum and vanadium. In annealed state, the alloy contains about 7% β-phase (Kβ = 0.25). Alloy VT6S has the following chemical composition (wt. %): Al, 5.0–6.5; V, 3.5–4.5. The content of impurities should not exceed (wt. %): C, 0.08; Fe, 0.25; Si, 0.15; Zr, 0.30; O2, 0.15; N2, 0.05; H2, 0.015; the sum of the other impurities, 0.30. The mechanical properties of sheets and rolled rods from alloy VT6S according to the current documentation are given in Table 42. Table 42 Mechanical properties of sheets and rolled rods from VT6S according to technical specifications. Semiproduct

State of tested specimens

Thickness, mm

Mechanical properties σB, MPa

δ, %

ψ, % KCV, MJ/m2 no less than

Sheet

annealed

1.0–6.0 6.0–10.5

850–1000 850–1000

12 10

– –

quenched and aged

1.0–10.5

1050

8

–

– – –

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(continued)properties of sheets and rolled rods from VT6S according to technical Table 42 Mechanical specifications. Semiproduct

State of tested specimens

Thickness, mm

Mechanical properties σB, MPa

δ, %

ψ, % KCV, MJ/m2 no less than

Rod

annealed

10–60 65–100 110–150

850–1000 850–1000 770–1000

10 10 7

30 25 22

0.4 0.4 0.4

The physical properties of alloy VT6S are as follows: • density: 4.45 g/cm3 • thermal conductivity vs temperature: Temperature, ºC λ, Cal/cm s deg

20 100 200 300 400 500 600 700 800 900 0.020 0.022 0.025 0.028 0.031 0.035 0.140 0.151 0.160 0.170

• heat capacity at various temperatures: Temperature, ºC C, Cal/(g deg)

100 200 300 400 500 600 700 800 900 0.131 0.140 0.151 0.160 0.170 0.181 0.190 0.211 0.221

• linear expansion coefficient within the temperature range: Temperature, ºC 20–100 100–200 200–300 300–400 400–500 500–600 600–700 α×106, deg–1 8.4 9.0 9.6 10.2 10.8 11.4 11.3

• normal spectral emissivity of etched sheets from air-heated VT1-0 (temperatures, 100 and 150°C) is, respectively, 0.200 and 0.245 • specific electrical resistance at 20°C: 142·10 –6 Ω cm. Typical mechanical properties of alloy VT6S at room and elevated temperatures are given in Table 43 and Fig. 36. Table 43 Mechanical properties of alloy VT6S according to technical specifications. Properties

Impact strength, kgf/cm2 Notch sensitivity at static load, kgf/mm2 Long-term strength for 100 h, kgf/mm2 (no less than) Creep limit for 100 h, kgf/mm2 (no less than) Crack sensitivity at bending impact, kgf/cm2

Semiproduct

Temperature, ºC 20

200

350

rod, 20 mm dia sheet, 3.0 mm sheet, 3.0 mm

6 1.3 –

– – 58

– – 50

sheet, 3.0 mm sheet, 3.0 mm

– 4

– –

32 –

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δ5 , %

Edyn, kgf/mm2

120

30

13000

20

11000

10

9000

0

7000

100

σB, σ0.2, kgf/mm2

80 σB

σ0.2

60

E 40 δ 20 0 −100 0

200 400 Temperature, °C

Figure 36 Change of the mechanical properties of titanium alloy VT6-S (sheet, 3 mm) at short-term rupture vs test temperature. 60 55 1

50 σ−1, kgf/mm2

2 45 3 40

35 4 30 104

105 106 107 Number of cycles before breakdown

108

Figure 37 Endurance curves for smooth and notched specimens from alloy VT6-S (sheet, 3 mm): 1, smooth specimens at 20°C; 2, notched specimens at 20°C; 3, smooth specimens at 350°C; 4, notched specimens at 350°C.

Brinell hardness (HB 10/3000/30) of the alloy varies within the limits of 241–321 kgf/mm2. Figure 37 shows the results of fatigue tests for smooth and notched specimens (rod, 25-mm diameter with σB = 100 kgf/mm2) at temperatures of 20 and 350°C. Endurance was determined by the method of bending with rotation at 320°C; and by the method of bending, at 350°C.

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Alloy VT6S is used to prepare sheets, forgings, stampings, rods, shapes, and other semiproducts. The alloy satisfactorily yields to fusion welding (argon arc, submerged, and electroslag) and resistance welding (seam, spot). The ultimate strength of a welded joint made by fusion welding is no less than 90% of that of the base metal. After welding, it is recommended to perform annealing to release internal stresses and to stabilize the structure. The alloy has a satisfactory ductility in hot deformation. Forging, stamping, and hot rolling of the alloy should be done at elevated temperatures. The basic operations of sheet forming should also be done with heating. Alloy VT6S is recommended for stamped-and-welded constructions operated long-term at temperatures up to 350°C and short-term up to 700–750°C. The alloy is widely used to produce high-pressure vessels. Alloy VT20 This alloy is based on the Ti–Al–Zr–Mo–V system. The content of β-stabilizing elements (molybdenum, vanadium) in it is small. This is virtually a single-phase α-alloy. As compared with other welded sheet alloys, it has a better high-temperature strength at temperatures up to 500°C. The alloy has a satisfactory thermal stability after prolonged holding at 500–550°C. Its chemical composition is as follows (wt. %): Al, 5.5–7.5; Mo, 0.5–2.0; V, 0.8–1.8; Zr, 1.5–2.5. The content of impurities should not exceed (wt. %): C, 0.10; Fe, 0.30; Si, 0.15; O2, 0.15; N2, 0.05; H2, 0.015; the sum of the other impurities, 0.30. The mechanical properties of sheets and rolled rods according to the current documentation are given in Table 44. Table 44 Mechanical properties of sheets and rolled rods from alloy VT20 according to technical specifications. Semiproduct

State of tested specimens

Thickness, mm

Mechanical properties σB, MPa

δ, %

ψ, % KCV, MJ/m2 no less than

Sheet

annealed

0.8–1.8 1.8–4.0 4.0–10.5

950–1150 950–1150 950–1150

12 10 8

– – –

– – –

Sheet

as received

0.8–4.0

1000–1200

9

–

–

Rod

annealed

4.0–10.5 less than 25 more than 25

1000–1200 950–1150 950–1150

6 10 10

25 25 18

– 0.3 0.4

The physical properties of alloy VT20 are as follows. • density: 4.5 g/cm3

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• thermal conductivity at various temperatures: Temperature, ºC λ, Cal/cm s deg

100 200 300 400 500 600 700 800 900 0.021 0.024 0.026 0.029 0.033 0.036 0.040 0.043 0.047

• heat capacity at various temperatures: Temperature, ºC C, Cal/(g deg)

100 200 300 400 500 600 700 800 900 0.131 0.140 0.151 0.160 0.170 0.181 0.201 0.211 0.221

• linear expansion coefficient within the temperature range 20–500°C: 8.8 × 10 –6 °C. • normal spectral emissivity of etched sheets from air-heated VT20 (temperatures, 100 and 500°C) is, respectively, 0.21 and 0.25. • specific electrical resistance at 20°C: 163·10 –6 Ω cm. Typical mechanical properties of alloy VT20 at room and elevated temperatures are given in Table 45 and Fig. 38. Table 45 Mechanical properties of alloy VT20. Properties

Semiproduct

Impact strength, kgf/cm2 rod, 20-mm dia sheet, 2 mm Notch sensitivity at static load, kgf/mm2 sheet, 2 mm Endurance limit based on 107 cycles, kgf/mm2 (no less than) sheet, 2 mm Long-term strength for 500 h, kgf/mm2 (no less than) sheet, 2 mm Creep limit for 100 h, kgf/mm2 (no less than) Crack sensitivity at bending impact, sheet, 2 mm kgf/cm2

Temperature, ºC 20

350

500

550

4.5 1.2 42

– – 38

– – 36

– – –

–

75

49

20

–

55

17

–

3.5

–

–

–

Brinell hardness (HB 10/3000/30) varies within the limits of 255–341 kgf/mm2. Alloy VT20 is intended for producing sheets and plates, and can also be used for fabricating forgings, stampings, rods, shapes, and other semiproducts. The alloy is distinguished by a decreased ductility. For instance, sheet forming should be carried out with heating up to 800–900°C; as cold work, the alloy does not yield to sheet forming. In hot forging, stamping, and pressing, the alloy has a satisfactory ductility.

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100

σB, σ0.2, kgf/mm2, δ5, %

90 80

σB

70

σ0.2

Edyn, kgf/mm2

VALENTIN N. MOISEYEV

12000 11000

60 E

50

10000 9000

40

8000

30

δ5

20 10 0

7000

6000 100 200 300 400 500 600 Temperature, °C

Figure 38 Change of the mechanical properties of titanium alloy VT20 (sheet, 2 mm) at short-term rupture vs test temperature.

The alloy welds well by argon arc, resistance and submerged welding. The strength of the weld is 0.9–0.95 of that of the base metal. To release the stress, the welded constructions should be annealed. The corrosion resistance under atmospheric conditions and in corrosive media is high. The alloy is recommended for producing stamped-and-welded constructions operated long-term at temperatures up to 500°C and short-term up to 800°C. Alloy ST5 This alloy is based on the Ti–Al–Sn–Zr–V system. The alloy contains a minor amount of β-stabilizing elements; it is virtually a single-phase alloy. It has a good high-temperature strength and decreased ductility. By its mechanical and process properties, the alloy is close to VT20. Alloy ST5 has the following chemical composition (wt. %): Al, 4.5–5.5; Zr, 1.0–3.0; Sn, 2.0–4.0; V, 4.5–5.5. The content of impurities should not exceed (wt. %): C, 0.10; Fe, 0.30; Si, 0.15; O2, 0.15; Nb, 0.05; H2, 0.015. The mechanical properties of sheets and rolled rods according to the current documentation are given in Table 46. The physical properties of alloy ST5 are as follows: • specific weight: 4.47 g/cm3 • specific electrical resistance at 20°C: 158·10 –6 Ω cm Typical mechanical properties of alloy ST5 at room and elevated temperatures are given in Fig. 39. Alloy ST5 is intended for producing sheets and plates and can also be used for producing forgings, stampings, rods, shapes, and other semiproducts.

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Table 46 Mechanical properties of sheets and rolled rods from alloy ST5 according to technical specifications. Semiproduct

State of tested specimens

Thickness, mm

Mechanical properties σB, MPa

δ, %

ψ, %

KCV, MJ/m2

no less than Sheet

annealed

0.8–1.8 1.8–4.0 4.0–10.5

900–1100 900–1100 900–1100

14 12 10

– – –

– – –

Rolled rod

annealed

10–60 65–100 105–150

900–1100 900–1100 900–1100

12 12 10

30 25 25

0.35 0.40 0.35

Plastic deformation (forging, stamping, and rolling) should be done as hot work. Alloy ST5 is distinguished by a decreased ductility in sheet stamping. Cold stamping of sheets is not recommended, it should be done as hot work.

δ5, %

Estat, kgf/mm2

120

30

12000

20

10000

10

8000

100

σB, σ0.2, kgf/mm2

σB 80 σ0.2 60 E 40 δ 20 0 −100 0

200 400 Temperature, °C

0 600

6000

Figure 39 Change of the mechanical properties of titanium alloy ST5 (sheet, 3 mm) at shortterm tension vs test temperature.

The alloy welds well by all types of welding applicable for titanium – argon arc, fusion, submerged, and resistance welding. The strength of the joint done by fusion welding is 0.9 of that of the base metal. Fusion-welded components should be annealed to release internal stresses. Corrosion resistance of the alloy is high. Its machinability is satisfactory. Alloy ST5 is recommended for producing stamped-and-welded constructions operated long-term at temperatures up to 500°C.

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A common characteristic of this group of alloys is their predominantly α-structure with a minor amount of β-phase. Owing to this, they weld well, are thermally stable and, as a rule, not liable to hardening heat treatment. Medium-strength alloys are widely used in various industries. They can be used to fabricate both load-bearing parts and welded components from sheet semiproducts. Alloys of this group can be used in units operated long-term at temperatures up to 400–500°C.

2.4 HIGH-STRENGTH ALLOYS This group covers thermally hardened titanium alloys whose high strength is achieved by quenching and aging. An exception is alloy VT22 whose ultimate strength is no less than 1100 MPa as annealed. High-strength alloys include grades VT6, VT14, VT16, VT23, VT22, VT22I, VT35, VT32, and TS6 (thermally hardened), and grade VT22 (annealed). As alloys VT6, VT14, VT23, and VT22 are used both as thermally hardened and as annealed, we will consider both sets of their properties. Alloys VT35, VT32, VT15, and TS6 are intended to be used exclusively thermally hardened. Their use as annealed is not rational, as after annealing they are not as strong and, at the same time, rather expensive because they contain a considerable amount of such alloying elements as molybdenum, vanadium, and chromium. Alloy VT6 This alloy is an analog of the Ti–6Al–4V alloy used worldwide. In Russian practice, the alloy is intended mainly for producing forgings, stampings, plates, sheets, rods, shapes, and some other semiproducts. The alloy can be used as annealed, as quenched, and as aged. Due to the low hardenability of the alloy, the components intended for hardening heat treatment should be no more than 30–40 mm in cross-section. Alloy VT6 has the following chemical composition (wt. %): Al, 5.5–7.0; V, 4.2–6.0. The content of impurities should not exceed (wt. %): C, 0.10; Fe, 0.30; Si, 0.15; O2, 0.20; N2, 0.05; H2, 0.015. The mechanical properties of sheets and rolled rods from alloy VT6 according to the current technical documentation are given in Table 47. The physical properties of alloy VT6 in annealed and thermally hardened states are the same and equal. • density: 4.43 g/cm3 • thermal conductivity at various temperatures: Temperature, ºC λ, Cal/cm s deg

20 0.020

100 0.022

200 0.026

300 0.027

400 0.029

500 0.033

600 0.037

700 0.040

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Table 47 Mechanical properties of sheets and rolled rods from alloy VT6 according to technical specifications. Semiproduct

State of tested specimens

Thickness, mm

Mechanical properties σB, MPa

δ, %

ψ, % KCV, MJ/m2 no less than

Sheet Rod

annealed annealed

quenched and aged

1.0–10.5 10–60 65–100

90–110 920–1070 920–1070

8 10 10

– 30 25

– 0.40 0.30

110–150 110–150

850–1070 1100

6 6

20 20

0.30 0.30

• heat capacity at various temperatures: Temperature, ºC C, Cal/(g deg)

100 0.131

200 0.140

300 0.160

400 0.170

500 0.190

600 0.211

• linear expansion coefficient within the temperature range: Temperature, ºC α×106, deg–1

20–100 8.4

100–200 9.3

200–300 9.8

300–400 10.1

Brinell hardness (HB 10/3000/30) of the alloy in annealed and thermally hardened states varies within the range of 255–341 kgf/mm2 and 293–361 kgf/mm2, respectively. Normal spectral emissivity of air-heated polished metal (temperatures, 200 and 500°C) is, respectively, 0.10 and 0.24. Typical mechanical properties of alloy VT6 in annealed and thermally hardened states at room and elevated temperatures are given in Fig. 40 and Table 48. Figure 41 presents the tension curves of annealed rods at yield strength at various temperatures. The ultimate strength at an instantaneous rupture at room temperature is 100 kgf/mm2. Figure 42 shows the change of the long-term strength (for 1000 h) of annealed rods 20 mm in diameter, with σB = 100 kgf/mm2, as a function of test temperature. Alloy VT6 is well hot deformed: forged, stamped, and rolled. Sheet stamping is also done as hot work. The alloy welds satisfactorily by all types of welding applicable for titanium alloys. Heat treatment after welding is obligatory to restore plasticity of the welded joint. The ultimate strength of the joint done by fusion welding is no less than 0.9 of that of the base metal. Plasticity of the welded joint is close to that of the base metal. The alloy is satisfactorily machined, the machining can be done both on as-annealed material, and in a thermally hardened state.

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σB

90

σ0.2

σB 70

σ0.2

σfc 50

E

δ

δ (a)

0

30 13000

σfc

E

30 10

δ5 , %

σB, σ0.2, σfc, kgf/mm2

110

200

400

20

11000

10

9000

(b)

600 0 200 Temperature, °C

400

Edyn, kgf/mm2

130

0 600

7000

Figure 40 Change of the mechanical properties of as-annealed (a) and thermally hardened (b) titanium alloy VT6 (rod, 20 mm in diameter) at short-term tension vs test temperature.

VT6 has a high corrosion resistance in as-annealed and thermally hardened state in humid atmosphere, seawater, and other aggressive media. Table 48 Mechanical properties of a rod (25 mm dia) from alloy VT6. Properties

Annealed

Quenched and aged Temperature, ºC

20

350

400

450

20

350

400

450

Impact strength, kgf/cm2 Notch sensitivity at static load, kgf/mm2 Long-term strength for 100 h, kgf/mm2 (no less than)

4 1.35

– –

– –

– –

3 1.25

– –

– –

– –

–

62

60

42

–

83

75

55

Creep limit for 100 h, kgf/mm2 (no less than) Endurance limit based on 107 cycles, kgf/mm2 (no less than) Crack sensitivity at bending impact, kgf/cm2

–

54

36

14

–

63

36

–

53

43

42

39

55

45

44

–

3.5

–

–

–

2.5

–

–

–

The alloy is recommended for producing welded constructions operated long-term at temperatures up to 400°C and short-term up to 750°C. Alloy VT14 This is the first thermally hardened alloy produced by national industry. Owing to its high strength in annealed state, it is used as annealed, too.

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100

87

20°C 20 σ0.2 20 σfc

σ, kgf/mm2

80

60

250 σ0.2

250

250 σfc 350 450 σ0.2 σ0.2

40

350 450°C

350 σfc 450

σfc 20

0

0.2 0.4

0.6

0.8 1.0 δ, %

Figure 41 Tension curves at yield strength for annealed titanium alloy VT6 (rod, 20 mm in diameter) at room and elevated temperatures. 70

σ1000, kgf/mm2

60

40

20

0 300 500 Temperature, °C

Figure 42 Long-term strength (σ1000) of annealed alloy VT6 (rod, 20 mm in diameter) vs test temperature.

The alloy is based on the Ti–Al–Mo–V system and its Kβ = 0.45. The chemical composition of VT14 is as follows (wt. %): Al, 3.5–6.3; Mo, 2.5–3.8; V, 0.9–1.9. The content of impurities should not exceed (wt. %): C, 0.10; Fe, 0.30; Si, 0.15; Zr, 0.30; N2, 0.05; H2, 0.015; the sum of the other impurities, 0.30. In sheets of up to 10 mm in thickness, the content of aluminum should be 3.5–4.5%; in other semiproducts, 4.5–6.3%. The mechanical properties of sheets and rolled rods from alloy VT14 according to the current documentation are given in Table 49.

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Table 49 Mechanical properties of sheets and rolled rods from alloy VT14 according to technical specifications. Semiproduct

Thickness, mm

State of tested specimens

Mechanical properties σB, MPa

δ, % ψ, % KCV, MJ/m2 no less than

Sheet

annealed

0.6–5.0 5.0–10.5

900–1070 850–1070

8 8

– –

– –

quenched and aged

0.6–1.5 1.5–5.0 5.0–7.0 7.0–10.5

1100 1200 1100 1120

5 6 4 4

– – – –

– – – –

annealed

10–60 65–100 110–150

900–1070 900–1070 880–1000

10 9 8

30 25 22

0.50 0.50 0.45

quenched and aged

10–60 65–100

1120 1100

12 8

30 25

0.25 0.20

Rod

The major physical properties of alloy VT14 in annealed and thermally hardened state are the same. • density: 4.32 g/cm3 • thermal conductivity at various temperatures: Temperature, ºC λ, Cal/cm s de

20 8.4

100 9.2

200 10.5

300 11.7

400 13.0

500 13.8

600 15.5

700 16.8

800 18.4

900 20.1

• heat capacity vs temperature: Temperature, ºC C, Cal/(g deg)

100 0.503

200 0.545

300 0.578

400 0.628

500 0.670

600 0.712

700 0.838

• linear expansion coefficient within the temperature range: Temperature, ºC α×106, deg–1

20–100 8.8

100–200 8.4

200–300 9.1

300–400 9.7

400–500 9.3

• specific resistance has the following values: Temperature, ºC ρ×10–6, Ohm cm

–100 150

–60 153

0 157

25 159

100 163

200 168

300 172

350 173

400 175

450 176

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σB

70 E

σ0.2

σ0.2

σfc

σfc

δ

δ

13000

20

11000

10

9000

E

30 10 0

30

σB

90

50

Estat, kgf/mm2

σB, σ0.2, σfc, kgf/mm2

110

δ5, %

130

(a) 200

400

(b)

600 0 200 Temperature, °C

400

0 600

7000

Figure 43 Change of the mechanical properties of alloy VT14 (sheet, 2 mm) at short-term rupture vs test temperature.

Typical mechanical properties of alloy VT14 in annealed and thermally hardened state are given in Fig. 43 and Table 50. Table 50 Mechanical properties of alloy VT14. Properties

Semiproduct

Annealed

Quenched and aged

Temperature, ºC 20

350

400

20

350

400

450

rod, 25 mm dia sheet, 2.0 mm

6 1.35

– –

– –

3 1.20

– –

– –

– –

sheet, 2.0 mm

40

–

34

42

–

38

–

sheet, 2.0 mm

–

63

60

–

42

68

54

sheet, 2.0 mm Creep limit for 100 h, kgf/mm2 (no less than) Crack sensitivity at bending sheet, 2.0 mm impact, kgf/cm2

–

48

–

–

53

35

–

4.5

–

–

3.0

–

–

–

Impact strength, kgf/cm2 Notch sensitivity at static load, kgf/mm2 Endurance limit based on 107 cycles, kgf/mm2 (no less than) Long-term strength for 100 h, kgf/mm2 (no less than)

Brinell hardness (HB 10/3000/30) in annealed and thermally hardened states varies within the range of 255–341 kgf/mm2 and 302–388 kgf/mm2, respectively. The creep curves shown in Fig. 44 are obtained on annealed rods 20 mm in diameter at a temperature of 300°C. At room temperature, the rod had ultimate strength of 94 kgf/mm2.

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Total deformation, %

VALENTIN N. MOISEYEV

0.7

σ = 53 kgf/mm2

0.6

50

0.5 0.4

48

0.3 400

45

800 1200 Time, h

1600

2000

0.8

(a)

σ = 55 kgf/mm2

0.7 53

0.6

6 res 0.16 res 0.135

50 res 0.090

0.5 0.4 0

20

res 0.061 47 40 60 80 100

Total deformation, %

Total deformation, %

Figure 44 Creep curves for annealed rods (20 mm in diameter) from alloy VT14 at 300°C.

0.6

00

(b)

0.5 0.4

res 0.2

37

.180

res 0

35

0.3 0.2 0

20

40

60

80 100

Duration of test, h

Figure 45 Creep curves for thermally hardened sheets (2 mm thick) from alloy VT14 at temperatures of (a) 350°C, (b) 400°C.

Figure 45 presents creep curves of thermally hardened sheets 2.0-mm thick from alloy VT14 at temperatures of 350 and 400°C. The numerator shows the total deformation for 100 h, and the denominator gives residual deformation. The sheets were hardened according to the following regime: 880°C, 20 min; water quenching; aging at 500°C, 16 h at σB = 128 kgf/mm2. It should be noted that in a thermally hardened state the alloy has sufficiently high values of high-temperature strength up to 400–430°C. At higher temperatures, it was observed to lose strength in long-term tests. For instance, in 100 hours a thermally hardened sheet at 450°C at a residual deformation of 0.5% has a creep limit of only 15 kgf/mm2. At the same time, its long-term strength is preserved sufficiently 450 = 54 kgf/mm2. high – σ0.2/100 Figure 46 presents endurance curves of thermally hardened sheets 2.0-mm thick at temperatures of 20–400°C. The sheets were thermally hardened according to the following regime: 880°C, 20 min; water quenching; aging at 500°C, 16 h at σB = 128 kgf/mm2. As seen in the plot, the alloy preserves comparatively high endurance within the temperature range of 20 up to 400°C. In quenched or annealed state, alloy VT14 has a satisfactory ductility, and acquires a high strength after quenching and aging. The alloy has a limited hardenability, so parts of blanks of more than 40–60 mm in cross-section should not be subjected to thermal hardening.

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50 48 σ−1, kgf/mm2

46

20°C

44 42 400°C 40 38 36 34 105 2 4 6 106 2 4 6 107 2 3 Number of cycles before breakdown

Figure 46 Endurance curves for alloy VT14 thermally hardened for σB = 1280 MPa at temperatures of 20 and 400°C.

The alloy is produced as sheets, band, plates, forgings, stampings, rods, shapes, and other semiproducts. Sheet stamping of the alloy in annealed or quenched states with some deformations can be done as cold work. The main operations of stamping are to be done as hot work. Forging, stamping, and hot rolling of the alloy should be performed within the temperature range of 1050–750°C; in this case, 50–60% deformation should be done → β transformation (lower than 950°C). Warm at a temperature lower than α+β ← rolling of the alloy is done at a temperature of 750–550°C. Ultimate strength of the welded joint is no less than 0.9 of that of the base metal. The alloy is satisfactorily machined. Machining could be done both in annealed and thermally hardened states. Alloy VT14 has a high corrosion resistance under atmospheric conditions and in most aggressive media, both in annealed and thermally hardened states. The alloy is recommended for production of forged-and-welded constructions operated long-term at temperatures up to 400°C and short-term up to 750°C. Alloy VT16 This grade is more alloyed as compared with VT14, is of martensite type and is based on the Ti–Al–Mo–V system. Its coefficient of β-phase stability is Kβ = 0.8 and it is distinguished by high cold ductility due to the decreased aluminum content. The chemical composition of alloy VT16 is as follows (wt. %): Al, 1.6–3.0; Mo, 4.5–5.5; V, 4.0–5.0. The content of impurities should not exceed (wt. %): C, 0.10; Fe, 0.25; Si, 0.15; Zr, 0.30; O2, 0.15; N2, 0.05; H2, 0.015; the sum of the other impurities, 0.30.

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The mechanical properties of rolled rods according to the current technical documentation are given in Table 51. Table 51 Mechanical properties of sheets and rolled rods from alloy VT16 according to technical specifications. Semiproduct

State of tested specimens

Thickness, mm

Mechanical properties σB, MPa

δ, %

ψ, %

KCV, MJ/m2

no less than Hot-rolled rod

quenched and aged

4.0–20 20–30 30–40

1050–1200 1050–1200 1050–1200

12 12 10

50 40 30

0.3 0.3 0.3

Ground rod

annealed

4.0–10.0

830–950

14

63

–

The physical properties of alloy VT16 in annealed or thermally hardened states are practically the same. • density: 4.65 g/cm3 • thermal conductivity at various temperatures: Temperature, ºC λ, Cal/cm s deg

20 10.0

100 10.9

200 12.1

300 13.4

400 14.6

500 15.9

600 16.7

700 18.0

800 19.6

900 21.6

• heat capacity vs temperature: Temperature, ºC C, Cal/(g deg)

100 200 300 400 500 600 700 800 900 0.481 0.503 0.545 0.587 0.670 0.712 0.796 0.838 0.880

• linear expansion coefficient within the temperature range: Temperature, ºC 20–100 100–200 200–300 300–400 400–500 500–600 600–700 700–800 9.1 9.8 10.4 10.5 10.3 7.7 10.2 13.4 α×106, deg–1

• specific resistance at various temperatures: Temperature, ºC ρ×106, Ohm cm

20 111

120 124

200 132

260 138

350 145

420 149

480 152

550 156

680 157

Normal spectral emissivity of etched sheets from air-heated VT16 (at 100 and 900°C) is, respectively, 0.49 and 0.68; of cold-rolled sheets at the same temperatures, 0.20 and 0.69, at 100 and 900°C, respectively. Change of the mechanical properties as a function of test temperature at an instantaneous rupture is shown in Fig. 47. Specimens from alloy VT16 (rod, 20 mm

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σB, σ0.2, σfc, kgf/mm2

σ0.2

80 σfc

ψ

60

δ5, ψ, %

σB

100

Estat, kgf/mm2

120

30 12000 E 20 10000

40 δ

20 0

0

10

0 200 400 600 Temperature, °C

8000 6000

Figure 47 Change of the mechanical properties of titanium alloy VT6 (rod 20 mm in diameter thermally hardened for σB = 1150 MPa) at short-term rupture vs test temperature.

in diameter) were processed according to the following regime: annealing at 780, 810°C for 1 h; water quenching; aging at 500°C for 8 h. The mechanical properties of alloy VT16 are given in Table 52. Table 52 Mechanical properties of quenched and aged alloy VT16. Properties

Semiproduct

Temperature, ºC 20

250

300

350

400

Brinell hardness (HB 10/3000/30), kgf/mm2 Notch sensitivity at static load, kgf/mm2 Long-term strength for 500 h, kgf/mm2 (no less than)

rod, 20 mm dia

303–415

–

–

–

sheet, 2.0 mm

1.15

–

–

–

sheet, 2.0 mm

–

–

70

60

47

Endurance limit based on 107 cycles, kgf/mm2 (no less than), smooth specimen notched specimen

rod, 20 mm dia

44

–

33

–

–

rod, 20 mm dia

32

–

30

–

–

Change of the mechanical properties of 2-mm sheets as a function of test temperature is shown in Fig. 48. The sheets were processed according to the following regime: 800°C, 20 min; water quenching; aging at 500°C, 12 h. The ultimate strength for 100 h and creep limit at residual deformation of 0.2% for 100 h and long-term strength at one-second load were determined on 2-mm-thick sheets thermally hardened at σB of about 130 kgf/mm2 according to the following regime: 800°C, 20 min; water quenching; aging at 500°C, 12 h. The values of long-term strength are given in Table 52. The creep curves at 300, 350, and 400°C for 100 h are given in Fig. 49.

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σ0.2

σB

90

δ5, ψ, %

σB, σ0.2, σfc, kgf/mm2

110

Estat, kgf/mm2

130

11000

25

10000

20

9000

15

8000

10

σfc 70 E 50 δ

30 10

0

100

200 300 400 500 Test temperature, °C

7000 600

0

Figure 48 Change of the mechanical properties of titanium alloy VT16 (sheet 2 mm) thermally hardened for σB = 1300 MPa) at short-term rupture vs test temperature. 1.0 0.9 75 0.8 Total deformation, %

res 0.516

0.8

res 0.198

0.7

σ = 77 kgf/mm2

70 0

20

60

40

58 res 0.172

0.6

res 0.195 (a)

0.7

σ = 60 kgf/mm2 res 0.203

0.5 80 100 20 0 Duration of test, h

(b) 40

60

80 100

0.7 0.6

res 0.275 σ = 35 kgf/mm2

0.5

res 0.204

30 0.4 27

res 0.187

0.3 (c)

0.2 0

60 80 20 40 Duration of test, h

100

Figure 49 Creep curves for 100 h for sheets (2 mm thick) from alloy VT16 thermally hardened for σB = 1300 MPa at temperatures of (a) 300°C, (b) 350°C, (c) 400°C.

The scale bar of the plot and the numerator show total creep strain. The denominator indicates the residual deformation after 100 h of tests. As it follows from the data, in long-term operation under creep conditions (at residual deformation of 0.2%) the alloy can be efficiently used at temperatures up to 350°C.

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95

1

52

σ−1, kgf/mm2

48

44

40

36

3

2 4

32 104 2 3 4 5 6 8 105 2 3 4 5 6 8 106 2 3 4 5 6 8 107 2 3 Number of cycles before breakdown

Figure 50 Endurance curves for as-thermally hardened alloy VT16 (σB = 1350 MPa) obtained on smooth and notched specimens at 20 and 400°C: (1) smooth specimens at 20°C, (2) notched specimens at 20°C, (3) smooth specimens at 400°C, (4) notched specimens at 400°C.

Figure 50 presents endurance curves for smooth and notched specimens from alloy VT16 at temperature of 20 and 400°C at bending through an angle. The specimens for fatigue tests were prepared from a rod 25 mm in diameter, thermally hardened at σB = 1300 MPa. The ratio of fatigue limit based on 107 cycles to ultimate strength is 0.4. The fatigue limit ratio of the notched to smooth specimen is 0.7. At a temperature of 400°C, the value of endurance on smooth specimens progressively decreases with the number of cycles. This is determined by the development of the yield of metal at these temperatures. In a notched specimen, metal is significantly hardened in the notch spot, and the notched specimen at 400°C behaves similar to that tested at 20°C. At present, alloy VT16 is used to fabricate rods and wire for producing fixing components by cold deformation. Alloy VT16 can also be successfully used to produce sheets, band, foil, thin-walled pipes, shapes, i.e., semiproducts produced by intensive cold deformation. Usually, fixing elements from alloy VT16 are quenched and aged at σB = 1100–1200 MPa. At present, the annealed state of the alloy followed by mechanical hardening is widely used for bolts produced by cold heading, reduction of cross-sectional area and knurling without recrystallization annealing. In this state, the alloy has σB = 1050–1150 MPa and is little sensitive to stress concentrators: notch, warp, etc.

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Alloy VT15 This is the first Russian titanium alloy based on β-structure. It has been designed similar to alloy V-120 VSA. Its distinctive feature is high ductility in as-quenched state characteristic of cubic-lattice metals and high strength after aging. The chemical composition of alloy VT15 is as follows (wt. %): Al, 2.3–3.3; Mo, 6.8–8.0; Cr, 9.5–11.5. The content of impurities should not exceed (wt. %): C, 0.10; Fe, 0.30; Si, 0.15; O2, 0.12; N2, 0.05; H2, 0.012; the sum of the other impurities, 0.30. The mechanical properties of sheets and forgings according to the current technical documentation are given in Table 53. Table 53 Mechanical properties of sheets and rolled rods from VT15 according to technical specifications. Semiproduct

State of tested specimens

Thickness, mm

Mechanical properties σB, MPa

δ, %

ψ, % KCV, MJ/m2 no less than

Sheet

quenched quenched and aged

Rod Forging

quenched quenched and aged

1.0–3.0 3.1–4.0 1.0–3.0

850–1000 850–1000 1300

16 16 4

– – –

– – –

3.1–4.0 10–100 10–100

1300 900–1000 1350

4 12 4

– 25 6

– 0.4 –

The physical properties of alloy VT15 in thermally hardened state are as follows: • density: 4.89 g/mm2 • thermal conductivity at various temperatures: Temperature, ºC λ, Cal/cm s deg

20 6.7

100 8.0

200 9.6

300 11.3

400 13.0

500 14.7

• heat capacity vs temperature: Temperature, ºC C, Cal/(g deg)

100 200 300 400 500 600 700 800 900 0.503 0.545 0.587 0.650 0.670 0.712 0.755 0.796 0.838

• linear expansion coefficient within the temperature range: Temperature, ºC α×106, deg–1

20–100 8.9

100–200 9.4

200–300 9.5

300–400 10.0

400–500 10.5

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σB σ0.2

110

σfc

90

E

70 50 30

δ 0

Estat, kgf/mm2

σB, σ0.2, σfc, kgf/mm2

130

δ5, %

150

30

12000

20

10000

10

8000

0 200 400 600 Temperature, °C

6000

Figure 51 Change of the mechanical properties of titanium alloy VT15 (sheet 2 mm) in thermally hardened state at short-term rupture vs test temperature.

• specific electrical resistance at 20°C: 155·10 –6 Ω cm. Typical mechanical properties of alloy VT15 in thermally hardened state are given in Fig. 51 and Table 54. Table 54 Mechanical properties of alloy VT15. Properties

Semiproduct

Temperature, ºC 20

350

400

500

Impact strength, kgf/cm2 Notch sensitivity at static load, kgf/mm2 Crack sensitivity at bending impact, kgf/cm2 Endurance limit based on 107 cycles, kgf/mm2 (no less than) Long-term strength for 500 h, kgf/mm2 (no less than)

rod, 20 mm dia sheet, 2.0 mm

2.0 1.05

– –

– –

– –

sheet, 2.0 mm

1.5

–

–

–

sheet, 2.0 mm

28

–

26

–

sheet, 2.0 mm

–

95

74

35

Creep limit for 100 h, kgf/mm2 (no less than)

sheet, 2.0 mm

–

53

–

–

Brinell hardness (HB 10/3000/30) of the alloy in thermally hardened state varies within the limits of 341–444 kgf/mm2. Alloy VT15 is recommended to be used only in quenched and aged states, when it has an advantage over low-alloyed structural alloys. It has a good hardenability and can be air hardened. The alloy is fabricated as sheets, band, foil, forgings, stampings, rods, shapes, and other semiproducts.

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Forging, stamping, hot rolling are performed at temperatures of 1100–850°C; warm rolling of sheets, at 800–650°C. Thin sheets, band, and foil are cold rolled. Sheets from alloy VT15 are well cold stamped. Sheet stamping associated with large deformations should be done at temperatures of 750–550°C. Alloy VT15 is satisfactorily welded by all types of welding applicable for titanium (fusion welding and resistance welding). Welded joints have a good plasticity both immediately after welding and after annealing. Hardening heat treatment of welded joints is not recommended due to their sharp embrittlement. Welded joints of alloy VT15 are thermally unstable in long-term heating and can lose plasticity. The alloy is satisfactorily machined, though slightly less as compared with other titanium alloys. It has a high corrosion resistance under atmospheric conditions and in most aggressive media. Under some conditions, it is more advantageous than other titanium alloys. Alloy VT15 failed to find wide use due to its low thermal stability under stress. It can be operated long-term only at temperatures up to 250–300°C.

Alloy ST6 This alloy based on β-structure is an analog of VT15. Its distinctive feature is that the isomorphic β-stabilizing elements are molybdenum and vanadium rather than only molybdenum as in VT15. The chemical composition of ST6 is as follows (wt. %): Al, 2.5–3.5; Mo, 4.5–5.0; V, 5.5–6.5; Cr, 10.5–11.5. The content of impurities should not exceed (wt. %): C, 0.10; Fe, 0.30; Si, 0.15; O2, 0.12; N2, 0.05; H2, 0.012; the sum of the other impurities, 0.30. The mechanical properties of sheets and forgings from alloy ST6 according to the current documentation are given in Table 55. Table 55 Mechanical properties of sheets and rolled rods from ST6 according to technical specifications. Semiproduct

State of tested specimens

Thickness, mm

Mechanical properties σB, MPa

δ, % ψ, % KCV, MJ/m2 no less than

Sheet

Rod, forging

quenched

1.0–3.0 3.1–6.0

850–1050 850–1050

18 18

– –

– –

quenched and aged

1.0–3.0 3.1–6.0

1350–1450 1350–1450

4.0 4.0

– –

– –

quenched quenched and aged

10–100 10–100

850–1050 1350–1500

18 4.0

25 6

0.45 –

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99

δ5, %

Estat, kgf/mm2

150

30

12000

20

10000

10

8000

σB, σ0.2, σfc, kgf/mm2

130 σB

110

σ0.2

σfc 90

E

70 δ

50 30

0

0 200 400 600 Temperature, °C

6000

Figure 52 Change of the mechanical properties of titanium alloy ST6 (sheet 2 mm) in thermally hardened state at short-term rupture vs test temperature.

• specific weight of alloy ST6: 4.80 g/cm3 • specific electrical resistance at 20°C: 160·10 –6 Ω cm Typical mechanical properties of alloy ST6 in thermally hardened state are given in Fig. 52. Brinell hardness (HB 10/3000/30) in thermally hardened state varies within the limits of 341–444 kgf/mm2. Impact strength is 2.0 kgf/cm2. Notch sensitivity at static load is 1.05. Alloy ST6 has a good ductility in annealed and quenched states, and after quenching and aging its ultimate strength sharply increases. The alloy should be used only in a thermally hardened state. In this case, it has noticeable advantages over other less alloyed titanium alloys. As alloy VT15, it has a good hardenability and can be hardened when air cooled. The alloy is used to produce various semiproducts: sheets, band, foil, forgings, stampings, rods, shapes, etc. Forging, stamping, and hot rolling of VT15 should be done within the temperature range of 1100–850°C. Warm rolling of sheets is performed at 800–850°C. Thin sheets, band, and foil are cold rolled with deformation of 40–60% between annealings. Sheets are well cold stamped. Stamping with large deformations should be accompanied with heating up to 750°C. Alloy ST6 satisfactorily yields to fusion welding (argon arc, submerged welding) and resistance (spot, seam) welding. Immediately after welding and after annealing, welded joints have a strength and plasticity close to those of the base metal. Welded joints should not be subjected to hardening heat treatment due to their sharp embrittlement. ST6 is as machinable as alloy VT15. It has a high corrosion resistance under atmospheric conditions and in most aggressive media. The alloy is recommended for stamped-and-welded constructions operated long-term at temperatures from –70 up to 300°C and short-term up to 500–600°C.

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Titanium alloy ST6, as alloy VT15, found no wide commercial use mainly due to its low thermal stability and is used only up to 250–300°C. Alloy VT22 This is a high-alloyed transition alloy with the temperature of martensite transformation close to room temperature (Kβ = 1.1). The alloy is based on the Ti–Al–Mo–V–Fe–Cr system and has the following chemical composition (wt. %): Al, 4.4–5.9; Mo, 4.0–5.5; V, 4.0–5.5; Cr, 0.5–2.0; Fe, 0.5–1.5. The content of impurities should not exceed (wt. %): C, 0.15; Si, 0.15; Zr, 0.30; O2, 0.20; N2, 0.05; H2, 0.015. The mechanical properties of rolled rods and plates from alloy VT22 according to the current documentation are given in Table 56. Table 56 Mechanical properties of sheets and rolled rods from alloy VT22 according to technical specifications. Semiproduct

State of tested specimens

Thickness, mm

Mechanical properties σB, MPa

δ, %

ψ, % KCV, MJ/m2 no less than

Rod

annealed

10–30 35–60 65–100 110–150

1100–1250 1100–1250 1100–1300 1100–1300

10 9.0 8.0 7.0

30.0 25.0 16.0 15.0

0.30 0.30 0.25 0.25

quenched and aged

10–40 42–60

1100–1300 1100–1300

7.0 6.0

18.0 16.0

0.20 0.18

Rolled plate

annealed

12–60

1100–1250

6.0

16.0

–

Stamping

annealed

up to 100

1100–1300

7.0

16.0

0.25

101–150 up to 250

1100–1300 1100–1300

6.0 6.0

16.0 16.0

0.25 0.20

The physical properties of alloy VT22 in annealed and thermally hardened states are approximately the same. • density: 4.65 g/cm3 • thermal conductivity at various temperatures: Temperature, ºC λ, Cal/cm s deg

20 8.4

100 9.2

200 10.4

300 11.7

400 13.4

500 14.6

600 15.9

700 17.1

800 18.4

900 19.7

• heat capacity vs temperature: Temperature, ºC C, Cal/(g deg)

100 200 300 400 500 600 700 800 900 0.544 0.585 0.628 0.670 0.710 0.753 0.836 0.876 0.921

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90 σfc 80

σ0.2

60 50 12000

E

40 11000 30 10000

70 60

δ5

50 40

0 200

12000

101

(b) E

11000

160

10000

140

9000

120

8000

100 σ fc

δ5, %

100

ψ

13000

σB,σ0.2,σfc, kgf/mm2

σB, σ0.2, σfc, kgf/mm2

110

Estat, kgf/mm2

σB

δ5,ψ, %

(a)

120

Estat, kgf/mm2

TITANIUM ALLOYS: RUSSIAN AIRCRAFT AND AEROSPACE APPLICATIONS

25 100

σB

20 80

σ0.2

20 9000

80

10 8000 0 7000

60

ψ, %

01 Chapter 2

15 60 10 40

ψ δ5

5

20

0

0

0 200 400 600 Temperature, °C

400 600 Temperature, °C

Figure 53 Change of the mechanical properties of as-annealed (a) and thermally hardened (b) titanium alloy VT22 (rod, 25 mm in diameter) at short-term tension vs test temperature.

• linear expansion coefficient within the temperature range: Temperature, ºC 20–100 100–200 200–300 300–400 400–500 500–600 600–700 700–800 7.8 8.3 8.7 9.2 9.7 9.2 10.6 14.5 α×106, deg–1

• normal spectral emissivity of polished vacuum-heated alloy VT22 at temperatures of 700 and 1400°C is, respectively, 0.22 and 0.30. • specific electrical resistance at 20°C: 153·106 Ω cm. Typical mechanical properties of alloy VT22 in annealed and thermally hardened states are given in Fig. 53 and Table 57. Table 57 Mechanical properties of alloy VT22. Properties

Semiproduct

Annealed

Quenched and aged

Temperature, ºC 20 Impact strength, kgf/cm2 Notch sensitivity at static load, kgf/mm2 Endurance limit based on 107 cycles, kgf/mm2 (no less than) Long-term strength for 100 h, kgf/mm2 (no less than) Creep limit for 100 h, kgf/mm2 (no less than)

300 350

20

350 400

rod, 25 mm dia rod, 25 mm dia

4 1.25

– –

– –

2.5 1.15

– –

– –

rod, 25 mm dia

58

–

46

58

48

–

rod, 25 mm dia

–

888

–

–

100

85

rod, 25 mm dia

–

78

–

–

62

32

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Table 57 Mechanical (continued) properties of alloy VT22. Properties

Semiproduct

Annealed

Quenched and aged

Temperature, ºC 20

300 350

20

350 400

Crack sensitivity at bending rod, 25 mm dia impact, kgf/cm2

3.0

–

–

2.0

–

–

Critical coefficient of stress rod, 25 mm dia intensity, K1C, kgf/cm3

250–280

–

–

220–260

–

–

Fatigue crack growth rate

0.90–1.20

–

–

1.15–1.50

–

–

rod, 25 mm dia

The tension curves for annealed alloy VT22 at yield strength at various temperatures are given in Fig. 54. 120

100

σ, kgf/mm2

80 60

σ20 0.2

20°C

20 σfc 400 σ0.2 σ0.2

300

σfc300

400

400 σfc

40

20 0

0.2

1.0

1.5

δ, %

Figure 54 Tension curves at yield strength for as-annealed alloy VT22 (sheet, 3 mm) at various temperatures.

Brinell hardness (HB 10/3000/30) in annealed and thermally hardened states varies within the range of 285–363 kgf/mm2 and 341–444 kgf/mm2, respectively. Alloy VT22 is intended for producing forgings, stampings, plates, rods, shapes, and pipes. It can also be used to prepare sheet semiproducts. Large-size and weight forgings and stampings can be manufactured (up to several tons in the annealed state with σB ≥ 1400 MPa). The most widespread and recommended level of strength for alloy VT22 is σB = 1150–1250 MPa. Forging and stamping of the alloy should be done within the temperature range of 1000–750°C. To have a quality micro- and macrostructure, no less than 30–50% deformation should be performed at temperatures below the polymorphic transformation (850–750°C).

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Alloy VT22 is satisfactorily welded by fusion welding (argon arc and submerged) and resistance (seam, spot) welding. Annealing is recommended to increase the plasticity of the welded joint. The alloy is satisfactorily machined. It has a high corrosion resistance under atmospheric conditions and in most aggressive media, as other titanium alloys. The alloy is intended for producing high-load parts and stamped-and-welded constructions operated long-term at temperatures up to 350–400°C and short-term up to 750–800°C. At present, alloy VT22 is widely used in aerospace industry to produce highload components and constructions, including welded items. In the aircraft industry, it is used for landing gear, high-lift device components, engine mount struts, and other constructions. In engine-building, for disks and blades of fans and low-pressure compressors. Metallurgical plants supply semiproducts from VT22 with several levels of mechanical properties, the most important of which are those presented in Table 58. Table 58 Properties and applications of alloy VT22. σB, MPa

Application

δ, %

ψ, %

KCV, MJ/m2

No less than Passenger aircraft industry Engine building Military aircraft industry

1100–1250 1200–1300 1150–130

7 7 7

16 18 16

0.3 0.25 0.25

Alloy VT22I This is largely an analog of alloy VT22, but it differs by a greater ductility in cold deformation. Besides, alloy VT22I is intended for production of precision stampings by the method of isothermal deformation. The method enables hot deformation at temperatures of 700–750°C, which makes possible the use of cheaper materials for press tools. The chemical composition of alloy VT22I (wt. %): Al, 2.5–3.5; Mo, 4.0–5.5; V, 4.0–5.5; Cr, 0.5–2.0; Fe, 0.5–1.5. The content of impurities should not exceed (wt. %): C, 0.10; Si, 0.15; Zr, 0.30; O2, 0.15; N2, 0.05; H2, 0.015. The mechanical properties of rods and stampings from alloy VT22I according to the current technical documentation are given in Table 59. The physical properties of alloy VT22I in a thermally hardened state are as follows. • density: 4.66 g/cm3 • thermal conductivity at various temperatures: Temperature, ºC λ, Cal/cm s deg

20 8.4

100 9.2

200 10.3

300 11.6

400 13.4

500 14.5

600 15.7

700 17.0

800 28.2

900 19.5

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Table 59 Mechanical properties of rolled rods and stampings from VT22I according to technical specifications. Semiproduct

State of tested specimens

Thickness, mm

Mechanical properties σB, MPa

δ, %

ψ, %

KCV, MJ/m2

No less than Rod

annealed

10–60 65–100 110–150

1100–1250 1100–1250 1100–1250

8 7 6

20 18 16

0.3 0.3 0.3

Stamping, rolled plate

annealed

60 160 110 150

1100–1250 1100–1250 1100–1250

8 7 6

20 18 16

0.3 0.3 0.3

• heat capacity vs temperature: Temperature, ºC C, Cal/(g deg)

100 200 300 400 500 600 700 800 900 0.545 0.583 0.627 0.668 0.700 0.750 0.835 0.874 0.920

• linear expansion coefficient within the temperature range: Temperature, ºC 20–100 100–200 200–300 300–400 400–500 500–600 600–700 700–800 7.8 8.2 8.6 9.1 9.6 10.0 10.5 14.0 α×106, deg–1

• specific electrical resistance of alloy VT22I in a thermally hardened state at 20°C is 152·106 Ω cm. • normal spectral emissivity of polished vacuum-heated alloy VT22I (temperatures, 700 and 1400°C) is, respectively, 0.22 and 0.30. Typical mechanical properties of alloy VT22I in a thermally treated state are given in Fig. 55 and Table 60. Brinell hardness (HB 10/3000/30) in a thermally treated state is within the range of 340–400 kgf/mm2. The alloy is intended for producing sheets, band, thin-walled pipes, and shapes. Rods and forgings from VT22I are often used as fabricating precision forgings by the method of isothermal deformation under conditions of superplasticity. The alloy is satisfactorily welded by all types of welding applicable for titanium. An obligatory heat treatment is required after welding. The alloy is satisfactorily machined. As other titanium alloys, it has a high corrosion resistance under atmospheric conditions, in sea water, and in most aggressive media. Alloy VT22I is intended for producing high-load parts and components (including welded constructions) operated long-term at temperatures up to 300–350°C and short-term up to 500–600°C.

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σ0.2 σB

90 70

δ

50 30 −100

0

150

10000

130

9000

60 8000 σfc

40

20 (a) 0 100 200 300 400 500 Temperature, °C

Estat,

110

11000

kgf/mm2

σB, σ0.2, σfc, kgf/mm2

130

δ5, %

E

170 E

80 9000

σfc

60

90 ψ

70 50 30 −100

11000

100 10000

σB σ0.2

110

12000

40 δ

20 (b) 0 0 100 200 300 400 500 Temperature, °C

Estat, kgf/mm2

150

12000

σB, σ0.2, σfc, kgf/mm2

170

δ, ψ, %

180

Figure 55 Change of the mechanical properties of thermally hardened alloy VT22-I (rod, 60 mm in diameter) at short-term tension vs test temperature.

Table 60 Mechanical properties of alloy VT22I in a thermally treated state (rolled rod, 60 mm in diameter). Properties

Test temperature, ºC 20

0.35 Impact strength, KCV, MJ/m2 Notch sensitivity at static load, kgf/mm2 1.35 Endurance limit based on 107 cycles, KCV, 580 MJ/m2 (no less than) 240–270 Critical coefficient of stress intensity, K1C, kgf/cm3 Fatigue crack growth rate 1.00–1.35 Long-term strength for 100 h, kgf/mm2 (no less – than) Creep limit for 100 h, MJ/m2 (no less than) – Crack sensitivity at bending impact, kgf/cm2 1.5

300

350

400

– – 520

– – 480

– – –

–

–

–

– 920

– 950

– 800

750 –

600 –

300 –

Alloy VT23 This is a martensite-type alloy with Kβ = 0.8. It is based on the Ti–Al–Mo–V–Cr–Fe system and is intended to be used as-annealed and thermally hardened. Is hardenable in cross-section up to 80 mm2. Alloy VT23 has the following chemical composition (wt. %): Al, 4.0–6.3; Mo, 1.5–2.5; V, 4.0–5.0; Cr, 0.8–1.4; Fe, 0.4–0.8. The content of impurities should not exceed (wt. %): C, 0.10; Si, 0.15; Zr, 0.30; O2, 0.15; N2, 0.05; H2, 0.015.

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The mechanical properties of sheets and rods from alloy VT23 according to the current documentation are given in Table 61. Table 61 Mechanical properties of sheets and rolled rods from alloy VT23. Semiproduct

State of tested specimens

Thickness, mm

Mechanical properties σB, MPa

ψ, % KCV, MJ/m2

δ, %

No less than Sheet

annealed

1.0–5.0 5.0–10.5

1100–1250 1050–1250

7 7

– –

– –

Rod

annealed

10–60 65–100 110 –150

1100–1250 1100–1250 1050–1250

10 9 8

25 18 16

0.3 0.3 0.3

The physical properties of alloy VT23 in annealed and thermally hardened states are virtually the same. • density: 4.57 g/cm3 • thermal conductivity of the alloy at various temperatures: Temperature, ºC λ, Cal/cm s deg

25 100 200 300 400 500 600 700 800 900 0.020 0.023 0.027 0.029 0.032 0.034 0.037 0.040 0.045 0.048

• linear expansion coefficient within the temperature range: Temperature, ºC 20–100 20–200 20–300 8.5 8.6 8.7 α×106, deg–1

20–400 8.8

20–500 8.9

20–600 9.0

20–700 9.3

20–800 9.4

• specific electrical resistance at 20°C: 1.27·10 –6 Ω cm. Typical mechanical properties of a rod, 25 mm in diameter, in annealed and thermally hardened states are given in Fig. 56 and Table 62. Brinell hardness (HB 10/3000/30) in the annealed state and quenched and aged state is 270–365 and 300–420 kgf/mm2, respectively. Alloy VT23 is manufactured as sheet semiproducts (sheets, plates), rods, forgings, shapes, pipes, and other semiproducts. It is welded by all kinds of welding applicable for titanium. Hot plastic deformation should be performed by providing no less than 50% of the treatment at → β transformations. This is required to temperatures below the boundary of α+β ← obtain micro- and macrostructure determining the level of mechanical properties, especially in martensitic alloys. Machinability of alloy VT23 and its corrosion resistance are the same as in titanium alloys VT6 and VT14.

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13000 120 σ0.2

60 30

13000 σ0.2

12000

E

δ

20 10 0 0

107

σB

11000 100 30

E

40 20

0

σB, σ0.2, kgf/mm2

Estat, kgf/mm2

σB

δ5 , %

80

δ5, %

σB, σ0.2, kgf/mm2

TITANIUM ALLOYS: RUSSIAN AIRCRAFT AND AEROSPACE APPLICATIONS

9000

80

20

7000

60

10

40

0

(a) 5000 100 200 300 400 500 Temperature, °C

10000

δ

0

Estat, kgf/mm2

01 Chapter 2

8000

(b) 6000 100 200 300 400 500 Temperature, °C

Figure 56 Change of the mechanical properties of titanium alloy VT23 (sheet 2 mm) at shortterm tension vs test temperature: (a) annealed state, (b) quenched and aged state.

Table 62 Mechanical properties of a rod 25 mm in diameter from alloy VT23. Properties

Annealed

Quenched and aged Temperature, ºC

20

300

350

400

20

300

350

400

Impact strength, kgf/cm2 Notch sensitivity at static load, kgf/mm2 Endurance limit based on 107 cycles, kgf/mm2 (no less than) Long-term strength for 100 h, kgf/mm2 (no less than)

4.5 1.3

– –

– –

– –

3.0 1.2

– –

– –

– –

49

46

43

–

50

50

45

–

–

77

74

60

–

103

95

80

Creep limit for 100 h, kgf/mm2 (no less than) Crack sensitivity at bending impact, kgf/cm2

–

72

38

–

–

86

58

30

3.0

–

–

–

2.0

–

–

–

Alloy VT32 This is a medium-alloyed pseudo-β-alloy (Kβ = 1.7–1.8) based on the Ti–Al–Mo–V–Fe–Cr system. It is the second-generation β-alloy that, in contrast with alloys V-120 VSA, VT15, and ST6, is alloyed with β-titanium–isomorphic alloying elements molybdenum and vanadium, and with only a minor amount of eutectoid-forming elements iron and chromium.

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The alloy has the following chemical composition (wt. %): Al, 2.0–4.0; Mo, 7.0–9.0; V, 7.0–9.0; Fe, 0.5–2.0. The content of impurities should not exceed (wt. %): C, 0.15; Si, 0.15; Zr, 0.30; O2,0.15; N2, 0.05; H2, 0.015. The mechanical properties of sheets and rods from alloy VT32 according to the current documentation are given in Table 63. Table 63 Mechanical properties of sheets and rolled rods from alloy VT32 according to technical specifications. Semiproduct

State of tested specimens

Thickness, mm

Mechanical properties δ, %

σB, MPa

ψ, %

KCV, MJ/m2

No less than Sheet

annealed thermally hardened

Rod

annealed thermally hardened

0.5–3.0 3.0–6.0 0.5–3.0 3.0–6.0

850–950 850–950 1200–1350 1200–1350

18 17 6 7

– – – –

– – – –

10–40 42–60 10–40 42–60

850–950 850–950 1200–1350 1200–1350

18 18 6 5

25 20 12 10

0.35 0.40 0.25 0.25

The physical properties of alloy VT32 in a thermally hardened state are as follows. • density, 4.830 g/cm2 • thermal conductivity at various temperatures: Temperature, ºC λ, Cal/cm s deg

25 6.7

100 7.95

200 9.63

300 11.7

400 13.4

500 15.5

600 17.6

400 0.649

500 0.700

600 0.780

• heat capacity at various temperature: Temperature, ºC C, Cal/(g deg)

50 0.542

100 0.552

200 0.577

300 0.610

• linear expansion coefficient within the temperature range: Temperature, ºC 20–100 20–200 20–300 8.1 8.3 8.4 α×106, deg–1

20–400 100–200 200–300 300–400 400–500 8.5 8.4 8.5 8.7 8.8

• specific electrical resistance at various temperatures: Temperature, ºC ρ×106, Ω cm

20 152

500 150

100 150

200 148

300 148

400 148

500 148

600 150

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E

8000

40 20 10

σfc δ

σ0.2

30 20

10 (a) 0 0 100 200 300 400 500 Temperature, °C

7000 Estat, kgf/mm2

σB

80 60

δ11, %

σB, σ0.2, σfc, kgf/mm2

100

δ5, δ11, %

9000

140 σB

120 100 σB, σ0.2, σfc, kgf/mm2

10000

109

E

40 20

25

σfc

80 60

σ0.2

12000 11000 10000

20 9000 15 8000

δ5

10 7000

Estat, kgf/mm2

01 Chapter 2

δ11

5 (b) 10 0 −100 0 100 200 300 400 500 Temperature, °C

Figure 57 Change of the mechanical properties of titanium alloy VT32 (sheet 1–2 mm) in a thermally hardened state at short-term rupture vs test temperature.

Typical mechanical properties of alloy VT32 (sheet, 1–2 mm) in a thermally hardened state at various temperatures are given in Fig. 57 and Table 64. Table 64 Mechanical properties of alloy VT32 (sheet, 1–2 mm) in a thermally hardened state at various temperatures. Properties

Temperature, ºC 20

Notch sensitivity at static load, kgf/mm2 1.20 Crack sensitivity at bending impact, kgf/cm2 1.5 7 2 40 Endurance limit based on 10 cycles, kgf/mm (no less than) Critical coefficient of stress intensity, K1C, kgf/cm3 350 Fatigue crack growth rate 1.00–1.15 Long-term strength for 500 h, kgf/mm2 (no less than) – Creep limit for 100 h, kgf/mm2 (no less than) –

350

400

– – –

– – –

– – 96 72

– – 70 35

Alloy VT32 is intended for use in a thermally hardened state. A feature of the alloy is its capability of hardening at small cooling rates (about 3–4°C/min and higher). Hardening treatment could then be done by stepwise heating and cooling directly in a furnace; argon-vacuum furnaces could be used for this purpose. This enables thermal hardening of large-size nonrigid constructions, including welded units, without oxidation, warpage, and, in practice, without residual stresses. Alloy VT32, as all pseudo-β-titanium alloys, is distinguished by a good ductility, and after a hardening heat treatment it acquires a high strength. The alloy is supplied as semiproducts, as-annealed with σB = 800–950 MPa, σ5 ≥ 18%, and in a thermally hardened state with σB = 1200–1300 MPa and σ5 ≥ 6%.

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Semiproducts produced are sheets, band, foil, thin-walled pipes, shapes, rods, and wire. Plates, forgings, stampings, and other semiproducts are also manufactured. The as-annealed and as-quenched alloy can be intensively cold deformed in sheet stamping, heading, and other operations. Alloy VT32 is well welded by all kinds of welding applicable for titanium alloys. Welded joints have good plasticity both after welding and after annealing. The strength of the joints is 0.85 of that of the base metal. Welded joints can be subjected to thermal hardening treatment in argon vacuum furnaces with satisfactory plasticity of the weld. VT32 is satisfactorily machined, has a high corrosion resistance under atmospheric conditions, sea water at temperatures up to 350°C and most aggressive media. Its corrosion resistance is much higher than that of most commercial titanium alloys.

Alloy VT35 This is a relatively low alloyed (Kβ = 1.5) pseudo-β-titanium alloy based on the Ti–Al–V–Cr–Sn system with minor additions of molybdenum and zirconium. This is a second-generation pseudo-β-alloy distinguished with a minor amount of eutectoid-forming element chromium that stabilizes the β-phase in titanium. The chemical composition of alloy VT35 is as follows (wt. %): Al, 2.0–4.0; V, 14.0–16.0; Cr, 2.0–4.0; Mo, 0.5–2.0; Sn, 2.0–4.0. The content of impurities should not exceed (wt. %): C, 0.10; Si, 0.15; Zr, 0.30; O2, 0.15; N2, 0.05; H2, 0.015. The mechanical properties of sheets and rods from alloy VT35 according to the current documentation are given in Table 65. Table 65 Mechanical properties of sheets and rolled rods from VT35 according to technical specifications. Semiproduct

State of tested specimens

Thickness, mm

Mechanical properties σB, MPa

δ, %

ψ, % KCV, MJ/m2 No less than

Sheet

annealed thermally hardened

Rolled rod

annealed thermally hardened

0.5–3.0 3.0–6.0 0.5–3.0 3.0–6.0

750–850 750–850 1200–1350 1200–1350

17 15 5 6

– – – –

– – – –

10–40 42–60 10–40 42–60

750–850 750–850 1200–1350 1150–1300

18 16 6 6

30 25 12 10

0.4 0.4 0.62 0.2

The physical properties of alloy VT35 in a thermally hardened state are as follows.

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• density: 4.774 g/cm3 • thermal conductivity at various temperatures: Temperature, ºC λ, Cal/cm s deg

20 5.7

100 6.8

200 6.5

300 8.7

400 9.8

500 16.2

600 23.6

700 27.7

800 33.2

• heat capacity at various temperatures: Temperature, ºC C, Cal/(g deg)

20 100 200 300 400 500 600 700 800 0.527 0.563 0.412 0.547 0.538 0.785 1.030 1.110 1.240

• linear expansion coefficient at various temperatures: Temperature, ºC α×106, deg–1

20–100 20–200 20–300 20–400 20–500 300–400 400–500 8.6 9.0 9.3 9.7 9.9 10.8 10.8

• specific electrical resistance at 20°C: 149·106 Ω cm Typical properties of alloy VT35 (sheet, 1–2 mm) in a thermally hardened state at various temperatures are given in Table 66 and Fig. 58. Table 66 Mechanical properties of alloy VT35 (sheet, 1–2 mm) in a thermally hardened state at various temperatures. Properties

Temperature, ºC

Notch sensitivity at static load, kgf/mm2 kgf/cm2

Crack sensitivity at bending impact, Endurance limit based on 107 cycles, kgf/mm2 (no less than) Critical coefficient of stress intensity, K1C, kgf/cm3 Fatigue crack growth rate Long-term strength for 100 h, kgf/mm2 (no less than) Creep limit for 100 h, kgf/mm2 (no less than)

20

350

400

1.15 1.15 37

– – –

– – –

334 1.06–1.19 – –

– – 100 80

– – 80 34

Brinell hardness (HB 10/3000/30) varies within the limits of 355–440 kgf/mm2. Alloy VT35 has a high ductility in an annealed state or in a quenched state, and after aging acquires a high strength. The alloy contains a comparatively large amount of expensive alloying elements, vanadium and tin; it is recommended to be used exclusively in a thermally hardened state. Only then does it have a noticeable advantage over other alloys, due to its high ductility, good hardenability, and the capability of hardening when air cooled.

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11000 10000

σB 120 σfc 100 80 60 −100

σ0.2

δ11 δ5 0

10 5 0

9000 8000 Estat, kgf/mm2

140

δ5, δ11, %

σB, σ0.2, σfc, kgf/mm2

160

100 200 300 400 Temperature, °C

Figure 58 Change of the mechanical properties of titanium alloy VT35 (sheet 1–2 mm) in a thermally hardened state at short-term rupture vs test temperature.

The alloy is intended mainly for sheet semiproducts – sheets, band, foil, and also for thin-walled pipes, shapes, and wire. Forging, stamping, pressing and some other operations are done as hot work. Thin sheets, band, foil, thin-walled pipes, shapes, and wire are produced using intensive cold deformation. Sheet stamping could also be done as cold work. Alloy VT35 is satisfactorily welded by all kinds of welding applicable for titanium. Welded joints have a good plasticity both immediately after welding and after annealing. Hardening heat treatment of welded joints is not recommended, as their plasticity sharply decreases. The alloy is satisfactorily machined, though to a smaller extent than α- or (α+β)-titanium alloys. The alloy has a high corrosion resistance under atmospheric conditions, sea water, and most aggressive media. By corrosion resistance, it has some advantages over α- and (α+β)-alloys. Alloy VT35 is recommended for producing stamped-and-welded constructions operated long-term at temperatures up to 350°C and short-term (several hours) up to 500–650°C. Alloy VT37 This alloy should be considered as the development of transition alloys, of the type of VT22. The alloy has the chemical composition similar to that of VT22, but is additionally alloyed with tin and zirconium. This significantly increases the mechanical and high-temperature strength of α- and β-solid solutions in an annealed state. The effect of dispersion hardening is preserved by quenching and aging. The characteristics of crack resistance, sensitivity to stress concentrators, cyclic strength

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and other features that determine the service life and reliability are retained at a sufficiently high level. Alloy VT37 has the following chemical composition (wt. %): Al, 4.4–5.9; Mo, 4.0–5.5; V, 4.0–5.5; Cr, 0.5–2.0; Fe, 0.5–2.0; Zr, 2.5–3.5; Sn, 2.0–3.0. The content of impurities should not exceed (wt. %): C, 0.10; Si, 0.15; O2, 0.15; N2, 0.05; H2, 0.015; the sum of the other impurities, 0.30. The mechanical properties of rolled rods and forgings from alloy VT37 according to the current technical documentation are given in Table 67. Table 67 Mechanical properties of rolled rods and forgings from alloy VT37. Semiproduct

State of tested specimens

Thickness, mm

Mechanical properties δ, %

σB, MPa

ψ, % KCV, MJ/m2 No less than

Rod

thermally hardened

10–60 65–100 105–200

1200–1350 1200–1350 1200–1350

7 6 6

18 16 14

0.2 0.2 0.25

Forging

thermally hardened

10–60 65–150 155 –250

1200–1350 1200–1350 1200–1350

7 7 6

18 16 16

0.2 0.2 0.2

The physical properties of alloy VT37 in a thermally hardened state are given below. • density: 4.79 g/cm3 • thermal conductivity at various temperatures: Temperature, ºC λ, Cal/cm s deg

100 7.95

200 9.21

300 10.9

400 12.1

500 13.8

600 15.5

700 16.7

800 17.6

• heat capacity vs temperature: Temperature, ºC C, Cal/(g deg)

100 200 300 400 500 600 700 800 0.592 0.624 0.650 0.672 0.690 0.736 0.828 0.956

• linear expansion coefficient at various temperatures: Temperature, ºC 20–100 20–200 20–300 20–400 20–500 100–200 200–300 300–400 400–500 α×106 9.0 9.3 9.5 9.8 10.1 9.5 10.0 10.6 11.2

• specific electrical resistance at various temperatures: Temperature, ºC ρ×106, Ω cm

–196 164

–70 166

20 167

100 168.5

200 170

300 171.5

400 173

500 174.5

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11000 10000

σB 120 σfc 100

δ11

80 60 −100

σ0.2

8000

10 5

δ5 0

9000

0

Estat, kgf/mm2

140

δ5, δ11, %

σB, σ0.2, σfc, kgf/mm2

160

100 200 300 400 Temperature, °C

Figure 59 Change of the mechanical properties of titanium alloy VT37 (rod, 60 mm in diameter) in a thermally hardened state at short-term rupture vs test temperature.

Typical mechanical properties of alloy VT37 are given in Fig. 59 and Table 68. Alloy VT37 can be operated long-term at temperatures of 350°C, preserving high characteristics of temperature resistance. The alloy is intended mainly for largesize disks and blades of fans and compressors, and also for high-load parts and units in aerospace and other mechanical engineering industries. A feature of alloy VT37 is a possibility of its hardening by air quenching or by the regime of “self-quenching” in cooling of semiproducts or components in argon vacuum furnaces at a rate of no less than 4–5°C/min. This approach makes it possible to avoid oxidation, residual stresses, and warpage of parts and units. Table 68 Mechanical properties of alloy VT37 in a thermally hardened state. Properties

Impact strength, kgf/cm2 Notch sensitivity at static load, kgf/mm2 Crack sensitivity at bending impact, kgf/cm2 Critical coefficient of stress intensity, K1C, kgf/cm3 Endurance limit based on 107 cycles, kgf/mm2 (no less than), K = 1.0 K = 2.6 Long-term strength for 100 h, kgf/mm2 (no less than) Creep limit for 100 h, kgf/mm2 (no less than)

Semiproduct

Temperature, ºC 20

300

350

400

rod, 25–60 dia rod, 25–60 dia rod, 25–60 dia

3.0 1.3 1.0

– – –

– – –

– – –

rod, 25–60 dia

180

–

–

–

rod, 25–60 dia

62–65

–

–

–

rod, 25–60 dia rod, 25–60 dia

38–40 –

– 105

– 100

– 90

rod, 25–60 dia

–

90

63

–

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By its characteristics of mechanical and high-temperature strength in large-size semiproducts and components at temperatures up to 350°C, alloy VT37 exceeds the existing Russian and Western titanium alloys. It should be noted that alloy VT37, as are all high-strength titanium alloys, is rather sensitive to the quality of the structure formed in hot deformation. Taking this into account, it is recommended to perform final hot deformation at temperatures not higher than 900°C and deformation no less than 30–40% to obtain the quality macroand microstructure. The alloy is satisfactorily welded by all kinds of welding applicable for titanium, and is satisfactorily machined. It has a high corrosion resistance under atmospheric conditions, in sea water, and in most aggressive media. At present, commercial tests of titanium alloy VT37, mainly in the aircraft industry, have been completed. The mechanical properties of thermally hardened semiproducts up to 200–250 mm in cross-section are as follows: σB = 1250–1350 MPa, δ = 16%, a = 3.0 kgf/cm3. High-strength titanium alloys used both as-annealed and thermally hardened can be successfully operated long-term in constructions at temperatures up to 400°C. An exception could be alloys VT15 and TS6, which contain a large amount of the eutectoid-forming element chromium and are liable to embrittlement at long-term heating. Their working temperature threshold is close to 200–250°C. Applications of high-strength thermally hardened titanium alloys are at present rather limited. They are mainly used for separate pieces and small constructions. This is due to the difficulties with hardening heat treatment of large-size, nonrigid and even thick-walled constructions. The problem is referred to as warpage during quenching and aging, limited hardenability, oxidability, etc. A significant obstacle for widespread use of thermally hardened titanium alloys is insufficient plasticity of welded joints of some alloys. Nevertheless, it could be said without exaggeration that high-strength thermally hardened titanium alloys are materials for the future, because even today it is possible to achieve ultimate strengths of 1800–2000 MPa at a specific weight 1.7 times lower than in steels. All high-strength titanium alloys considered above are two-phase (α+β)-alloys covering alloys based both on α- and β-structure. The content of the β-stabilizing element varies in these alloys within rather wide limits (from 4 up to 18%), which makes it possible to obtain various physicomechanical and process properties. The high strength of this group of alloys is achieved not only by alloying α- and β-solid solutions, but also by dispersion strengthening in quenching and aging. In annealed and quenched states, the process properties of these alloys are on the level of medium-strength alloys, and sometimes, of highly ductile alloys. In a thermally hardened state (postquenching and aging) the alloys are not to be heated at temperatures over 550–650°C. Owing to clearly expressed heterophase character, the welding and thermal stability problems of these alloys are more problematic than in the first two groups. Still, they are successfully solved by choosing corresponding heat treatment regimes. The basic feature of high-strength titanium alloys as construction material is their increased sensitivity to stress concentrators. Therefore, when designing pieces or particular units from these alloys, it is necessary to take into account some

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restrictions recommended for these alloys (higher requirements to the quality of surface, increased radius of transition from some to other cross-sections, etc.), similar to those for high-strength steels.

References 1. Moiseyev, V.N. (2002) Half a century of Russian titanium. Natsionaln. Metallurgiya 3: 25–29 (in Russian). 2. Moiseyev, V.N. (2002) Role of VIAM in the development of the first Soviet aircraft from titanium. In Aviation Materials. Moscow: MISIS–VIAM, pp. 100–111 (in Russian). 3. Moiseyev, V.N. (1958) New titanium alloys for aircraft industry. In Production and Processing of Titanium and its Alloys. Moscow: Central Research Institute of Nonferrous Metallurgy, pp. 13–20 (in Russian). 4. Moiseyev, V.N. and Luzhnikov, L.P. (1960) Sheet titanium alloys. In Advanced Scientific & Technical Experience and Production. Moscow: VIAM, issue 8, theme 15, pp. 1–19 (in Russian). 5. Luzhnikov, L.P. and Moiseyev, V.N. (1961) Alloys of the Ti–Al–Mn system. Metalloved. Term. Obrabotka Metallov 7: 29–36 (in Russian). 6. Glazunov, S.G., Moiseyev, V.N., and Danilov, Yu.S. (1965) Deformed welded titanium alloys. Deformed mid-strength titanium alloys. Structural materials. Moscow: Sov. Entsyklopediya, pp. 330–335 (in Russian). 7. Moiseyev, V.N. (2001) Low-strength high-ductility titanium alloys. Mid-strength alloys. High-strength alloys. In Nonferrous Metals and Alloys (Encyclopedia). Moscow: Mashinostroenie, Vol. II-3, pp. 281–312 (in Russian). 8. Moiseyev, V.N. and Shokholova, L.V. (1969) Thermal stability of structural titanium alloys OT4 and OT4-1 in prolonged operation at elevated temperatures. In Titanium Alloys and Their Use. Moscow: Department of Scientific and Technical Information, All-Russian Institute of Aircraft Materials, p. 4 (in Russian). 9. Moiseyev, V.N. and Sholokhova, L.V. (1972) Comparison of the effect of iron, chromium, manganese, molybdenum, and vanadium on the properties of low-alloyed titanium alloys. Moscow: Department of Scientific and Technical Information, All-Russian Institute of Aircraft Materials, pp. 160–168 (in Russian). 10. Moiseyev, V.N. and Sholokhova, L.V. (1967) Strengthening heat treatment of alloy OT4. Aviatsion. Promyshlennost 3: 51–57 (in Russian). 11. Moiseyev, V.N., Sholokhova, L.V., and Terent’eva, L. N. (1969) Heat treatment of titanium alloy OT4. In Production of Titanium Alloys. Moscow: All-Russian Institute of Light Alloys, No 5, pp. 152–155 (in Russian). 12. Moiseyev, V.N. and Sholokhova, L.V. (1967) Effect of structure on the properties of alloy OT4. In Applications of Titanium Alloys, Part 1. Moscow: Department of Scientific and Technical Information, All-Russian Institute of Aircraft Materials, pp. 58–62 (in Russian). 13. Moiseyev, V.N. and Sholokhova, L.V. (1972) Comparison of the effect of iron, chromium, manganese, molybdenum, and vanadium on the properties of low-alloyed titanium alloys. Moscow: Department of Scientific and Technical Information, All-Russian Institute of Aircraft Materials, pp. 160–168 (in Russian). 14. Moiseyev, V.N. and Terent’eva, L.N. (1972) Substitution of molybdenum and vanadium for manganese in OT4-type alloys. In Titanium Alloys. Moscow: Department of Scientific and Technical Information, All-Russian Institute of Aircraft Materials, pp. 168–174 (in Russian). 15. Moiseyev, V.N., Glazunov, S.G., and Sholokhova, L.V. (1974) Substitution of vanadium for manganese in OT4-type alloys. Aviatsion. Promyshlennost 6: 65–67 (in Russian).

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16. Prokhodtseva, L.V. et al. (1974) Anisotropy of the mechanical properties of sheets from titanium–aluminum–manganese alloys. Problemy Prochnosti 5: 81 (in Russian). 17. Moiseyev, V.N. et al. (1977) Substitution of vanadium and iron for manganese in OT4-type alloys. In Titanium Alloys. Moscow: Department of Scientific and Technical Information, All-Russian Institute of Aircraft Materials, pp. 65–72 (in Russian). 18. Moiseyev, V.N. and Sholokhova, L.V. (1983) Structural titanium pseudo-α-alloys. In Aviation Materials. Heat Treatment of Titanium Alloys. Moscow: Department of Scientific and Technical Information, All-Russian Institute of Aircraft Materials, pp. 93–100 (in Russian). 19. Moiseyev, V.N. and Luzhnikov, V.P. (1960) Sheet titanium alloys. Advanced scientific and technical and production experience. Use of plastics and new materials in mechanical engineering. Moscow: VIAM, issue 8, theme 15, pp. 1–19 (in Russian). 20. Moiseyev, V.N. (1960) New heat-resistant titanium alloy OT4-2. Aviatsion. Promyshlennost 12: 51–53 (in Russian). 21. Luzhnikov, L.P. and Moiseyev, V.N. (1961) Alloys of the Ti–Al–Mn system. Metalloved. Term. Obrabotka Metallov 7: 29–36 (in Russian). 22. Moiseyev, V.N. and Chinenov, A.M. (1965) Thermally strengthened welded titanium alloys little sensitive to overheating. Metalloved. Term. Obrabotka Metallov 5: 43–45 (in Russian). 23. Moiseyev, V.N. (1961) Thermally strengthened titanium alloy VT14. Aviatsion. Promyshlennost 2: 54–58 (in Russian). 24. Glazunov, S.G. and Moiseyev, V.N. (1961) Heat treatment, structure and properties of alloy VY14. Titanium in industry. Moscow: Oborongiz, pp. 232–244 (in Russian). 25. Moiseyev, V.N., Glazunov, S.G., and Khorev, A.I. (1961) Improvement of the composition and properties of thermally strengthened titanium alloy VT14. Advanced scientific and technical and production experience, new materials and alloys for mechanical engineering industry. Moscow, TsITEI, issue 3, pp. 4–15 (in Russian). 26. Glazunov, S.G., Moiseyev, V.N., and Mikhailov, B.M. (1964) Non-alloyed titanium-clad alloy VT15. In Applications of Titanium Alloys. Moscow: Department of Scientific and Technical Information, All-Russian Institute of Aircraft Materials, pp. 62–69 (in Russian). 27. Glazunov, S.G. and Moiseyev, V.N. (1965) Deformed high-strength titanium alloys. Structural materials. Moscow: Sov. Entsiklopediya, p. 328 (in Russian). 28. Moiseyev, V.N. (1965) High-strength titanium alloy VT16. Aviatsion. Promyshlennost 9: 63–66 (in Russian). 29. Dolzhansky, Yu.M., Moiseyev, V.N., et al. (1972) Studies of the statistical regularities of the effect of alloying elements on the mechanical properties of Ti–Al–Mo–V alloys (of VT16 type). Izv. Vuzov, Tsvetn. Metallurgiya 4: 132–137 (in Russian). 30. Moiseyev, V.N. (1967) High-strength titanium alloy VT22. Aviatsion. Promyshlennost 3: 7–10 (in Russian). 31. Moiseyev, V.N. (1968) New commercial high-strength titanium alloys. Advanced scientific and technical and production experience. Moscow: State Institute of Sci. & Techn. Information, pp. 1–29 (in Russian). 32. Glazunov, S.G. et al. (1975) Studies of the statistical regularities of the effect of alloying on the mechanical properties of critical-composition alloys. In Production of Titanium Alloys. Moscow: All-Russian Institute of Light Alloys, No 7, pp. 58–64 (in Russian). 33. Moiseyev, V.N., Znamenskaya, E.V., and Tarasenko, G.N. (1975) Assessment of the serviceability of titanium alloys with ultimate strength of over 150 kg/mm2. In Production of Titanium Alloys. Moscow: All-Russian Institute of Light Alloys, pp. 64–72 (in Russian). 34. Kasparova, O.V. et al. (1975) Studies of the possibility for increasing the strength of annealed alloy VT22 by alloying. In Production of Titanium Alloys. Moscow: All-Russian Institute of Light Alloys, pp. 72–77 (in Russian). 35. Matveyenko, A.F. et al. (1977) Effect of plastic deformation and heat treatment on the properties of sheets of alloy VT15. In Titanium Alloys. Moscow: Department of

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Scientific and Technical Information, All-Russian Institute of Aircraft Materials, pp. 181–189 (in Russian). 36. Moiseyev, V.N. (1983) High-strength titanium alloys and prospects of their development. Aviation materials. Heat treatment of titanium alloys. Moscow: Department of Scientific and Technical Information, All-Russian Institute of Aircraft Materials, pp. 101–112 (in Russian). 37. Moiseyev, V.N. and Sysoyeva, N.V. (1995) High-strength titanium alloy produced by granular metallurgy. Powder alloys for aircraft industry (abstracts of papers). Moscow: All-Russian Institute of Light Alloys, pp. 68, 69 (in Russian). 38. Moiseyev, V.N. and Sysoyeva, N.V. (1988) Effect of intermetallide strengthening on the mechanical properties of alloy VT22 produced by rapid crystallization. Powder alloys for aircraft industry (abstracts of papers). Moscow: All-Russian Institute of Light Alloys, pp. 13–15 (in Russian). 39. Antipov, A.I., Moiseyev, V.N., and Moder, N.I. (1997) Strengthening of titanium alloy VT35 in aging. Metalloved. Term. Obrabotka Metallov 12: 2–5 (in Russian). 40. Moiseyev, V.N. (2001) High-strength titanium alloy VT16 for manufacturing fixing items by cold deformation. Metalloved. Term. Obrabotka Metallov 2: 28–32 (in Russian). 41. Moiseyev, V.N. (2002) High-strength titanium alloys for aerospace industry. Moscow: MISIS–VIAM, pp. 115–121 (in Russian).

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3

High-Temperature Titanium Alloys

3.1 GENERAL CHARACTERISTICS This group includes alloys representing α- and (α+β)-solid solutions heterogenized to this or that extent by chemical compounds, which are an important factor of increasing their high-temperature strength. All commercial Russian and Western high-temperature titanium alloys used in mechanical engineering satisfy this definition. The solid-solution matrix is heterogenized using silicon, carbon, boron, aluminum, and other elements in amounts slightly exceeding the limit of solubility in solid solution. The characteristics of the high-temperature strength of titanium alloys are also seriously affected by the chemical composition of the solid-solution matrix. Therefore, alloying of high-temperature alloys is preferentially done by elements increasing their recrystallization temperature, by aluminum; to a smaller extent, by tin and zirconium; theoretically, by gallium and some other elements. The effect of high-temperature strength increase can be also obtained by alloying solid solutions (matrix) with such elements as molybdenum, tungsten, niobium, chromium, iron, and others. Commercial high-temperature titanium alloys based on solid solutions and with an intermetallide type of strengthening are efficiently used at temperatures not higher than 550–600°C. At higher temperatures, their characteristics of long-term hightemperature strength and creep are significantly decreased, and they become noncompetitive with other high-temperature alloys. Of great interest in recent years have been titanium alloys representing chemical compounds of titanium with aluminum of the type of α2-phase (Ti3Al) and γ-phase (TiAl). These alloys have satisfactory high-temperature strength up to 750–800°C, but are distinguished by a rather low ductility at room temperature. This does not enable their use as industrial alloys at present. Another group of high-temperature alloys to be operated at temperatures up to 750–800°C is also considered. These are β-titanium alloys with the intermetallide type of hardening. These alloys have a satisfactory ductility but a higher specific weight as compared with alloys based on titanium aluminides. Alloys based on titanium aluminides and also β-alloys with the intermetallide type of hardening will be discussed in Chapter 4.

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This chapter considers titanium alloys based on α- and β-solid solutions with the dispersion intermetallide type of hardening, which in fact represent the entire range of Russian and Western high-temperature titanium alloys used commercially. Most high-temperature titanium alloys are used only as-annealed. Alloys containing an increased amount of β-phase (more than 0.3–0.4 Kβ) can be subjected to a hardening heat treatment and be used in quenched and aged state. Thus, high-temperature alloys can be divided, as construction alloys, into two groups by the ratio of α- and β-solid solutions: pseudo-α-alloys with Kβ < 0.25 and martensite-type alloys with Kβ = 0.3–0.9. Commercial Russian high-temperature titanium alloys are presented in Tables 69 and 70. Table 69 Chemical composition and classification of high-temperature commercial titanium alloys by the type of solid solution. Groups

Kβ

Pseudo-α-alloys

0.25

Grade

Mean chemical composition, wt. %

VT18U 6.8Al–2.5Sn–4.0Zr–0.7Mo–1.0Nb VT36 6.3Al–0.7Mo–3.6Zr–2.2Sn–0.15Si–5.0W

Martensite-type alloys 0.3–0.9 VT3-1 VT8 VT9 VT8-1 VT25U

6.8Al–2.5Mo–0.28Si–1.4Cu 6.4Al–3.3Mo–0.30Si–0.30Fe 6.4Al–3.3Mo–1.5Zr–0.28Si 6.3Al–3.3Mo–1.0Sn–1.0Zr–0.18Si 6.5Al–4.0Mo–1.8Sn–3.8Zr–0.18Si–1.0W

Table 70 Typical mechanical properties of disks made from high-temperature titanium alloys and their operational conditions. Grade

VT18U VT36 VT3-1 VT8 VT9

Operational conditions t, ºC

h

550–600 550–600 400–450

500 500 6000 3000 6000 500

500 500

VT8-1

450–500

VT25U

500–550

6000 3000 6000 3000

Mechanical properties σB, MPa δ, % ψ, %

KCV, J/m2

400

σ 100

500

σ 100

600

σ 100

MPa 910 1030 960

7 5 10

15 7 25

22.5 14.5 29.5

706 755 675

510 665 540

372 327 –

960 1040

10 8

25 22

34 30.0

568 795

440 588

980

10

25

34.0

750

480

1080

6

15

27.5

862

686

– 550 392 550 245 550 450

The recommended life of an alloy increases with a decrease of temperature. The operational temperature of an alloy is mainly limited by the characteristics of high-temperature strength and metal surface oxidation, and the life of an alloy

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depends on its thermal stability, i.e., the ability to preserve the properties within the admissible operational limits. Pilot productions use alloys VT8M, VT8M-1, VT25, and VT18, which are discussed in technical literature. These alloys are varieties of commercial alloys given in Tables 69 and 70. Alloy VT8M is an analog of alloy VT8 and differs by a lower aluminum content and a higher content of molybdenum, which ensures a greater processibility. The alloy can be cold deformed in manufacturing engine blades by cold forging. Alloy VT8M-1 is an analog of alloy VT8-1. It contains less aluminum and more molybdenum and is also intended only for production of engine blades. Alloy VT25, unlike alloy VT25U, contains less molybdenum and is not subjected to hardening heat treatment. The use of this alloy is limited. Alloy VT18, due to the high content of zirconium, is not commercially used at present.

3.2 MARTENSITE-TYPE HIGH-TEMPERATURE ALLOYS These are the most widespread commercial high-temperature titanium alloys. They are used both as-annealed and thermally hardened owing to the increased content of β-phase. However, the effect of increasing the high-temperature strength by thermal hardening due to dispersion of α- and β-solid solutions is valid at temperatures not higher than 450–500°C. Alloy VT3-1 Deformable high-temperature titanium alloy VT3-1 is based on the Ti–Al– Mo–Cr–Fe–Si system. It belongs to martensite-type two-phase (α+β)-alloys with Kβ = 0.8 and can be hardened by quenching and aging. Alloy VT3-1 was one of the first widespread high-temperature titanium alloys used in aircraft engine manufacturing. The chemical composition of alloy VT3-1 is as follows (%): Al, 5.5–7.0; Mo, 2.0–3.0; Cr, 0.8–2.0; Si, 0.15–0.40; Fe, 0.2–0.7. Impurities (no more than): C, 0.10; O2, 015; N2, 0.05; H2, 0.015. The alloy is manufactured as forgings, stampings, rolled and pressed shapes, rods, and other semiproducts. The mechanical properties of forgings, stampings, and rods are given in Table 71. Depending on the purpose and required properties, the semiproducts can be heat treated in the following regimes: • Isothermal annealing, which includes heating at 870°C, holding for 1 h, cooling down to 650°C, holding for 2 h, and then air cooling. This is the basic regime used for pieces operated long-term at elevated temperatures. The strength for stamped blades and rods of up to 100 mm dia is 1000–1200 MPa; and for discs, rings, and rods of over 100 mm dia, no less than 950 MPa.

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Table 71 Mechanical properties of as-annealed alloy VT3-1 according to technical specifications. Semiproduct

Thickness, mm

Mechanical properties σB, MPa

δ, %

ψ, % KCV, MJ/m2 No less than

Forgings, stampings Rolled rods

Up to 100 101–250

1000–1200 10.0 950–1200 8.0

25.0 20.0

0.3 0.3

Up to 60 65–100 110 –150

1000–1250 1060 1000–1250 10.0 950–1200 8.0

30.0 25.0 20.0

0.3 0.3 0.3

• Duplex annealing, which includes heating at 880°C, holding for 1 h, air cooling, then heating at 550°C, holding for 2–5 h, and air cooling. This regime makes it possible to increase the strength properties of the alloy at 20°C by 80–100 MPa as compared with isothermal annealing, at an insignificant decrease of ductility. In essence, this is a hardening heat treatment with annealing in air. • Hardening heat treatment (water quenching and aging) includes heating at 858°C, holding for 1 h, water hardening followed by aging at 500°C for 4–6 h, and air cooling. Hardening and aging are recommended to be done for semiproducts or components of no more than 80 mm in cross-section. In some cases, semiproducts from alloy VT3-1 are made using a hightemperature thermomechanical treatment (HTMT), which consists of water quenching after a high-temperature deformation followed by aging at temperatures of 550–600°C for 5 h and air cooling. HTMT, as hardening heat treatment by water quenching and aging, makes it possible to increase the strength of small-cross section semiproducts to a guaranteed ultimate strength of 1200–1250 MPa. Alloy VT3-1 is well deformed by forging and stamping within the temperature range of 1050–850°C. Herewith, 40–50% of the deformation at the last stage of treatment should be done at temperatures lower than that of α↔β transformation (970°C). This alloy satisfactorily welds by all welding methods used for titanium. To restore the plasticity of the welded joint in argon arc welding, it should be obligatorily annealed to stabilize the structure. The ultimate strength of the welded joint is no less than 0.9 of that of the base metal. VT3-1 has a high corrosion resistance under atmospheric conditions, seawater, and in most aggressive media. It is satisfactorily machined, both in annealed and thermally hardened states. The alloy is recommended mainly for fabricating disks, blades, rings, and other aircraft-engine components operated at temperatures up to 400–450°C. It can also be used for other applications, including welded constructions. An obligatory heat treatment is required after welding.

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The physical properties of the alloy in annealed and thermally hardened states are the same. The density of the alloy at 20°C is 4500 kgf/m3. The temperature of → β transformation is 920±20°C. α+β ← The linear expansion coefficient within the temperature range is as follows: Temperature, ºC α×106, deg–1

20–100 100–200 200–300 300–400 9.2 9.5 9.9 10.2

400–500 10.7

500–600 11.2

Thermal conductivity as a function of temperature: Temperature, ºC λ, W(m deg)

25 8.0

100 8.8

200 10.1

300 11.3

400 12.6

500 14.2

600 15.5

Heat capacity as a function of temperature is as follows: Temperature, ºC C, Cal/(g deg)

100 0.461

200 0.503

300 0.545

400 0.608

500 0.670

600 0.712

Specific electrical resistance at 20°C: 136·10 –6 Ω cm. The mechanical properties of alloy VT3-1 at various temperatures are determined on rods (20 mm in diameter). The tension curves of as-annealed specimens at yield strength are given in Fig. 60. The change of mechanical properties as a function of instantaneous rupture temperature in as-annealed and thermally hardened specimens from alloy VT3-1 are shown in Fig. 61. Figure 62 presents the change of long-term strength of annealed rods from VT3-1 at temperatures of 20, 300, 400, and 450°C as a function of test temperature and time. At room temperature, the rods had the ultimate strength of 1060 MPa. The creep characteristics of annealed specimens from titanium alloy VT3-1 are given in Table 72. As it follows from the data, an increase of temperature from 400 up to 450°C is accompanied by an almost double decrease of creep resistance. A change of endurance of smooth and notched (r = 0.75 mm) specimens from alloy VT3-1 in annealed and thermally hardened states is given in Fig. 63. The other typical properties of annealed rods from VT3-1: • impact strength (KCV), 0.4 MJ/m2 • notch sensitivity at static load, 1.5 • endurance limit (2·107 cycles) at 20°C, 520 MPa at 400°C, 480 MPa • shearing modulus, 46,000 MPa • Poisson ratio, 0.3

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100 20°C 100

σ, kgf/mm2

80

200 300 400 450 500

60

40

600

20

0

0.2

0.4

0.6

0.8 1.0 δ, %

Figure 60 Tension curves at yield strength for titanium alloy VT3-1 (rod, 20 mm in diameter) at room and elevated temperatures.

40

Edyn

Edyn

(a)

11000 Estat

120

σB

100

60

20 20

σ0.2

σB

80

σfc

σ0.2

σfc

(b)

ψ

ψ δ

9000 7000 E, kgf/mm2

σB, σ0.2, σfc, kgf/mm2

140

δ, ψ, %

160

δ

100 200 300 400 500 600

20

100 200 300 400 500 600

Temperature, °C

Figure 61 Change of the mechanical properties of titanium alloy VT3-1 (rod, 20 mm in diameter) at short-term rupture vs test temperature: (a) after annealing, (b) after quenching and aging.

Alloy VT3-1 is one of the most widespread alloys used in engine building. In some cases, it is also used as a structural material in various fields of mechanical engineering.

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108 300°C

σ, kgf/mm2

100

400°C

92

300°C

84

400°C

76 68 60

450°C

52 44 102

103

104

Time, h

Figure 62 Long-term strength of annealed alloy VT3-1 (rod, 12 mm in diameter) vs test temperature and time.

Table 72 Creep tests of annealed alloy VT3-1. Temperature, ºC

Stress, MPa

Time, h

Residual deformation, %

300

700 650 600 600 620 550

200 200 500 2000 2000 2000

0.30 0.20 0.06 0.07 0.15 0.04

400

500 450 450 430 430 420 370

100 100 500 100 500 500 2000

0.16 0.08 0.22 0.10 0.22 0.18 0.23

450

280 250 230 230 200 200 230 200

100 500 100 500 100 500 2000 2000

0.18 0.30 0.12 0.18 0.12 0.18 0.40 0.19

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σ−1, kgf/mm2

σ−1, kgf/mm2

VALENTIN N. MOISEYEV

50

68 Smooth specimens

64

2

46

1

60

1

42 38

56 2

52

Notched specimens 34

Notched specimens

48

30

44

1 26

40 (a)

Smooth specimens

54

105

106

107

N

1 2

2 22 (b)

105

106

107

N

Figure 63 Endurance of smooth (a) and notched (rn = 0.75 mm) (b) specimens from alloy VT3-1 (rod, 20 mm in diameter) vs number of load cycles: (1) a thermally hardened state, (2) annealed state.

Alloy VT8 Deformable titanium alloy VT8 is based on the Ti–Al–Mn–Si system. It belongs to two-phase martensite-type (α+β)-alloys and its Kβ = 0.3. The alloy is used mainly as-annealed, but can be subjected to hardening heat treatment (quenching and aging) in small cross-sections of up to 40 mm. The chemical composition of titanium alloy VT8 is as follows (in %): Al, 5.8–7.0; Mo, 2.8–3.8; Si, 0.20–0.40. Impurities (in %, no more than): C, 0.10; Fe, 0.30; O2, 0.15; N2, 0.05; H2, 0.015. The alloy is produced as forgings, stampings, shapes, rolled and welded rings, and other semiproducts. The mechanical properties of forgings, stampings, and rods according to the current technical documentation are given in Table 73. Semiproducts and constructions from alloy VT8 are subjected to heat treatment according to the following regimes: • annealing which includes heating at 920°C, holding for 1–4 h, air cooling, then heating at 590°C, holding for 1–2 h, air cooling. This is the most widespread heat treatment regime used for pieces operated long-term at temperatures up to 500°C, • thermal hardening, which consists of heating up to a temperature of 920°C, water quenching followed by aging at 550°C for 1–6 h and air cooling, • thermal hardening, which includes water quenching after high-temperature deformation within the temperature range of 800–950°C followed by aging at 570°C for 2 h and air cooling. Hardening heat treatments of alloy VT8 are efficient only for pieces and constructions operated at temperatures up to 450°C. The physical properties of alloy VT8:

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Table 73 Mechanical properties of as-annealed alloy VT8 according to the current technical specifications. Semiproduct

Thickness, mm

Mechanical properties σB, MPa

δ, %

ψ, %

KCV, MJ/m2

No less than Forgings, stampings

Up to 100 101–150 151–250

1000–1250 950–1200 950–1200

9 7 6

25 16 16

0.3 0.3 0.3

Rolled rods

Up to 60 65–100 115 –150

1000–1250 1000–1250 950–1200

9 9 7

30 25 19

0.3 0.3 0.3

• density at 20°C: 4480 g/cm3 → β transformation temperature: 1000±20°C • α+β ← • linear expansion coefficient for as-annealed alloy as a function of temperature: Temperature, ºC α×106, deg–1

20–100 20–200 8.3 8.6

20–300 8.7

20–400 8.8

20–500 9.1

20–600 9.5

Thermal conductivity of the annealed alloy as a function of temperature: Temperature, ºC λ, W(m deg)

25 7.1

100 8.4

200 9.6

300 11.3

400 12.6

500 14.2

600 15.5

Specific electrical resistance after annealing, at temperature of 20°C: 161·10 –6 Ω cm. As all titanium alloys, VT8 is nonmagnetic. The mechanical properties of annealed rods from alloy VT8 at elevated temperatures are given in Fig. 64. The tension curves of as-annealed specimens at yield strength are shown in Fig. 65. Long-term strength of the annealed alloy is given in Fig. 66. The values of long-term strength and creep limit for annealed and thermally hardened alloy VT8 are given in Table 74. The tests were performed on specimens of 11 mm in diameter (the specimens were made from a forged rod 22 mm in diameter). As it follows from the data of Table 74, the high-temperature strength of alloy VT8 sharply decreases within the temperature range of 500–550°C. At 500°C, the value of creep limit significantly decreases with the time of the test. The endurance limit of alloy VT8 was determined after annealing on smooth and notched (r = 0.75 mm) specimens. The tests were carried out at temperatures of 20 and 500°C at a symmetrical load based on 1·105 to 2·108 cycles. The fatigue strength curves are shown in Fig. 67.

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Edyn

12000

Estat

10000

σB 100

σ0.2

80

σfc

60

ψ

8000 E, kgf/mm2

δ, ψ, %

σB, σ0.2, σfc, kgf/mm2

VALENTIN N. MOISEYEV

40 20

δ

20

300 400 500 600 700 800 Temperature, °C

Figure 64 Change of the mechanical properties of alloy VT8 (rod, 20 mm in diameter) at short-term rupture vs test temperature.

100

20°C

80

σ, kgf/mm2

400 60

500 550 600

40

20

0

0.2 0.4

0.6

0.8 δ, %

Figure 65 Tension curves at yield strength for annealed alloy VT8 (rod, 20 mm in diameter) at room and elevated temperatures.

The mechanical properties of alloy VT8 were determined at low temperatures of –70 and –196°C on annealed rods 25 mm in diameter. The results obtained are presented in Table 75. The other typical mechanical properties of annealed rods from alloy VT8: • Poisson ratio: 0.35 • shearing modulus: 44,000 MPa • notch sensitivity (notch angle, 60 deg; radius, 0.1 mm): 1.35

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σ, kgf/mm2

120

129

20°C

100

100

80 450

60

500°C

40

102

103 Time, h

104

105

Figure 66 Long-term strength of annealed specimens from alloy VT8 (rod, 22 mm in diameter) at instantaneous loadings at temperatures of 600, 700, and 800°C for up to 300 s.

Table 74 Long-term strength and creep strength of alloy VT8. State

Annealed

Thermally hardened

Temperature, ºC

450 500 550 500

Long-term strength, MPa, for time (h)

Creep limit at residual deformation, MPa, for time (h)

100

500

2000

10000

700–750 500–550 380 580

630–700 400–450 – 480

610 320 – 360

550 – – –

100

500

2000

480 380 250 200 80–100 – – –

300 100 – –

Table 75 Mechanical properties of annealed alloy VT8 at low temperatures Temperature, ºC 20 –70 –196

σB, MPa

σ0.2, MPa

δ, %

ψ, %

KCV, KJ/m2

1100 1300 1650

1020 1200 1550

11 9 6

35 30 25

0.50 0.44 0.29

Along with alloy VT8, its analog VT8M is commercially used. It differs by a reduced content of aluminum, 5.2–5.8%, and silicon, 0.10–0.30%, instead of 0.2–0.4% and an increased content of molybdenum, 3.5–4.5%. Alloy VT8M has the same mechanical properties at room and elevated temperatures as alloy VT8, but differs by an increased cold ductility. VT8M is intended for engine blades fabricated using cold roll forging.

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65

(a)

60

σ−1, kgf/mm2

55 50

45 40

35 104

105

65

106

107

108

N

107

108

N

(b)

60 55

σ−1, kgf/mm2

50 45

40

35

30

104

105

106

Figure 67 Endurance curves for alloy VT8 at temperatures of 20°C (a) and 500°C (b): (1) smooth specimens, (2) notched specimens (rn = 0.75 mm).

Alloy VT9 The deformable high-temperature titanium alloy VT9 is based on the Ti–Al–Mo– Zr–Si system. It belongs to two-phase martensite-type (α+β)-alloys with Kβ = 0.30. The alloy is mainly used as-annealed, but can be thermally hardened by water quenching and aging in small cross-sections (up to 40 mm). Chemical composition (in %): Al, 5.8–7.0; Mo, 2.8–3.8; Zr, 1.0–2.0; Si, 0.20– 0.35. Impurities (in %, no more than): C, 0.10; Fe, 0.25; O2, 0.15; N2, 0.05; H2, 0.015.

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The alloy is fabricated as forgings, stampings, rolled and pressed rods, shapes, and some other semiproducts. The mechanical properties of forgings, stampings, and rods according to the current documentation are given in Table 76. Table 76 Mechanical properties of as-annealed alloy VT9 according to the current documentation. Semiproduct

Thickness, mm

Mechanical properties σB, MPa

δ, %

ψ, %

KCV, KJ/m2

No less than Forgings, stampings

less than 100 101–150 151–250

105–125 100–125 95–125

9 6 6

25 14 14

0.3 0.3 0.3

Rolled rods

less than 60 65–100 110 –150

105–125 105–125 100–125

9 9 7

25 25 16

0.3 0.3 0.3

Semiproducts and pieces from alloy VT9 are subjected to heat treatment by the following regimes: • annealing, which includes heating at 950°C, holding for 1 h, air cooling; then heating at 530°C, holding for 6 h, air cooling. This is the basic heat treatment regime used for semiproducts and pieces from alloy VT9 • hardening heat treatment consisting of water quenching from a temperature of 925°C followed by aging at 500–600°C for 1–6 h and then air cooling • hardening heat treatment consisting of water quenching after hot deformation from temperatures of 920–800°C followed by aging at 500–600°C for 1–6 h and then air cooling. Hardening heat treatment makes it possible to increase the strength but slightly decreases ductility. Admissible working temperature of the thermally hardened alloy is about 50°C as low as that of the annealed alloy. The physical properties of annealed alloy VT9: • density at 20°C: 4480 kg/m3 → β transformation temperature: 1000±20°C • (α+β) ← • linear expansion coefficient: Temperature, ºC α×106, deg–1

20–100 20–200 8.50 8.76

20–300 9.01

20–400 9.27

20–500 9.51

20–600 9.76

• thermal conductivity as a function of temperature: Temperature, ºC λ, W(m deg)

25 7.11

100 8.37

200 9.63

300 11.3

400 12.6

500 14.6

600 16.3

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VALENTIN N. MOISEYEV

20°C 100

σ, kgf/mm2

80 400 500 550 600

60

40

20

0

0.2

0.4

0.6

0.8 δ, %

160 140 120

10500 σB σ0.2

80

13500 12000

Estat

100

60 δ, ψ, %

Edyn

9000 E, kgf/mm2

σB, σ0.2, σfc, kgf/mm2

Figure 68 Tension curves at yield strength for annealed alloy VT9 (rod, 20 mm in diameter) at room and elevated temperatures.

σfc ψ

40 δ

20

100 200 300 400 500 600 Temperature, °C

Figure 69 Mechanical properties of as-annealed alloy VT9 (rod, 20 mm in diameter) at short-term rupture vs test temperature.

• heat capacity at various temperatures: Temperature, ºC C, kJ/(kg deg)

100 0.544

200 0.584

300 0.628

400 0.668

• specific electrical resistance at 20°C: 165·10 –6 Ω cm.

500 0.712

600 0.816

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133

(a)

70

500°C

σ, kgf/mm2

60 50 550°C

40 30

600°C

20 10

650°C

σ, kgf/mm2

20

60

40

80 100

h

100 80 60 50 40 30 20 50

(b) 450°C

500°C 100

200 300

500

1000

2000

5000 h

Figure 70 Long-term strength curves of annealed alloy VT9 (rod, 15 mm in diameter): a) smooth specimens; b) notched specimens.

0.8

σ = 40 kgf/mm2

δ, %

0.6 0.4

σ = 39 kgf/mm2

σ = 38 kgf/mm2

0.2 20

40

60 80 Time, h

100

Figure 71 Curves of creep strength of annealed alloy VT9 (rod, 15 mm in diameter) as a function of temperature and time of test.

The tension curves of annealed specimens at yield strength are shown in Fig. 68. The mechanical properties of annealed rods 14–25 mm in diameter are given in Fig. 69. The long-term strength of annealed VT9 rods 15 mm in diameter is given in Fig. 70. The change of creep limit of the annealed alloy (forged rod) at residual deformation of 0.2% is shown in Fig. 71. Endurance tests on as-annealed smooth and notched (rn = 0.75 mm) specimens were performed on Weller machines at a rate of 3000 rpm. The circumferential notch was made by a cutter. The notched specimen was 5 mm in cross-section, as the

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65

(a)

σ−1, kgf/mm2

60 55 50

45 104

105

106

107

50

N

(b)

σ−1, kgf/mm2

45 40

35

30

25 104

105

106

107

108

N

Figure 72 Fatigue curves of annealed alloy VT9 at a temperature of 20 and 500°C: (a) at 20°C, (b) at 500°C; ( ) smooth specimens, ( ) notched specimens.

°

•

smooth specimens. The endurance limit was determined based on 2·107 cycles at 20 and 500°C. The results obtained are presented in Fig. 72. The mechanical properties of alloy VT9 (annealed rod 14 mm in diameter) at low temperatures down to –196°C are given in Table 77. Other mechanical properties of annealed alloy VT9 are as follows: • shearing strength: 70–80 MPa • torsional strength: 90 MPa • torsional yield strength: 69 MPa • torsional limit of proportionality: 47 MPa • shearing modulus: 44,000 MPa • Poisson ratio: 0.35 Alloy VT9 is intended mainly for aircraft-engine disks, blades, and rings operated at temperatures up to 500°C.

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Table 77 Mechanical properties at stress and impact strength of alloy VT9 at temperatures of 20, –70, and –196°C. Temperature, ºC

136.7 12.8 28.5 –

185 – – –

σB

Notched specimens

1.5 – – 0.50

n

σB

Smooth specimens

170 – – –

------

Notched specimens

113.0 14.4 42.2 –

–196

Smooth specimens

σB, MPa δ, % ψ, % KCV, MJ/m2

–70

Notched specimens

Smooth specimens

Properties

20

1.36 170.7 – 5.8 – 23.9 0.40 –

197.5 – – –

n

σB

-----σB

n

σB

-----σB

1.16 – – 0.25

Alloy VT25U The deformable high-temperature titanium alloy VT25U is based on the Ti–Al–Zr– Mo–Sn–W–Si system. It is a two-phase martensite-type (α+β)-alloy with Kβ = 0.4. The alloy is used as-annealed and in thermally hardened (quenched and aged) state in semiproducts of up to 60 mm in cross section. The chemical composition of VT25U is as follows (in %): Al, 6.0–7.0; Sn, 1.0–2.5; Zr, 3.0–4.5; Mo,3.5–4.5; Si, 0.10–0.25; W, 0.4–1.5. The content of impurities (in %, no more than): C, 0.10; O2, 0.15; N2, 0.05; Fe, 0.03; H2, 0.015. The alloy is produced as forgings, stamping, forged and rolled rods, and other semiproducts. The mechanical properties of forgings, stampings, and rods according to the current technical documentation are given in Table 78. Table 78 Mechanical properties of forgings, stampings, and rods from alloy VT25U according to the current documentation. Semiproduct

Thickness, mm

Mechanical properties σB, MPa

δ, %

ψ, %

KCV, MJ/m2

No less than Forgings Stampings Rolled rods Rolled rods

Up to 100 102–150 Up to 40 Up to 60

1100–1250 1100–1250 1100–1250 1100–1250

8 7 9 6

17 15 25 20

0.3 0.3 0.3 0.25

Semiproducts and pieces from alloy VT25U are subjected to heat treatment in accordance with the following regimes: annealing at a temperature of 940–970°C for

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VALENTIN N. MOISEYEV 100

20°C

σ, kgf/mm2

80

400 500 600

60 40 20

0

0.2

0.4

0.8

0.6

1.0 δ, %

160

14000

140

12000

120 100 80

σB

E

11000 10000

σ0.2

E, kgf/mm2

σB, σ0.2, kgf/mm2

Figure 73 Tension curves at yield strength for annealed VT25U (rod, 25 mm in diameter).

δ, ψ, %

60 40

ψ

20 δ 0 100 200 300 400 500 600 Temperature, °C

Figure 74 Mechanical properties of titanium alloy VT25U (rod, 20 mm in diameter) at short-term rupture vs test temperature.

1–4 h, air cooling, heating at 520–550°C, holding for 4–6 h, air cooling. This regime of heat treatment guaranties the ultimate strength of 1100 MPa for semiproducts. The hardening heat treatment consists of heating up to 900°C, holding for 1–4 h, water cooling followed by aging at 550–570°C for 4–8 h, air cooling. This regime of heat treatment ensures the ultimate strength of ≥ 1200 MPa for semiproducts of small cross section. As-annealed alloy VT25U has the following physical properties: • density at 20°C: 4620 kgf/m3 → β transformation temperature: 970°C • (α+β) ← • linear expansion coefficient within the temperature range: Temperature, ºC α×107, deg

20–100 20–200 8.2 8.4

20–300 8.7

20–400 9.0

20–500 9.4

20–600 9.4

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• thermal conductivity as a function of temperature: Temperature, ºC λ, W(m deg)

20 7.12

100 7.95

200 9.21

300 10.5

400 11.7

500 13.4

600 14.6

300 0.604

400 0.632

500 0.660

600 0.704

500 187.9

600 187.7

• heat capacity as a function of temperature: Temperature, ºC C, kJ/(kg deg)

20 0.536

100 0.552

200 0.578

• specific electrical resistance: Temperature, ºC ρ×106, Ω m

20 181.0

100 183.5

200 181.0

300 187.1

400 187.7

The tension curves of as-annealed alloy VT25U at yield strength are given in Fig. 73. The change of mechanical properties of the annealed alloy are shown in Fig. 74. The value of long-term strength and creep limit of alloy VT25U at temperatures of 450, 500, and 550°C and different test times are presented in Table 79. Specimens for tests were cut from annealed forgings and stampings of up to 100 mm in thickness. Table 79 Long-term strength and creep limit of as-annealed alloy VT25U. Properties

Temperature, ºC 450

500

550

800900 650 600

650700 550 500

460480 280330 230

– –

380 –

220 120

Long-term strength, MPa for 100 h for 500 h for 1000 h Creep limit at residual deformation of 0.2%, MPa for 100 h for 500 h

Endurance of as-annealed alloy VT25U was determined on smooth and notched (rn = 0.5 mm) specimens based on 5·108 cycles at temperatures of 20–500°C. The endurance curves of as-annealed alloy VT25U (rod 14–25 mm in diameter) are presented in Fig. 75. Typical mechanical properties of alloy VT25U (forgings and stampings up to 60 mm thick and rods 60 mm in diameter) in a thermally hardened state (after quenching and aging) are as follows:

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VALENTIN N. MOISEYEV

σ, kgf/mm2

90

450°C 500

70 550

50 30

600

10 20 40 60 80 100

200 Time, h

300

Figure Endurance curves for alloy VT25U at 20 and 500°C: _ _ _75_, notched specimens, ( ) 20°C, ( ) 500°C.

°

•

400

___, smooth specimens,

• ultimate strength, MPa: 1250–1350 • yield strength, MPa: 1100–1200 • relative elongation, %: 5–8 • reduction of area, %: 10–15 • impact strength, KCV, MJ/m2: 0.20–0.35 • ultimate strength at 500°C, MPa: 900–1100 • ultimate strength at 550°C, MPa: 850–1000 • long-term strength at 500°C for 100 h: 710–750 • long-term strength at 550°C for 100 h: 450–480 Alloy VT25U is an improved version of alloy VT25 intended for high-load pieces and constructions in aircraft engine building, because it can be subjected to a more efficient hardening treatment as compared with VT25. Alloy VT25 is rarely used at present in new constructions. By its physicomechanical properties, it practically does not differ from alloy VT25U. The difference in the chemical composition of these two alloys is the content of molybdenum. In VT25U, it is increased up to 3.5–4.5%, which makes it possible to harden the alloy by quenching and aging up to σB no less than 1200–1250 MPa in cross-sections of no more than 60 mm. Alloy VT8-1 The deformable high-temperature titanium alloy VT8-1 is based on the Ti–Al–Mo– Sn–Zr–Si system. It belongs to two-phase martensite-type (α+β)-alloys and has Kβ = 3. The alloy is mainly used as-annealed, but at small thicknesses (up to 40 mm) can be subjected to hardening heat treatment. The chemical composition of the alloy is as follows (in %): Al, 5.8–6.8; Sn, 0.4–1.5; Zr, 0.5–1.5; Mo, 2.8–3.8; Si, 0.10–0.25. Impurities (in %, no more than): C, 0.10; Fe, 0.30; N2, 0.05; H2, 0.015; O2, 0.15.

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The alloy is manufactured as forgings, stampings, rolled and forged rods, and other semiproducts. The mechanical properties of forgings, stampings, and rods according to the current technical documentation are given in Table 80. Table 80 Mechanical properties of as-annealed alloy VT8-1 according to the current documentation. Semiproduct

Thickness, mm

Mechanical properties σB, MPa

δ, %

ψ, %

KCV, MJ/m2

No less than Forgings Stampings Rolled rods

Up to 100 101–150 Up to 60

100 100 105

9 8 10

25 20 25

0.3 0.3 0.3

Semiproducts and components from alloy VT8-1 are subjected to heat treatment in accordance with the following regimes: • annealing at 930°C, holding for 1–4 h, air cooling, then heating at 580°C, holding for 1–2 h, air cooling. This is the most widespread regime of annealing. • hardening heat treatment that consists in water quenching followed by aging at 560°C for 2–6 h and air cooling. Physical properties of alloy VT8-1: • density at 20°C: 4485 kg/m3 → β transformation temperature: 1005±20°C • (α+β) ← • linear expansion coefficient of as-annealed alloy as a function of temperature: Temperature, ºC α×106, deg–1

20–100 20–200 8.4 8.7

20–300 8.9

20–400 9.0

20–500 9.3

20–600 9.6

• thermal conductivity of the annealed alloy as a function of temperature: Temperature, ºC λ, W/(m deg)

25 7.1

100 8.4

200 9.6

300 11.3

400 12.6

500 14.4

600 16.0

heat capacity at various temperatures: Temperature, ºC C, kJ/(kg deg)

100 0.545

200 0.586

300 0.627

400 0.666

500 0.710

600 0.317

• specific electrical resistance of the annealed alloy at 20°C: 163·10 –6 Ω cm; the alloy is nonmagnetic

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VALENTIN N. MOISEYEV

δ, % σ = 38 kgf/mm2

0.4 0.2 20

40

60

80

100 h

Figure 76 Tension curves at yield strength for annealed specimens from alloy VT8-1. 65

σ, kgf/mm2

60 55 50 45 40 104

105

106

107

108

N

Figure 77 Change of the mechanical properties of alloy VT8-1 (rod, 20 mm in diameter) at short-term rupture vs test temperature.

Mechanical properties of alloy VT8-1: The tension curves of annealed specimens from alloy VT8-1 (rod, 14–25 mm in diameter) at yield strength are given in Fig. 76. The mechanical properties of the annealed alloy (rod, 20 mm in diameter) at various temperatures are given in Fig. 77. The high-temperature strength characteristics of the annealed alloy (forgings, stampings, rods up to 100 mm in thickness) at various temperatures are given in Table 81. The endurance limit of the alloy was determined on annealed smooth and notched (rn = 0.75 mm) specimens based on 2·107 cycles (Table 82). Alloy VT8-1 is intended for manufacturing engine disks and blades operated at temperatures up to 500°C. Alloy VT8M-1, which is an analog of VT8-1, is used for fabricating engine blades and fixtures by cold roll forging. VT8M-1 has the physicomechanical properties close to VT8-1 but its chemical composition is slightly different. In VT8M-1, the content of aluminum is decreased and the content of molybdenum is increased, which enables its intensive deformation at room temperature (up to 40% of cold upsetting). The chemical composition of alloy VT8M-1 is as follows (in %): Al, 4.8–6.0; Sn, 0.3–1.5; Zr, 0.3–1.5; Mo, 3.5–4.5; Si, 0.08–0.25. The content of impurities is as in alloy VT8-1. Alloy VT8M-1 is recommended for long-term operation at temperatures up to 400°C and up to 450°C for no more than 1000 h.

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Table 81 High-temperature strength characteristics of annealed titanium alloy VT8-1. Properties, MPa

σB σ100 σ0.2/100 σ0.2/500

Temperature, ºC 450

500

550

820 760 500 390

770 580 300 –

730 410 110 –

Table 82 Endurance limits of smooth and notched specimens from alloy VT8-1 at various temperatures. Specimen

Smooth Notched

Temperature, ºC 20

500

540 MPa 390 MPa

450 MPa 310 MPa

3.3 HIGH-TEMPERATURE PSEUDO-α-ALLOYS These are alloys containing minor amounts of β-stabilizing elements (Kβ ≤ 0.25) close to their solubility in α-titanium. A particular feature of these alloys is that they can be used at temperatures up to 550–600°C. Alloy VT18U The deformable high-temperature alloy VT18U is based on the Ti–Al–Zr–Sn– Mo–Nb–Si system. It can be assigned to α-titanium alloys (Kβ = 0.15), because the β-stabilizing elements used for alloying are in practice within the limits of solubility in α-solid solution. The alloy is commercially used exceptionally in the as-annealed state. The chemical composition of titanium alloy VT18U is as follows (in %): Al, 6.2–7.3; Mo, 0.4–1.0; Zr, 3.5–4.5; Sn, 2.0–3.0; Si, 0.1–0.25; Nb, 0.5–1.5. Content of impurities (%, no more than): C, 0.10; Fe, 0.2; O2, 0.4; N2, 0.04; H2, 0.015. The alloy is manufactured as forgings, stampings, rods, forged and rolled rings. The mechanical properties of forgings and rods according to the current technical documentation are given in Table 83. The major heat treatment regime for semiproducts from alloy VT18U is annealing at a temperature of 900–980°C, holding for 1–4 h, and air cooling. The hightemperature strength of the alloy is slightly increased by duplex annealing, which includes heating at a temperature of 900–980°C, holding for 1–4 h, air cooling, then heating at 600°C, holding for 6 h, and air cooling.

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20°C 100

σ, kgf/mm2

80 400 500 550 600

60

40

20

0

0.2

0.4

0.6

0.8 1.0 δ, %

Figure 78 Tension curves at yield strength of annealed alloy VT18U.

Table 83 Mechanical properties of as-annealed alloy VT18U. Semiproduct

Thickness, mm

Mechanical properties σB, MPa

δ, %

ψ, %

KCV, MJ/m2

No less than Forgings, stampings Rolled rods

Up to 100 10–35

950–115 1000–1150

6 10

17 25

0.25 0.25

Physical properties: • density at 20°C: 4550 kg/m3 → β transformation temperature: 1000–1030°C • (α+β) ← • linear expansion coefficient within the temperature range: Temperature, ºC α×106, deg–1

20–100 20–200 8.0 8.25

20–300 8.5

20–400 8.7

20–500 9.0

20–600 9.1

• specific electrical resistance at 20°C: 170·10 –6 Ω cm • thermal conductivity as a function of temperature: Temperature, ºC λ, W/(m deg)

25 7.1

100 7.9

200 9.6

300 10.9

400 12.2

500 13.8

600 15.1

700 16.7

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100

12000

E

11000

120

σB

10000

σ0.2

80 60

δ, ψ, %

14000

E, kgf/mm2

σB, σ0.2, kgf/mm2

160

ψ

40 δ

20 0

143

0 100 200 300 400 500 600 Temperature, °C

Figure 79 Mechanical properties of alloy VT18U (rod, 20 mm in diameter) at short-term rupture vs test temperature.

20°C

σ, kgf/mm2

100 80

400 500

60

600 40 700 20 0

0.2

0.4

0.6

0.8

1.0 δ, %

Figure 80 Long-term strength of annealed alloy VT18U at various temperatures and loading times up to 500 h.

• heat capacity as a function of temperature: Temperature, ºC C, kJ/(kg deg)

100 0.503

200 0.545

300 0.608

400 0.670

500 0.712

600 0.755

700 0.795

The mechanical properties were determined on forged annealed rods 14–25 mm in diameter. The tension curves at yield strength are presented in Fig. 78. The change of mechanical properties of annealed alloy VT18U as a function of temperature at instantaneous rupture is shown in Fig. 79. The long-term strength of the alloy at temperatures of 550, 600, and 650°C as a function of temperature is given in Fig. 80.

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12000 11000 10000 9000 8000 7000 6000 5000

110 90

ψ

70 E 50

δ E, kgf/mm2

δ, ψ, %

σB, σ0.2, σfc, kgf/mm2

VALENTIN N. MOISEYEV

σB 30

σ0.2 σfc

10 20 500

600

700

800

Temperature, °C

Figure 81 Endurance curves for as-annealed alloy VT18U (rod, 20 mm in diameter): ______, smooth specimens, _ _ _, notched specimens (rn = 0.75 mm), o, 20°C, ×, 600°C.

The results of creep tests for annealed rods from alloy VT18U are presented in Table 84. Table 84 Creep limit of alloy VT18U at residual deformation of 0.2% at various temperatures and test times. Test time, h

Temperature, ºC 500

550

600

650

Creep limit, σ0.2, MPa 100 500

370 –

200 115

100 50

70 30

The results of endurance tests of smooth and notched (rn = 0.75 mm) specimens at temperatures of 20 and 600°C are given in Fig. 81. Mechanical properties of as-annealed alloy VT18U at low temperatures are presented in Table 85. Table 85 Mechanical properties of alloy VT18U at low temperatures. σB, MPa

δ, %

ψ, %

KCV, MJ/m2

20

110

10

25

0.25

–70 –196

125 155

7 4

15 10

0.15 –

Temperature, ºC

Alloy VT18U is a modification of titanium alloy VT18 developed earlier, which is not commercially manufactured or used at present. Still, the reader can come across information about it in the literature and should be aware of its existence.

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Alloy VT18, in contrast with alloy VT18U, had the chemical composition (in %): Al, 7.2–8.3; Mo, 0.2–1.0; Zr, 7.8–12.0; Si, 0.05–0.3; Nb, 0.7–1.4, and a lower ductility and thermal stability, especially in tests under stress. Alloy VT36 The deformable high-temperature titanium alloy VT36 is based on the Ti–Al–Mo– Zr–Sn–W–Si system. It is a two-phase (α+β)-alloy with Kβ = 0.2. The admissible concentration range of alloying elements and impurities is the same as in other commercial titanium alloys. The alloy is used only as-annealed and is manufactured as forgings, stampings, rods, and other semiproducts. The mechanical properties of forgings and stampings according to the current documentation are presented in Table 86. Table 86 Mechanical properties of as-annealed alloy VT36 according to the current documentation. Semiproduct

Thickness, mm

Mechanical properties σB, MPa

δ, %

ψ, %

KCV, MJ/m2

No less than Forgings, stampings Rods

Up to 100 Up to 60

1000–1200 1000–1200

5 6

7 8

0.15 0.15

The major heat treatment regime for alloy VT36 is annealing at a temperature of 900–980°C for 1–4 h and air cooling. The physical properties of alloy VT36 • density at 20°C: 4590 kg/m3 • α/β transformation temperature: 1000–1025°C • linear expansion coefficient within the temperature range: Temperature, ºC α×106, deg–1

20–100 20–200 8.2 8.4

20–300 8.6

20–400 8.8

20–500 9.0

20–600 9.2

• thermal conductivity as a function of temperature: Temperature, ºC λ, W/(m deg)

25 7.1

100 8.2

200 9.5

300 11.1

400 12.4

500 14.0

600 15.3

• heat capacity as a function of temperature: Temperature, ºC C, kJ/(kg deg)

100 0.544

200 0.580

300 0.625

400 0.653

500 0.688

600 0.690

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50 500°C

σ, kgf/mm2

40 30 600°C 20 650°C 10 100

0

400

300 200 Time, h

500

Figure 82 Change of the mechanical properties of annealed alloy VT36 (rod, 20 mm in diameter) at short-term rupture vs test temperature.

• specific electrical resistance at 20°C: 164·10 –6 Ω cm. The alloy is nonmagnetic. The mechanical properties of annealed alloy VT36 at various temperatures are given in Fig. 82. The high-temperature characteristics of as-annealed alloy are presented in Table 87. Table 87 Long-term strength and yield strength of annealed alloy VT36. Properties

Temperature, ºC 300

Long-term strength, 808–820 MPa, for 100 h Creep limit at residual deformation 0.2%, for 100 h – for 500 h –

400

500

770–755

550

600

650

735–750 510–520 330–340 210–230

– –

440 300

260 100

150 60

70 –

The endurance of annealed alloy VT36 was determined on smooth and notched specimens with the stress concentrators of 2.33 and 3.35 based on 1·104 and 2·107 cycles at temperatures of 20 and 600°C. The results of tests are presented in Table 88. Table 88 Endurance of alloy VT36 at various temperatures and test bases (in MPa). Specimen

Smooth

Notched with α = 2.33

Notched with α = 3.35

– –

440 200

Based on cycles 20ºC 1×104 1×107

900 480

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(continued)of alloy VT36 at various temperatures and test bases (in MPa). Table 88 Endurance Specimen

Smooth

Notched with α = 2.33

Notched with α = 3.35

270 160

– –

Based on cycles 600ºC 1×104 1×107

500 260

References 1. Glazunov, S.G. (1958) Titanium-based high-temperature alloys. Moscow: Oborongiz (in Russian). 2. Neugodova, V.N. (1960) Development of high-temperature titanium alloys operated at temperatures above 400°C. In Titanium and its Alloys. Moscow: Oborongiz, pp. 9–20 (in Russian). 3. Glazunov, S.G., Solonina, O.P., and Kokhova, G.M. (1960) High-temperature titanium alloys VT3 and VT3-1. In Titanium and its Alloys. Moscow: Oborongiz, pp. 30–42 (in Russian). 4. Neugodova, V.N. and Neugodova, Z.N. (1961) Structure and properties of alloy VT8. In Titanium in Industry. Moscow: Oborongiz, pp. 176–184 (in Russian). 5. Neugodova, V.N. (1961) Development of high-temperature titanium alloy VT9. In Titanium in Industry. Moscow: Oborongiz, pp. 185–202 (in Russian). 6. Glazunov, S.G. and Kurayeva, V.P. (1961) Titanium alloy VT10 with increased creep limit. In Titanium in Industry. Moscow: Oborongiz, pp. 212–226 (in Russian). 7. Solonina, O.P. and Kurayeva, V.P. (1964) High-temperature titanium alloys operated at 600°C. In Applications of Titanium Alloys. Moscow: Department of Scientific and Technical Information, All-Russian Institute of Aircraft Materials, pp. 70–75 (in Russian). 8. Kurayeva, V.P. and Solonina, O.P. (1964) α-Structure-based titanium alloys operated at 600°C. In Production of Titanium Alloys. Moscow: Metallurgiya, pp. 12–16 (in Russian). 9. Boyarskaya, E.A., Elagina, L.A., and Brun, M.Ya. (1966) Effect of deformation conditions on the structure and mechanical properties of disks from titanium alloys VT8 and VT9. In Production of Titanium Alloys. Moscow: Department of Scientific and Technical Information, All-Russian Institute of Light Alloys, pp. 66–72 (in Russian). 10. Elagina, L.A. and Borzetsovskaya, K.M. (1969) Effect of structure on the properties of semiproducts from alloy VT18. In Production of Titanium Alloys. Moscow: Department of Scientific and Technical Information, All-Russian Institute of Light Alloys, pp. 10–17 (in Russian). 11. Moiseyev, V.N. (2001) Titanium and titanium alloys. In Nonferrous Metals and Alloys (encyclopedia). Moscow: Mashinostroenie, Vol. II-3, pp. 313–330 (in Russian). 12. Solonina, O.P. (1966) Structure and properties of semiproducts from alloy VT3-1. In Production of Titanium Alloys. Moscow: Department of Scientific and Technical Information, All-Russian Institute of Light Alloys, pp. 90–94 (in Russian). 13. Borzetsovskaya, K.M., Elagina, L.A., Kurayeva, V.P. et al. (1967) Process testing of high-temperature α-titanium alloy VT18. In Production of Titanium Alloys. Moscow: Department of Scientific and Technical Information, All-Russian Institute of Light Alloys, pp. 3–10 (in Russian). 14. Zvereva, Z.F., Kaganovich, I.N., Makhmutova, E.A. et al. (1971) Effect of the type of

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microstructure on the long-term and fatigue strength of semiproducts from titanium alloys VT8 and VT9. In Production of Titanium Alloys. Moscow: All-Russian Institute of Light Alloys, No 6, pp. 164–168 (in Russian). 15. Brun, M.Ya. and Bykova, L.A. (1971) Effect of the heat treatment mode on the structure and mechanical properties of alloy VT9. In Production of Titanium Alloys. Moscow: All-Russian Institute of Light Alloys, No 6, pp. 180–183 (in Russian). 16. Elagina, L.A. et al. (1969) Effect of heat treatment on the high-temperature strength and thermal stability of titanium alloy VT18. In Production of Titanium Alloys. Moscow: All-Russian Institute of Light Alloys, No 5, pp. 129–140 (in Russian). 17. Ilchenko, A.M., Akzhakov, V.M., and Loginov, N.Z. (1968) Effect of the structure of alloy VT8 on the sensitivity to stress concentrators under alternating loads. In Metal Materials in Aircraft Industry. Moscow: Department of Scientific and Technical Information, All-Russian Institute of Aircraft Materials, pp. 25–30 (in Russian). 18. Kurayeva, V.P., Solonina, O.P., Sazonova, T.P. et al. (1977) Effect of heat and thermomechanical treatments on the properties and structure of titanium alloys VT18 and VT18U. In Titanium Alloys. Moscow: Department of Scientific and Technical Information, All-Russian Institute of Aircraft Materials, pp. 198–204 (in Russian). 19. Nikishov, O.A., Solonina, O.P., Krivenko, M.P. et al. (1977) Effect of prolonged heating on the properties of high-temperature titanium alloys. In Titanium Alloys. Moscow: Department of Scientific and Technical Information, All-Russian Institute of Aircraft Materials, pp. 209–212 (in Russian). 20. Glazunov, S.G., Elagina, L.A., and Kotova, V.I. (1961) Alloys of the titanium–silicon and titanium–aluminum–silicon systems. In Titanium in Industry. Moscow: Oborongiz, pp. 41–72 (in Russian).

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Functional-Purpose Titanium Alloys

4.1 TITANIUM ALLOYS OPERATED AT LOW TEMPERATURES Prospects of using titanium alloys in cryogenic units made it necessary to study their properties at low temperatures. Titanium alloys were found to behave differently under these conditions. A common feature is an increased requirement of strength (ultimate strength and yield strength) with the test temperatures decreasing. However, the decrease of ductility characteristics depends to a significant degree on the composition and structure of an alloy. The effect of various alloying elements, which form substitution solid solutions with titanium, on impact strength of these alloys at temperatures from 20 down to –196°C, and in some cases to –253°C, was studied on binary titanium alloys with different contents of aluminum, molybdenum, vanadium, niobium, iron, chromium, and manganese, and also some more complex alloys. The list includes alloys containing elements that stabilize α- and β-phases in titanium. Eutectoid-type β-stabilizing elements were used, as well as elements isomorphic to β-titanium. Research was made into cold shortness of titanium alloys alloyed with α- and β-stabilizing elements simultaneously. Special attention was paid to the effects of such impurities as oxygen, nitrogen, carbon, and hydrogen. It was found that the content of aluminum from 0 to 3% significantly decreased the impact strength at a temperature of –196°C, and a further increase of aluminum content from 3 to 7% the rate of the decrease slows down. The eutectoid-forming elements iron and chromium are the strongest β-stabilizing elements that sharply decrease impact strength both at room temperature and at low temperatures (–70°C, –196°C). In alloys with elements isomorphic to β-titanium (tantalum, niobium, vanadium, and molybdenum) the cold shortness is less than in titanium alloys with eutectoid-forming elements. Simultaneous alloying of titanium with α- and β-stabilizing elements, for instance, with aluminum and molybdenum, was observed to increase the cold shortness, the total of which was the sum of actions of both elements.

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The physicomechanical properties of titanium alloys at low temperatures are greatly affected by gas impurities: oxygen, nitrogen, hydrogen. These impurities should be limited. Typical mechanical properties of titanium alloys recommended for cryogenic applications at temperatures of 20, –196, –253°C are given in Table 89. Table 89 Mechanical properties of alloys at low temperatures (rod, 20 mm in diameter). Temperature, ºC

Properties

Alloys VT1-0

OT4

VT5-1

VT6S

VT14

VT16

20

σB, MPa σ0.2, MPa δ5, % ψ, % KCV, MJ/m2

470 400 30 65 2.0

830 770 24 50 0.9

820 800 21 55 1.0

860 810 17 55 1.4

980 890 15 60 1.3

870 780 20 60 1.4

–196

σB, MPa σ0.2, MPa δ5, % ψ, % KCV, MJ/m2

920 700 48 60 2.2

1430 1400 13 19 0.5

1320 1310 16 27 0.4

1310 1270 16 48 0.6

1440 1380 10 45 0.4

1380 1310 15 40 0.6

–253

σB, MPa σ0.2, MPa δ5, % ψ, % KCV, MJ/m2

1310 920 24 17 1.3

1560 1410 16 10 0.4

1580 1400 15 9 0.4

– – – – –

– – – – –

– – – – –

Research into and practical work with titanium alloys has shown that there is no need to develop a special group of alloys for cryogenic applications. Available commercial alloys could be used taking into account the above conditions and keeping the gas impurities within the obligatory limits (wt. %): O2, 0.10; N2, 0.03; H2, 0.0; C, 008. Purchase orders for alloys intended to be used at cryogenic temperatures (below 100°C) should specify these requirements and the fabricated semiproduct should have a designation.

4.2 SPECIAL-PURPOSE CORROSION-RESISTANT TITANIUM ALLOYS Commercial titanium alloys have a high corrosion resistance under atmospheric conditions and in most aggressive media. The high corrosion resistance of titanium and titanium alloys is determined by the formation of a protective oxide film.

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However, in some especially aggressive media corrosion resistance of titanium proves not too high, for instance, in concentrated acids, so special titanium alloys are required. A similar problem arises in applications of high-strength titanium alloys in seawater environments, where crack resistance is of crucial importance. Medical applications of titanium alloys as corrosion-resistant and biocompatible material also require optimal chemical compositions, as traditional titanium alloys do not always meet new special requirements. There is an opinion that such alloys should contain noble metals (platinum, palladium, etc.) capable of shifting the stationary potential of the alloy to a positive side, when the rate of cathodic dissolution of titanium greatly decreases. Alloying with elements stabilizing the β-phase (molybdenum and nickel) decreases the capability of anodic dissolution of titanium and also contributes to the corrosion resistance of the alloy. Such alloying elements as chromium, zirconium, tantalum, niobium, and vanadium significantly increase the liability of titanium to passivity in a corrosive environment. Alloy 4202 This is a technical-grade titanium alloyed with 0.2% palladium. It has relatively low strength characteristics and excellent ductility. Applications of this alloy are pieces and constructions operated in corrosive media. The alloy is fabricated as sheet semiproducts, band, and foil, and also pipes, wire, etc. The physical properties of alloy 4202 are as follows: • density: 4.54 g/cm3 • thermal conductivity at various temperatures: Temperature, ºC λ, Cal/cm s deg

20 0.030

100 0.032

200 0.036

300 0.040

400 0.042

• heat capacity at various temperatures: Temperature, ºC C, Cal/(g deg)

20

100 0.125

200 0.126

300 0.139

400 0.150

500 0.159

• linear expansion coefficient within the temperature range: Temperature, ºC α×10 –6, deg–1

20–100 8.1

100–200 8.6

200–300 9.1

300–400 9.6

400–500 10.0

Typical mechanical properties of alloy 4202 at room and elevated temperatures are given in Fig. 83 and Table 90. The alloy is used in an as-annealed state. It welds well by all types of welding applicable for titanium. The strength of the welded joint is 0.95 of that of the base

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ψ

E

50

12000

80

10000

70

8000

δ, ψ, %

σB, σ0.2, kgf/mm2

60

90

40

σB

60

6000

30

σ0.2

50

4000

Estat, kgf/mm2

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40

20 δ

10

30 20

0 0

100 200 300 400 500 600 Temperature, °C

Figure 83 Change of the mechanical properties of alloy 4202 (sheet, 1 mm) at short-term rupture vs test temperature.

metal. Plasticity of the weld in hot deformation – forging, stamping, and rolling – is high. The major operations of sheet forming can be done as cold work. Table 90 Mechanical properties of alloy 4202 (sheet 1.0 mm). Properties

Temperature, ºC 20

200

300

400

Notch sensitivity at static load Endurance limit based on 107 cycles, MPa Resistance to repeated static loads at σ = 0.7σB, cycles Crack sensitivity at bending impact, kgf/cm2

1.45 240 17000

– 200 –

– 190 –

– 160 –

90

–

–

–

Long-term strength for 100 h, kgf/mm2 (no less than) Creep limit for 100 h, MPa (no less than)

–

250

170

–

–

180

130

–

Alloy 4202 is recommended for long-term applications in aggressive media at a temperature up to 200–250°C. Alloy 4201 This is a commercial supercorrosion-resistant alloy containing 33% molybdenum, and 0.2% palladium. The grade is high-alloyed with a stable β-structure. It can be used instead of tantalum, alloys based on nickel (of the type of hastalloy), and also

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Estat, kgf/mm2

σB 60

σ0.2

δ5, %

σB, σ0.2, kgf/mm2

80

E 40

12000

40 10000 δ

20 0

153

20 8000 0

20

6000

200 400 600 Temperature, °C

Figure 84 Change of the mechanical properties of annealed alloy 4201 (sheet, 1.5 mm) at short-term rupture vs test temperature.

noble metals platinum and gold. This alloy is recommended as a corrosion-resistant material for units operated at elevated temperatures in aggressive media containing sulfuric, hydrochloric, formic, and some other acids. The alloy can be fabricated as plates, sheets, band, foil, and also forgings, stampings, shapes, pipes and other semiproducts. It has a good ductility. Forging, stamping, pressing of the alloy is done as hot work, and rolling of thin sheets, band, and foil as cold work. Sheet forming of pieces of not complex shapes can be done as cold work, and of more complex shapes as hot work. The alloy welds well by all types of welding applicable for titanium: fusion welding and resistance welding. It is noticeably oxidized by heating in air at a temperature above 500°C. The physical properties of alloy 4201: • density: 5.69 g/cm3 • linear expansion coefficient at various temperatures: Temperature, ºC α×106, deg–1

20–100 11.2

20–200 13.3

20–300 14.3

20–400 15.0

20–500 15.2

, ºC α×106, deg–1

20–600 15.3

20–700 15.7

20–800 16.2

20–900 16.7

20–1000 17.3

• specific electrical resistance at 20°C: 100.5·10 –6 Ω cm The typical mechanical properties of alloy 4201 at room and elevated temperatures are given in Fig. 84 and Table 91. Alloy 4201 is a pilot-scale alloy and its use is restricted. It should be noted that several new commercial corrosion-resistant titanium alloys have appeared in Russia in the recent years. The alloys are intended for use in seawater as flexible pipelines, drill pipes, casing, etc. These are grade 9M (chemical ooo

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Table 91 Mechanical properties of alloy 4201 (sheet). Properties

Temperature, ºC 200

400

600

800

oooooooooooooooooooooo for 100 h

635

570

130

25

oooooooooooooooooooooo for 1000 h

630

–

–

–

Creep limit at σ0.2, MPa oooooooooooooooooooooo for 100 h oooooooooooooooooooooo for 1000 h Endurance limit based on 2.107 cycles, MPa

550 –520 290

– – –

45 – 290

– – –

Long-term strength, MPa

composition, Ti–3.2% Al–2.7% V–1.3% Mo–1.0% Zr–0.05% Pd) and grade 23M (chemical composition, Ti–6.0% Al–4.0% V–0.5% Pd). The typical mechanical properties of the alloys for pipes 186×17 mm are presented in Table 92. Table 92 Mechanical properties of alloys 9M and 23M. Properties (in an as-annealed state) Ultimate strength, MPa Yield strength, MPa Relative elongation, % Reduction in area, % Impact strength, MJ/m2 Resistance to repeated static loads, N = 104, r = 0.1 MPa Endurance, N = 2.106, r = 1.0 MPa

9M

23M

789–798 665–674 19–21 39–41 1.1–1.3 805 450

921–941 813–843 11–12.5 31–38 0.35 850 490

The pipes were fabricated by hot rolling on a Pilger mill followed by cold multiple-pass rolling with intermediate annealings. The pipes from alloy 23M were also fabricated by hot pressing. Corrosion resistance tests of alloys for general and functional-purpose corrosion showed their high resistance. A particular problem is the corrosion behavior of high-strength titanium alloys in aggressive media, for instance, in seawater. The values of the critical coefficient of stress intensity for corrosive media K1SCC are observed to be significantly decreased as compared with the critical coefficient of plane-strain intensity K1C determined under atmospheric conditions. This factor is of great importance for the service life and reliability of aerospace and other commercial constructions where titanium alloys are used. A high-strength titanium alloy, VT22ch, was developed, with the ultimate strength of 1050–1200 MPa, which has satisfactory characteristics of crack resistance in marine and atmospheric environments.

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The alloy has the chemical composition close to that of alloy VT22 and some restrictions with respect to impurities and alloying elements. Alloy VT22ch found use in high-load pieces and constructions in marine aircraft, for instance, Be-200, and in other fields. The mechanical properties of stampings and forgings up to 150 mm in cross section made from alloy VT22ch are given in Table 93. Table 93 Mechanical properties of semiproducts from alloy VT22ch. Properties

Ultimate strength, MPa Yield strength, MPa Limit of proportionality, MPa Relative elongation, % Reduction of area, % Impact strength, kgf/mm2 Resistance to repeated static loads, MPa, K = 2.2, Kt = 0.1, N = 1.104 cycles Endurance limit, MPa, Kt= 1.0, R = 1, N = 2.107 cycles Impact strength K1C, MPa oooooooooooo K1SCC, MPa oooooooooooo σ100, MPa oooooooooooo σ0.2/100, MPa

Temperature, ºC 20

250

350

1050–1120 1000–1120 895–1020 10–20 25–40 3.5–5.0 56

940–975 800–880 – 11–20 24–42 – 50

890–920 710–750 – 13–23 27–47 – 46

58

53

48

920 880

810 620

74 60

The physical and process properties of alloy VT22ch are close to those of alloy VT22. Titanium alloys, both corrosion-resistant and biocompatible, have attracted the attention of medical workers. Major applications of titanium alloys in medicine are endoprosthetic replacements, dental prosthesis, and other implants. A thin oxide film formed on the surface of titanium and its alloys provides ionic protection of metal from corrosion in many environments, including physiological media. Under these conditions, titanium alloys resist not only general corrosion but also local effects, unlike stainless steels. Titanium alloys are an excellent material, not only with respect to their compatibility with living tissue but also by their physicomechanical properties. They have a high specific strength, and their low elasticity modulus is closer to bone than that of steel. Titanium alloys withstand cyclic loads well, including those in contact with physiological solutions, which is important for endoprosthesis. Initially, both Russian and Western material for implants was titanium alloy Ti–6% Al–4% V (Russian grade VT6) and also technical-grade titanium. Later, there were concerns that vanadium and also aluminum have a negative effect on accretion of tissue to the implant. Therefore, alloying having no vanadium and aluminum with decreased elasticity modulus were developed for implants. The

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chemical composition and mechanical properties of some of them are given in Table 94. Table 94 Chemical composition and mechanical properties of alloys used for implants. Chemical composition, wt. %

Ti–6Al–4V Ti–6Al–7 Ti–15Mo–5 3Al Ti–3Al–15Mo–2.6Nb–0.2Si Ti–12Mo–6Zr–2Fe Ti–0.4O2 –0.1N–0.8Fe Ti–30Ta Ti–5Al–2.5Fe Bone tissue

Mechanical properties σB, MPa

σ0.2, MPa

E, MPa

δ5, %

ψ, %

KCV, MPa

890 1000 1120 820 1050 870 920 880 –

825 900 1000 780 960 810 840 820 250

110000 110000 850000 1100000 9800 110000 105000 110000 25000

10 13 12 14 – 11 16 10 0.5

25 25 20 22 – 20 32 26 –

520 540 546 – – 520 500 200

There are reassuring data on prospective applications of aluminides and nickelides as implant material. A peculiar feature of nitenol (a chemical compound of titanium and nickel) is shape memory. This effect enables its use in surgery for mechanical dilation of blood vessels by introducing wire into a blood vessel; the wire then transforms into a spiral and dilates the vessel. Implants are often manufactured by complex shape casting. For orthopedic implants, deformed metal is preferable, as it has a greater resistance to fatigue loads.

4.3 INTERMETALLICS-BASED TITANIUM ALLOYS In the recent past, chemical compounds were considered unsuitable for use as structural materials due to the unfavorable combination of properties, say, high strength along with brittleness. However, in some cases these compounds proved to have acceptable and sometimes set of physicomechanical properties. In titanium alloys, one can single out three groups: high-temperature alloys based on titanium aluminide, shape memory alloys based on titanium nickelide, and alloys with eutectoid used as fire-safe alloys in engine building. By additionally alloying chemical compounds, it proved possible to develop alloys based on titanium aluminides and nickelides, and alloys with eutectoid, which are close to commercial titanium alloys by a set of properties. A number of processes, some of them new, have been used for commercial production of intermetallide-based alloys: microgranulation, formation of rapidly cooled flakes and fibers, compaction of powdered and cast pieces in gasostat, isothermic formation with small rates, and some others.

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4.3.1 Titanium aluminides This system (Ti–Al) includes three chemical compounds corresponding to the stoichiometric formulas TiAl3 (phase α2), TiAl (phase γ) and TiAl3. The first two are of greatest interest as the base for developing light high-temperature alloys. Table 95 presents some physicomechanical properties of such alloys as compared with those of common commercial titanium and nickel alloys. The data show that aluminide-based alloys significantly exceed titanium alloys by some properties and are comparable with nickel alloys. This refers to the elasticity modulus and operational temperature, which is 100–200°C higher than in commercial titanium alloys. Table 95 Comparison of the properties of aluminide- and nickelide-based titanium alloys. Properties

Ti (base)

Ti3Al

TiAl

High-temperature nickel alloy

Density, g/cm3 Young’s modulus Operational temperature: by admissible creep, ºC by admissible oxidation, ºC Ductility at 20ºC, % Ductility at operational temperature, %

4.5 110–96

4.15–4.7 145–110

3.76 176

8.3 206

550 600 20 over 20

800 650 2–5 5–8

1050 1050 1–2 7–12

1100 1100 3–5 10–20

Owing to the high aluminum content, alloys based on α2- and γ-phases have a good heat resistance in air atmosphere and are not liable to intensive oxidation observed in commercial titanium alloys at temperature above 600°C and long-term operational heating. A major advantage of titanium aluminides as a structural material for aircraft industry is their high-temperature strength, increase elasticity modulus, and low density. These advantages enable them to compete not only with high-temperature titanium alloys but also with nickel-based alloys. The above is supported by a plot presenting the dependence of specific strength for various alloys on temperature (Fig. 85). The plot compares the properties of common titanium alloys, high-temperature nickel alloys, and intermetallides. One of the advantages of titanium aluminides as compared with hightemperature nickel alloys is saving of deficit alloying elements, such as tantalum, tungsten, rhenium, etc. Mass production of aluminide-based alloys can be cheaper than common titanium alloys, as 15 to 33% of titanium in them is replaced by cheap aluminum. Wide distribution of high-temperature alloys based on titanium aluminides is mainly prevented by low ductility at room temperature and poor machinability by pressure and cutting. Presumably, the low ductility of titanium aluminides at room temperature can be increased by extra alloying, which would lead to the formation of new plastic phases or by maximal refinement of macro- and micrograin by intensive hot deformation.

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K1C, MPa M NiAl TiAl

30

d3 = 20−300 µm

Ti3Al-Nb 25

d3<10 µm W

20

NiAl d3~10 µm

15 Al2O3 ZrO2 10

iAl

NbN

5

Si3N4 SiC

ZrO2 0

200

400

600

800

1000

t, °C

Figure 85 Dependence of specific strength on temperature for various alloys.

Titanium aluminides have satisfactory casting properties and in some cases can be used for casting complex shapes into graphite molds. To date, only one Russian titanium alloy VTI-1, based on aluminide Ti3Al, has been certified. The alloy represents the α2-phase and some amount of β-phase. Several other variants of alloys based on α2-phase are being considered and studied. Aluminide Ti–Al, or alloys on its basis, are intended mainly for fabricating complex shapes. Alloy VTI-1 is based on the Ti–Al–Mo–Zr system, represents a two-phase composition based on intermetallide Ti3Al and β-solid solution. A characteristic feature of alloy VTI-1 is the use of an ordered structure as the base, which ensures a high-temperature strength. The alloy has a low ductility and low crack resistance at room temperature, and also is highly resistant to hot stress. Stampings from alloy VTI-1 after thermal hardening (oil quenching followed by aging) provides at room temperature the strength of 900–1000 MPa, elongation of 1–2%, long-term strength of no less than 380 MPa at 650°C. Alloy VTI-1 is recommended for fabricating aircraft-engine compressor parts (blades, rings) operated at temperatures of 650–700°C, instead of, for instance, nickel high-temperature alloy EP 718. In the recent years, further development of titanium aluminides based on α2-phase has been discussed in a large number of publications. In Russian practice, to increase strength, part of the niobium in these alloys is replaced by molybdenum, tantalum, or chromium. Tantalum and molybdenum increase resistance to oxidation. There is no mass production from titanium aluminides yet. Some pieces from this material were fabricated and tested. The tests showed it to be promising.

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Stampings were prepared in accordance with the following protocol: pressing of ingots at a temperature of 1120°C to fabricate rods; cutting into pieces; stamping (upsetting) of pressed pieces in isothermal regime at temperatures of 1050–1070°C; heat treatment, which included heating at 1075°C, holding for 1 h, oil quenching, heating at 850°C, holding for 3 h, air cooling. The mechanical properties of annealed stampings from alloy VTI-1 are given in Table 96. Table 96 Mechanical properties of stampings from alloy VTI-1. Tempe- σB, rature, MPa ºC 20 600 650 700

900– 1000 800– 850 780– 840 770– 820

an·104, at·104, σ – 1 J/m3 J/m3

n

σ0.2, MPa

δ, %

ψ, %

σ100, MPa

Estat, MPa

σ–1

880– 900 700– 730 680– 720 670– 700

1.0– 1.8 1.8– 2.0 1.8– 2.0 1.8– 2.0

–

–

129000

7

3

1.8– 2.0 1.8– 2.0 1.8– 2.0

480– 520 380– 460 250– 300

–

15

–

460– 220– 490 280 – –

–

20

–

330

–

–

25

–

–

–

According to the results of studies of a batch of ingots, the properties of the alloy 650 are within the following limits: σ650 = 380–460 MPa, B = 780–840 MPa, σ100 650 σ0.2/100 = 160–180 MPa; ductility at room temperature δ = 1.8–2.0%, ψ = 1.8–2.0%. The physical properties of alloy VTI-1 in an as-annealed state are given in Table 97. Table 97 Physical properties of alloy VTI-1. Properties

Temperature, ºC 20

Density, kg/cm3 Thermal conductivity Heat capacity Linear expansion coefficient within temperature range, ºC

100

200

300

400

500

600

700

4.7–4.8 170.8 173.3 176.2 177.4 178.3 178.9 179.5 180.1 0.570 0.585 0.590 0.595 0.610 0.625 0.638 0.654 20–100 20–200 20–300 20–400 20–500 20–600 20–700 9.8 10.0 10.2 10.4 10.6 10.9 11.1

Alloy VTI-1 has satisfactory casting properties, which enables casting of thinwalled complex shapes. 4.3.2

Titanium nickelide

Titanium nickelide, in contrast to titanium aluminides that are at the state of tests and pilot production, has already been widely used commercially, and its applications become wider.

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This chemically equiatomic Ti–Ni compound has good mechanical properties and high ductility (Table 98). A major feature of titanium nickelide which determines its value as a structural material is the shape memory effect, i.e., the ability of metal whose shape was considerably changed to restore the initial configuration by heating up to a definite temperature. The restoration temperature should be higher than that of the martensite transformation of the alloy. Additional alloying of titanium nickelide with iron, copper, and other elements makes it possible to adjust the shape recovery temperature within wide limits depending on the requirements. The chemical compound TiNi belongs to the class of berthollides and its homogeneity range varies from 2 to 5%. Melting temperature is 1240°C. Table 98 Physical properties of titanium nickelide. Properties

Values

Crystalline structure and lattice periods, Å

Ordered (CsCl type), a = 3.015* Martensite, triclinic cell, a = 4.60, δ = 2.86** Martensite, triclinic cell, c = 4.11, α = 90.1º Martensite, triclinic cell, β = 90.9º, γ = 96.7º New phase with 54 atoms/cell, a = 9.03** 6.443 0.001 Density, g/cm3 measured Density, g/cm3 calculated 6.516 Melting temperature, ºC 1250–1310 Recrystallization onset temperature, ºC 500–550 Critical martensite-transformation points, ºC Ms + 65; Mf + 40 As + 95; Af + 105 T0 + 85; T0 +67.5 Transformation heat, ∆H, Cal/g atom 496–370 Heat content change in transition (49.8 at. % 822.6 Ni) ∆H100º – ∆H25º, Cal/g atom Entropy change in transformation, ∆S, 1.01 0.05 8*; 6** Heat capacity, cp, Cal/mol K 76; 65 Specific electrical resistance, ρ, µΩ cm Absolute thermal emf, mV/deg, at 100ºC 8 Absolute thermal emf, mV/deg, at 25ºC 12 Absolute thermal emf, mV/deg, at –196ºC 6 Specific susceptibility, emu/g at 100ºC 3.5×10–6* Specific susceptibility, emu/g at 25ºC 2.5×10–6** Hall coefficient 0.5*; 1.8** Thermal conductivity, W/cm deg 0.01 Linear expansion coefficient, deg –1 14.0×10–6*; 6.0×10–6** Young’s modulus, kgf/mm2 Shearing modulus, kgf/mm2 Poisson’s ratio, µ

7400*; 66800** 2500*; 2300** 0.33*; 0.48**

* High-temperature phase; ** low-temperature phase

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The crystalline structure is ordered B2 (CsCl type) with the degree of 0.8–0.9. Lattice parameter is 3.005–3.040 Å depending on heat treatment and chemical composition. The major physical properties of titanium mononickelide are in Table 98. The maximal value of work per unit of volume of the alloy obtained in restoration of the initial shape on the TiNi alloy (rod, 2 cm in diameter): preliminary deformation temperature, 25°C; deformation of about 7% is equal to 2.04 kgf m/cm3. Titanium nickelide belongs to medium-strength materials, which facilitates its use in constructions from aluminum alloys, titanium alloys, and structural steels. The chemical composition and process characteristics of alloys based on titanium nickelide are given in Table 99, and the mechanical properties in Table 100. Table 99 Chemical composition and process characteristics of alloys based on titanium nickelides. Grade TN1 TN1K TNM3

Chemical composition, wt. % Shape restoration (titanium, the remainder) temperature, ºC 53.5–56.5Ni 50.0–53.5Ni; 2.0–4.5Fe 48.0–52.5Ni; 3.0–6.0Cu

Restoration force, MPa

from –10 to +90 from –160 to –80 from +80

300 400 200

Table 100 Mechanical properties of alloys based on titanium nickelide. Grade TN1 TN1K TNM3

σB, MPa

σ0.2, MPa

δ, %

ψ, %

600–900 500–900 600–700

150–200 100–250 150–250

15–20 15–20 15–20

15–20 15–20 15–20

KCV, MJ/m2 HB, kg/mm2 0.3–0.4 0.3–0.5 0.4–0.6

150–200 200–250 150–200

4.3.3 Eutectoid-based alloys Eutectoid-based alloys are proposed as fire-safe alloys for fabricating parts of aircraft engines: housings, blades, etc. The structure of these alloys is about 50% eutectoid, which melts or loses strength at comparatively low temperatures and, thus, prevents fire of components and constructions under extreme conditions. These alloys contain about 17% copper, 15% chromium, and also other elements – aluminum, molybdenum, vanadium, niobium, tin, and zirconium. The mechanical properties of rods 20 mm in diameter from alloy VT-1 and sheets 2.0 mm in thickness from alloy VTT-3 are given in Table 101. Table 101 Mechanical properties of alloys VTT-1 and VTT-3. Alloy

VTT-1 VTT-3

σB, MPa

σ0.2, MPa

950–1150 900–1100 600–750 420–460

δ, %

4–8 10

ψ, %

10–20 –

σ 100 ,

350

σ 100 ,

450

σ 100 ,

500

σ–1 ,

MPa

MPa

MPa

2×107

720 320

550–600 –

300 –

45–48 –

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Of the same category is alloy Ti–35% V–15% Cr, which in an as-cast state is planned to be used as a fire-safe alloy instead of nickel alloys.

4.4 TITANIUM ALLOYS FOR PRODUCTION OF CAST SHAPES Commercial production of cast shapes from titanium alloys is associated with problems, as liquid titanium is highly active with respect to gases and all known molding refractory materials. The casting properties of metals largely depend on the crystallization temperature range and on the amount of eutectics. Titanium alloys, as a rule, have a small range of crystallization temperature and, from this point of view, are of interest as material for casting complex shapes. Eutectics in modern commercial titanium alloys is practically absent. The best casting properties – fluidity and the least value of shrinkage in hardening – are peculiar to alloys of titanium with aluminum, zirconium, vanadium, niobium, tantalum, molybdenum, and tungsten, that are crystallized by the scheme of continuous series of solid solutions with β-modification of titanium. In all these systems, the interval of crystallization does not exceed 50–70°C. By fluidity, titanium approximately corresponds to carbon steel, but in reality it is much more difficult to produce large-size thin-walled castings from titanium due to the very rapid cooling and solidification of metal. This is attributed to a comparatively low heat content of liquid titanium and the difficulty of high overheating under conditions of consumable-electrode arc melting. The linear shrinkage of titanium is close to that of carbon steel (about 1.5% in ceramic mold casting). In casting into metal molds the linear shrinkage is larger, or the order of 2%. Volumetric shrinkage is about 3%. Under real conditions, the shrinkage can be slightly smaller. The use of vacuum in melting and casting of titanium alloys rules out the formation of gas porosity or oxide and slag inclusions, and fluidity increases due to the absence of a surface oxide film in the liquid metal flow. No special casting alloys for complex shapes were developed in Russian and Western practice. Use was made of commercial titanium alloys, which was due to the general principles of developing both casting and deformable structural and hightemperature alloys. The chemical composition and impurity content in deformable and casting alloys are the same. The most optimal alloys for casting are single-phase α-alloys or alloys based on α-phase with a minor amount of β-phase. Single-phase α-alloys VT1L and VT5L were used for casting complex shapes the most, and also two-phase (α+β)-alloys based on α-titanium – VT6L, VT14L, VT3-1L, VT9L, and VT12L. The amount of β-phase in these alloys in an as-annealed state does not exceed 10%. In the recent years, it became clear that titanium alloys with β-structure, while having slightly decreased casting properties, for instance, alloy VT35, are distinguished with higher fatigue characteristics as compared with α- and (α+β)-alloys.

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A feature of titanium alloys in an as-cast state is high strength, which is quite comparable with that obtained on deformed metal. Plasticity characteristics (reduction of area, relative elongation, impact strength, etc.) are noticeably lower than those obtained on deformed metal (Table 102). This is explainable by the effect of large-crystalline cast structure that has a negative effect on the plasticity of titanium alloys. Nevertheless, even in titanium alloys comparatively alloyed with β-stabilizing elements (VT3-1) the mechanical properties in an as-cast state are comparatively high. Table 102 Mechanical properties of castings from titanium alloys according to the current documentation. Alloy grade

Mechanical properties σB, MPa

VT1L VT5L VT6L VT14L VT20L VT3-1L VT9L VT21L VT35L

350 700 850 900 950 950 950 1000 1100

σ0.2, MPa – 630 750 800 830 830 830 860 940

δ, %

ψ, %

KCV, MJ/m2

10 6 5 5 5 4 4 4 5

20 14 10 12 13 8 8 8 10

0.4 0.3 0.25 0.25 0.25 0.25 0.2 0.2 0.25

In casting titanium alloys in an as-overheated state, as in deformed alloys, the characteristics of ductility tend to worsen as strength increases. Table 103 presents typical mechanical properties of casting alloys at short-term tension. Table 103 Typical mechanical properties of casting titanium alloys at room and elevated temperatures. Alloy

TempeE, rature, ºC kg/mm2

σB, MPa

σ0.2, MPa

σfc, kgf/mm2

δ, %

ψ, %

σ

n

-----HB-

aH, kgf/cm2

σB

VT1L

20 400

11200 –

47 18

– –

– –

15 16

35 17

– –

12 –

VT5L

–70 20 300 400 500

– 11800 10500 9800 –

85 80 40 35 30

79 63 32 25 –

50 – 25 20 –

10 6 8 10 13

20 14 25 30 –

1.57 – – – –

1.5 3 – – –

VT6L

20 200 300 400

11500 – 9500 9100

88 60 55 50

80 55 50 45

62 50 40 30

4 4 5 5

10 10 12 12

– – – –

3 – – –

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Table 103 Typical mechanical properties of casting titanium alloys at room and elevated Table 103 (continued). temperatures. Alloy

TempeE, rature, ºC kg/mm2

σB, MPa

σ0.2, MPa

σfc, kgf/mm2

δ, %

ψ, %

σ

n

-----HB-

aH, kgf/cm2

σB

VT20L

20 300 400

11300 9700 9500

95 63 55

80 52 46

60 39 33

7 8 8

15 20 20

– – –

3.5 – –

VT3-1L

–70 20 200 300 350 400 450 500

– 10800 – – 9800 – – 8100

110 98 77 67 63 61 59 56

– 85 – – 49 – – 44

– 65 – – 32 – – 29

5 8 8 10 10 10 12 12

13 20 20 30 34 35 35 35

1.4 1.15 – – – – – –

– 3 – – – – – –

VT9L

–70 20 400 450 500

– 11400 – 9200 8800

110 99 73 68 63

– 83 58 52 50

– 63 – 35 30

3 4 9 10 10

7 8 18 20 20

1.4 1.4 – – –

– 3 – – –

VT21L

20 150 300 400 450 500 550 600

10300 9100 900 8800 8600 8200 8000 7800

103 78 68 65 63 57 55 50

80 60 52 48 45 44 40 40

65 34 33 32 30 28 25 22

4 4 4 4 4 4 4 4

12 15 15 15 15 15 15 20

– – – – – – – –

2.5 – – – – – – –

VT35L

–70 20 300 400 500 600

– 11200 – 9300 8800 –

120 107 70 65 60 53

110 86 – 54 50 –

– 64 – 37 32 –

2 4 8 6 9 7

3 8 15 13 17 15

1.4 – – – – –

1.5 2 – – – –

20 350

10800 9000

114 95

105 85

– –

8 10

25 36

– –

– –

The characteristics of long-term high-temperature strength and creep limit are presented in Table 104.

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Table 104 Characteristics of high-temperature casting titanium alloys (no less than). Alloy

VT5L VT6L VT14L VT20L VT3-1L VT9L VT21L VT35L

Time, h 100 500 100 500 100 1000 100 500 100 500 100 500 100 500 100

Long-term strength, MPa

Creep limit, MPa

300

350

400

450

500

550

300

350

400

500

550

40 40 53 53 60 60 – – – – 65 – – – –

– – – – – – 60 60 – – – – – – 94

35 – 47 40 – – – – 72 68 62 – 62 60 –

– – – – – – – – – – 60 55 – – –

– – – – – – 43 33 – – 50 43 42 – –

– – – – – – – – – – 35 – – – –

– – – – 47 43 – – – – 50 – – – –

– – – – – – 45 – – – – – – – 80

28 – – – – – – – 45 38 48 – 42 – –

– – – – – – 16 – – – 28 20 18 – –

– – – – – – – – – 20 10 – – –

Both long-term strength and creep limit of cast metal are close to those of deformed metal. A distinctive feature of casting titanium alloys are comparatively low-fatigue characteristics. Table 105 presents the values of endurance limits for smooth and notched specimens of casting titanium alloys. Table 105 Endurance limits of casting titanium alloys (no less than). Alloy

Endurance limit based on 2.107 cycles, MPa 20ºC

300ºC

400ºC

500ºC

smooth notched smooth notched smooth notched notched notched VT5L VT6L VT14L VT20L VT3-1L VT9L VT21L VT35

280 200 270 220 220 180 200 420

– – 230

– – –

180

200*

–

–

*Based on 500 h, 50 cycles/s.

– –

– – – 200* – 180* –

– –

– – –

180* –

– –

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σB, kgf/mm2

120 1 2 3

110

4 5

100

ψ,%

δ5, %

90

4 5

6 4 2 0

1 2 3 1 2 3 4 5

15 10 5 0 4

8

16 Aging time, h

32

Figure 86 Change of the mechanical properties of alloy VT14L water quenched from 850°C, 1 h, as a function of aging regime: (1) 475°C, (2) 500°C, (3) 525°C, (4) 550°C, (5) 600°C.

As it follows from the table, the fatigue limits of cast specimens are 2–3 times as small as those of deformed specimens; the stress concentrators (notches) have no significant effect on the endurance limits. Heat treatment of casting alloys is basically similar to that of deformed alloys. Castings from alloys VT1L and VT6L are, as a rule, not heat treated. These are single-phase alloys, so comparatively small residual stresses occur in their castings. Casting titanium alloys with increased content of β-phase need to be annealed to stabilize the structure and release residual stresses. In this case, annealing is done according to the regimes accepted for deformed metal. Casting titanium alloys with (α+β)-structure can be hardened by quenching and aging. The major regularities of the change of mechanical properties and phase composition of casting titanium alloys are the same as for deformed alloys. A distinctive feature is their low ductility in a thermally hardened state peculiar of the metal with overheated structure. Changes of the properties of titanium alloy VT14L depending on the heat treatment regimes are presented in Fig. 86. As seen in the figure, cast metal after quenching and aging has lower strength and ductility as compared with deformed metal. This is explainable by the fact that traditional hardening treatment of cast metal did not find commercial use. In the recent years, hydrostatic pressing is widely used to improve the properties and structure of casting titanium alloys. This technology is especially successful in fabrication of castings from alloy VT35L. Alloy VT35L has increased fatigue strength as compared with other casting titanium alloys. A characteristic of the alloy is also that it can be subjected to thermal hardening treatment (aging) from the initial as-cast state without any preliminary quenching. Thus, the complex shape of a casting can be preserved during the heat treatment without any process accessories. Table 106 presents the properties of alloy

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VT35L in the state after gasostat and heat treatment according to the following regime: heating at 850–900°C; pressing, 1500 atm for 1–2 h; then aging at 500–550°C for 8 h. Table 106 Mechanical properties of alloy VT35L after gasostat and heat treatments. σB, MPa

Temperature, ºC 20 350

σ0.2, MPa

1100–1190 1020– 1090 950 850

ΜΝ,

δ, %

ψ, %

K1C, MPa

MPa

σ100, σ0.2/100, MPa MPa

7–10 10

22–28 40

112 –

500 –

– 900

– 800

References 1. Glazunov, S.G. and Moiseyev, V.N. (1974) Titanium alloys operated at low temperatures. Structural titanium alloys. Moscow: Metallurgiya, pp. 215–227 (in Russian). 2. Borisova, E.A. and Glazunov, S.G. (1960) Effect of oxygen and hydrogen on the mechanical properties of technical-grade titanium at temperatures of –196°C up to +350°C. In Titanium and its Alloys. Moscow: Oborongiz, pp. 114–120 (in Russian). 3. Moiseyev, V.N. and Belyaev, S.E. (1970) Effect of alloying elements on the cold brittleness of titanium alloys. In Applications of Titanium Alloys. Moscow: Department of Scientific and Technical Information, All-Russian Institute of Aircraft Materials, pp. 61–71 (in Russian). 4. Kornilov, I.I. (1968) Titanium alloys for new applications. Moscow: Nauka, pp. 24–34 (in Russian). 5. Kornilov, I.I., Mikheyev, V.S., and Belousov, O.K. (1962) In Metal Chemistry and New Titanium Alloys. Moscow: USSR Academy of Sciences Publishers, issue VII, pp. 120–129 (in Russian). 6. Kolachev, B.A., Kondrasheva, N.N., and Skoltsov, V.I. (1998) Effect of cryogenic temperatures on the mechanical properties of alloy VT6ch. Titan 10: 42–46 (in Russian). 7. Borisova, E.A., Klimova, G.S., and Belyaev, S.E. (1967) Properties of titanium alloys at cryogenic temperatures. In Applications of Titanium Alloys, Part 1. Moscow: Department of Scientific and Technical Information, All-Russian Institute of Aircraft Materials, pp. 101–109 (in Russian). 8. Anoshkin, N.F., Oginskaya, E.I., and Lebedeva, E.S. (1993) Titanium alloys for items operated in aggressive media. Titan 2: 73–76 (in Russian). 9. Igolkin, A.I. (1993) Titanium in medicine. Titan 10: 86–90 (in Russian). 10. Naidich, I.M. (1988) Titanium alloys in chemico-pharmaceutical production. Titan 1: 28–30 (in Russian). 11. Tetyukhin, V.V. and Kirichenko, N.I. (1993) Development of corrosion-resistant titanium alloys. Titan 2: 70–72 (in Russian). 12. Tetyukhin, V.V. and Smirnov, V.G. (1996) New deformed titanium alloy for offshore oilproduction pipes. Titan 1: 37–40 (in Russian). 13. Tetyukhin, V.V. et al. (1996) Production of titanium-alloy pipes for inland and sea-shelf geological exploration and oil production. Titan 1: 41–44 (in Russian). 14. Moiseyev, V.N. (1960) Heat treatment and the mechanical properties of titanium alloys with 5–13% aluminum. Metalloved. Term. Obrabotka Metallov 6: 30–39 (in Russian). 15. Altunin, Yu.F. and Glazunov, S.G. (1961) Titanium–aluminum binary alloys. In Titanium in Industry. Moscow: Oborongiz, pp. 5–30 (in Russian). 16. Povarova, K.B., Maslenkov, S.B., and Filin, S.A. (1993) Izv. Akad. Nauk SSSR, Metally 1: 191–196 (in Russian).

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17. Povarova, K.B. and Bannykh, O.A. (1996) Principles of the development of new aluminide-based materials operated at high temperatures. In Processing of Light and Special-purpose Alloys. Moscow: All-Russian Institute of Light Alloys, pp. 56–70 (in Russian). 18. Fatkulina, L.P. et al. (1993) Titanium nickelide-based alloys possessing the shape memory effect and superelasticity. Titan 3: 17–20 (in Russian). 19. Borisova, E.A. and Sklyarov, N.M. (1993) Fire-safe titanium alloys. Titan 3: 21–24 (in Russian). 20. Borisova, E.A. and Sklyarov, N.M. (2000) Combustion and fire safety of titanium alloys. Moscow: All-Russian Institute of Aviation Materials, p. 86 (in Russian). 21. Glazunov, S.G. and Moiseyev, V.N. (1974) Casting titanium alloys. Structural titanium alloys. Moscow: Metallurgiya, pp. 228–236 (in Russian). 22. Bratukhin, A.G., Bibikov, E.D., Glazunov, S.G. et al. (1998) Production of complex cast shapes from titanium alloys. Moscow: All-Russian Institute of Light Alloys (in Russian). 23. Moiseyev, V.N., Zhikharev, A.I., Khromov, A.M. et al. (1970) New casting titanium alloy VT14L. Aviatsion. Promyshlennost 3: 83–85 (in Russian). 24. Moiseyev, V.N. (2001) Casting titanium alloys. In Materials in Mechanical Engineering Industry (Encyclopedia). Moscow: Mashinostroenie, vol. II-3, pp. 340–346 (in Russian). 25. Prostov, I.A. and Yasinsky, K.K. (1967) Mechanical properties of casting rings from titanium alloys. In Applications of Titanium Alloys, Part 1. Moscow: Department of Scientific and Technical Information, All-Russian Institute of Aircraft Materials, pp. 87–92 (in Russian).

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5

Technological Properties of Titanium Alloys

An important factor determining the prospects of structural materials are their technological features, which should be taken into consideration when choosing an alloy for an application. The greatest attention should be paid to the treatments of titanium alloys, which are accompanied by changes in the structure of metal, state of surface, and, as a consequence, decrease of physicomechanical properties. In this connection, it is useful to consider such issues as: • formation of the structure and mechanical properties by thermomechanical treatment in fabrication of semiproducts and pieces; • change of structure and properties of various classes of alloys by heat treatment (annealing, thermal hardening); • mechanical and chemical treatment of semiproducts and pieces from titanium alloys and their effect on the properties of metal.

5.1 FORMATION OF THE STRUCTURE AND PROPERTIES IN HOT DEFORMATION OF TITANIUM ALLOYS An important factor determining the mechanical properties of semiproducts from titanium alloys is their structure formed by hot deformation of cast metal (ingot) or a predeformed billet. Therefore, one of the major problems of semiproduct fabrication is to obtain a specified and stable structure which provides the required set of mechanical properties. The complexity of this problem is that commercial titanium → β) transalloys are a multiphase metal which undergoes a polymorphic (α ← formation in the course of heating and deformation, and also numerous metastable transformations as β-unstable phase and α′-, α″-, and ω-phases. On the other hand, there is much in common in structure formation during the thermomechanical treatment of α-, (α+β)-, or β-solid solution titanium alloys. Thermomechanical treatment of titanium alloys, starting from the ingot and ending with the fabricated item, passes numerous stages. These include consecutive heatings, deformations at various temperatures starting from those in the β-region, then in (α+β)-region or α-region (in α-alloys). Herewith, the structure is significantly affected by the rate of metal deformation and cooling after hot deformation. The end operation is, as a rule, heat treatment (annealing or thermal hardening).

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Θ, °C

C

D

1100 II

1100 I 900 D 800

II

tα + β → β

C III

10−3 10−2 10−1 1

10 102 . ε, σ −1

Figure 87 Structural state of the β-phase of deformed alloy VT22 in temperature vs deformation ratee coordinates.

Each of the operations had an effect on the structure and physicomechanical properties of metal. The effect can reversible and irreversible. As a rule, an initial billet can be an ingot or casting (we do not consider powder or granular metallurgy here). The first stage of casting metal deformation is performed at temperatures of the β-region. Its purpose is to refine the cast structure and eliminate its inhomogeneity. In a casting titanium alloy of any class depending on its chemical composition and diameter of an ingot, the size of grain varies from 1–3 mm up to 10 cm. Refinement of the cast structure is achieved by recrystallization that occurs in alloys as a result of deformation and heating at β-region temperatures. By selecting deformation and heat treatment regimes, the cast grain could be refined by more than one order of magnitude. Polycrystalline alloys with this large-grain structure in an as-cast state and in heating in the β-region are characterized by a significant inhomogeneity of deformation. The central areas of the grains are always less deformed than the boundary ones, and have therefore a less perfect dislocation structure. In boundary areas, already in the process of deformation, the largest and most disoriented subgrains are formed, including those between which the misorientations are close to high-angle ones. The occurrence of the latter is a sign of dynamic recrystallization. Formation of subgrains and grains in the boundary region is accompanied by migration of the initial boundary; it becomes wavy. Figure 87 shows a typical structural state β-phase diagram of deformed titanium alloy VT22. Line DD on the diagram shows the temperature vs rate boundary of the onset of dynamic recrystallization, the position of which reflects the known tendency: as the temperature is increased and the rate of deformation decreased (to the left and higher than line DD), the tendency to dynamic recrystallization increases. During the deformation by the regimes described by points on DCE, located to the right of line DD, there is no time for recrystallization to begin, the subgrain structure is very imperfect and the density of irregular dislocations is high. This structure is unstable at high temperatures characteristic of the β-region and continues to be rearranged during the cooling of deformed metal to form recrystallized grains. The

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lower the temperature and the higher the degree and rate of deformation are, the more complete recrystallization during the cooling is. On the diagram, the region of intensive recrystallization that develops during the cooling after deformation is to the right of and higher than line CC. Upon deformation by the regimes corresponding to this region, metal inevitably has a partially or totally recrystallized structure. A feature of hot-deformed alloys at unidirectional deformation in the β-region is the deformation of the axial texture, which prevents the development of recrystallization. This phenomenon is observed in all classes of solid-solution titanium alloys and contributes to greater structural inhomogeneity of commercial semiproducts. The cooling of a precritical-composition titanium alloy from the deformation temperatures in the β-region is inevitably accompanied by polymorphic β→α transformation. The precipitated α-phase is located first of all as a continuous layer by the boundaries of the β-grain. The layer delineates the boundaries and, thus, interrupts their changes. Then the α-phase emerges in the inner regions. The intragrain precipitations of α-phase have a lamellar shape. The plates are assembled into clusters (α-colonies) within which they have the same geometric and crystallographic orientation. α-Plates are separated by lamellas of the residual β-phase, whose relative thickness is proportional to the volume fraction of the residual β-phase in the alloy. An increase of the cooling rate is accompanied by thinning-out of the boundary α-layer and refinement of all intragrain structural elements. In the deformation in the (α+β)-region at regular rates recrystallization can occur during the cooling of metal. It is accompanied by the origination of new grains not only by the boundaries of the initial β-grain, but also in its inner regions. A feature of deformation at temperatures of the (α+β)-region is transformation of the initial lamellar structure into the globular one. The initial spheroidization of the lamellar structure is achieved after deformation in the (α+β)-region by no less than 50–60% followed by annealing. In titanium alloys highly alloyed with β-stabilizing elements (transition alloys, pseudo-β-alloys, Kβ > 1.0) the problem of transformation of the lamellar α-phase into the globular one is not so sharp, as their intragrain structure is sufficiently heterogenized by a large amount of β-phase and is not liable to the formation of a rough lamellar structure. Deformed semiproducts from titanium alloys of various classes fabricated by various technologies (forgings, stampings, rods, plates, sheets, etc.) have rather different micro- and macrostructures that has a significant effect on the mechanical properties of metal. Precritical-composition titanium alloys (Kβ ≤ 1.0) are classified by their intragrain microstructure into three types (Fig. 88). The first type called equiaxial (α+β)-structure represents an equally distributed mixture consisting of primary α-phase (light regions) and mixtures of α- and β-phases (dark regions). This structure is obtained by the deformation of the alloy at temperatures of (α+β)-phase with the deformation degree of no less than 50–60%. The second type of structure occurs in semiproducts deformed first at temperatures of the β-region and then at temperatures of (α+β)-region up to no less than 20–30%. A characteristic feature of this structure is its intragrain lamellar structure.

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(a)

(b)

(c)

Figure 88 Precritical-composition titanium alloys.

The third type of structure occurs in the deformation in the β-region with the insufficiently clear details of the cast structure. It is characterized by large grains of the primary β-phase and rough- or fine-needle intragrain structure. The effect of structure on the mechanical properties of rods, for instance, based on martensite-type alloy VT3-1 (Kβ = 0.6) is shown in Table 107. Table 107 Mechanical properties of rods with various microstructures from alloy VT3-1 (forged rods 14–35 mm in diameter). Type of structure

Heat treatment

σB, MPa

σ0.2, MPa

δ, %

ψ, %

an, kgf/cm2

1

annealing thermal hardening

1180 1420

1140 1350

16 7

47 23

3.9 2.3

2

annealing thermal hardening

1140 1330

1080 1280

17 9

47 28

4.5 9.5

3

annealing thermal hardening

1050 1230

990 1200

12 5

21 14

5.5 3.0

Note: Thermal hardening included heating at 840°C, water quenching and aging at 550°C, air cooling

Important factors in precritical-composition alloys are the size of micrograin and the micrograin structure. In hypercritical-composition alloys, the main parameter of structural quality is the size of micrograin. This is explained by the fact that the increased content of β-stabilizing elements contributes to the heterogenization of intragrain structure, and it had an insignificant effect on the physicomechanical properties of high-alloyed alloys. The major role is played by the size of micrograin. Typical microstructures, for instance, of alloy VT22, are shown in Fig. 89.

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2

3

5

6

173

4 4

Figure 89 Typical microstructures of alloy VT22 (×500).

The dependence between the mechanical properties and macrograin size of alloy VT22 is presented in Fig. 90. As it follows from the data of Fig. 90, the factor of micrograin size has a significant effect on the ductility of metal and depends on the level of strength. The character of structure in semiproducts from titanium alloys can be adjusted in the fabrication of semiproducts (during the forging, stamping, rolling, and pressing). However, the process used leaves specific traces on the structure, which are difficult to get rid of. One should also take into account the anisotropy of the mechanical properties, which depend on semiproduct fabrication technology. In practice, the structure of commercial semiproducts is controlled mainly by visual examination.

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ψ, % 50

σBmin(acc. to specs) σBmax(acc. to specs) D3 = 20 µm

40

70

30

200

20

ψmin(acc. to specs) D3

400

=1

10

20 µm

0 1000 1100

Figure 90

800 1200

600 1300 σB, MPa

Dependence of the mechanical properties on the size of micrograin of alloy VT22.

The macrostructure is assessed with the naked eye or at low magnification (10–30-fold). This control is required to assess the macrostructural homogeneity of a large-size portion of the semiproduct surface. The macrograin is revealed as part of an equal-contrast polished section determined by its crystallographic orientation. Particular elements of structure include initial β-grain, large coarse colonies of α-formations, and some others. One also takes into account such a characteristic of macrostructure as the tint of its particular regions to describe the extent of recrystallization in particular areas of the polished section. Usually, these macrostructural parameters are specified in the technical documentation for semiproducts. Microstructure characterizes metal locally, as a rule, inside particular grains, under an optical microscope. In some cases, electron microscopy should be used. The microstructural examination is to assess the type of microstructure (lamellar, globular or mixed), micrograin size and texture, character of intragrain structure, and to have an idea of the regimes of hot deformation and heat treatment of a semiproduct. The ranges of admissible and inadmissible microstructures, as a rule, given in the technical specifications for the supply of semiproducts by metallurgical plants. Still, the character of the structure does not always makes possible a judgment of the physicomechanical properties of metal.

5.2 HEAT TREATMENT OF TITANIUM ALLOYS To obtain optimal physicomechanical and process properties, pieces, and semiproducts from commercial alloys are subjected to various heat treatments. The most widespread of them are annealing, quenching, and quenching and aging (tempering).

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Annealing is used for all types of titanium alloys and is the only heat treatment for α- and β-alloys. Quenching and quenching and aging (tempering) are used only for (α+β)-alloys. Quenching alone is used comparatively rarely and is an operation increasing the ductility of some alloys, as a rule, at the intermediate stages of fabrication. Quenching and aging are a hardening heat treatment to significantly increase the hardening characteristics of two-phase (α+β)-titanium alloys. Some characteristics determining the particular heat treatments of commercial titanium alloys are presented in Table 108. The alloys are given in the order of increasing Kβ. In heat treatment of titanium, as in the other process operations associated with heating, one should take into account its rather active interaction with the atmosphere. The consequences of interaction with gases are much more serious for titanium than for other metals, because atmospheric oxygen not only forms scales, but also diffuses deeply into the crystalline lattice, thus evoking a sharp increase of Table 108 Heat treatments, polymorphic (α+β)-transformation temperatures, and Kβ of commercial titanium alloys. Alloy

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24

VT1-00 VT1-0 VT5-1 VT18-U OT4-0 VT20 OT4-1, OT4-1V OT4, OT4-V VT6 VT6S VT14 VT9 VT8-1 VT25-U VT8 VT36 VT3-1 VT23 VT16 VT22 VT22-I VT35 VT32 VT15

Heat treatment

Polymorphic transformation temperature, °C

Kβ

annealing annealing annealing annealing annealing annealing annealing annealing annealing annealing, quenching and aging annealing, quenching and aging annealing, quenching and aging annealing, quenching and aging annealing, quenching and aging annealing, quenching and aging annealing, quenching and aging annealing, quenching and aging annealing, quenching and aging annealing, quenching and aging annealing, quenching and aging annealing, quenching and aging quenching and aging quenching and aging quenching and aging

880–900 870–890 980–1030 990–1030 860–920 980–1020 910–950 920–960 960–1000 950–990 920–960 980–1020 970–1010

– – – 0.09 0.10 0.11 0.16 0.17 0.25 0.25 0.32 0.32 0.33 0.42 0.43 0.55 0.63 0.78 0.80 1.16 1.16 1.55 1.80 2.25

980–1020 970 950–980 890–930 840–880 860–890 820–860 750–770 760–90 750–800

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surface hardness. This is a factor to be considered in heat treatment operations in atmosphere furnaces. Therefore, titanium alloys are often heat treated in vacuum furnaces or protective-atmosphere (argon or helium) furnaces. The surfaces of alloys heat treated in air should be chemically cleaned or shotblasted. 5.2.1

Annealing

Titanium alloys are annealed to recrystallize and stabilize the structure and phase composition, to decrease strength and increase ductility, and also to release internal stresses that emerge as a result of pressing, welding, or machining. Annealing consists of heating up to temperatures close to or higher than the recrystallization temperature, holding at a given temperature followed by cooling (slow in the furnace, in air or stepwise). Annealing of α-titanium alloys is, as a rule, annealing of the first kind, because it is not associated with phase transformations. Annealing of (α+β)-titanium alloys is accompanied in all cases by a change of the α/β phase ratio, and it should be referred to as annealing of the second kind. Annealing with complete phase recrystallization for alloys with both α- and, especially, (α+β)-structure is considered to be undesirable, because it is accompanied by an irreversible structural change (growth of micrograin) and a significant decrease of strength and traditional characteristics of ductility (δ and ψ). Sometimes annealing with complete phase recrystallization is used to increase the characteristics of crack resistance of titanium alloys. Full and partial annealing could be made. The difference between them is that the former is done at lower temperatures and is intended mainly to release residual internal stresses on semiproducts and pieces from titanium alloys. Titanium pseudo-α-alloys and also low-alloyed martensite (α+β)-alloys (VT6 and VT14) are practically insensitive to the rate of cooling after annealing. At the same time, high-alloyed martensite alloys (VT16, VT23, and VT3-1), pseudoβ-alloys (VT15 and VT32), and especially transition alloys (VT22) are rather sensitive and, therefore, their cooling rate from the annealing temperature should be specified. For this purpose, for medium- and high-alloyed (α+β)-alloys annealing should be followed with cooling in furnace at a specified rate down to a definite temperature, and then in air; or stepwise annealing could be done, which could be duplex or isothermal. Pseudo-β-alloys should be cooled at an accelerated rate to preserve solid β-solution. Duplex annealing consists of heating up to temperatures higher than the recrystallization temperature, holding, air cooling followed by heating at a definite temperature, holding, and air cooling. Often this heat treatment is hardening. Isothermal annealing includes heating up to temperatures above the recrystallization temperature, holding, then transfer into a furnace with a definite temperature, holding, and air cooling. In annealing followed by a slow cooling in furnace, duplex annealing or isothermal annealing the temperature from which air cooling should be done should

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be low enough to provide sufficient stability of α- and β-constituents in the subsequent long-term operation of the alloy within the operating temperatures. As titanium alloys are most often intended for constructions operated long-term at elevated temperatures, their annealing regimes are chosen not only from the point of view of optimal strength/ductility ratios, but also from the point of view of their thermal stability, i.e., invariable physicomechanical properties in long-term operation. Therefore, this or that kind of annealing is chosen for titanium alloys depending on their composition and, sometimes, on the purpose of a semiproduct or piece. Depending on the cross-section of a semiproduct, the holding time during the annealing is recommended to be set based on the data given in Table 109. Table 109 Annealing regimes of titanium alloys according to the current technical documentation. Alloy

Rods, forgings, stampings, Sheets, thin-walled thick-walled pipes, shapes, and pieces therefrom pipes, shapes, and pieces therefrom

Notes

Annealing temperature, °C VT1-00 VT1-0 VT5-1 VT18-U

520–570 520–570 700–750 –

670–730 670–720 800–850 Heating up to 900–950°C, air cooling

OT4-0 VT20 OT4-1, OT4-1V OT4, OT4-V VT6, VT6S

590–640 700–800 640–690 660–710 750–800

690–740 700–850 740–790 740–790 750–900

VT14

740–760 790–810 640–660

740–760 790–810 640–660

Duplex annealing is admissible: heating at 900–950°C, air cooling; heating up to 550–650°C, holding for 2–8 h, air cooling

Admissible annealing regimes: 1. Heating up to 850°C, holding, cooling within the furnace to 750°C, holding for 30 min, air cooling 2. Heating up to 800°C, holding for 30 min, cooling with furnace to 500°C, then in air Isothermal annealing is admissible: heating up to 790–810°C; holding, cooling within the furnace or transfer to another furnace with temperatures up to 640–660°C, holding for 30 min, air cooling

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Table 109 Annealing regimes of titanium alloys according to the current technical Table 109 (continued). documentation. Alloy

Rods, forgings, stampings, Sheets, thin-walled thick-walled pipes, shapes, and pieces therefrom pipes, shapes, and pieces therefrom

Notes

Annealing temperature, °C Isothermal annealing is admissible: heating up to 950–980°C; holding, cooling within the furnace (with open door) or transfer to another furnace with temperatures up to 530–580°C, holding for 2–12 h, air cooling

VT9

–

Duplex annealing: heating up to 950–980°C, air cooling, holding at 530–580°C for 2–12 h

VT8-1

–

VT25-U

–

VT8

–

VT36

–

VT3-1

–

VT3-1

–

Duplex annealing: heating up to Tpt – (20–40)°C, air cooling; heating at 550– 590°C, holding for 1–4 h, air cooling Duplex annealing: heating up to Tpt – 30°C, air cooling; heating at 530–570°C, holding for 4–6 h, air cooling. The optimal cooling rate after the first annealing stage is 20–60°C/min Duplex annealing: heating at Isothermal annealing is admissible: heating up to 920–950°C, 920–950°C, air cooling, holding (1 h) at 570–600°C holding, cooling within the furnace (with open door) or transfer to another furnace with temperatures of 570–600°C, holding for 1 h, air cooling Heating up to Tpt – 30°C, holding, cooling with furnace to 650°C, holding 5 h, air cooling; heating up to 600°C, holding 15 h, air cooling Isothermal annealing: heating up to 870–920°C, holding, cooling within the furnace (admissible with open door) or transfer to another furnace with temperatures of 630–680°C, holding for 2 h, air cooling Duplex annealing: heating For heavy-load components, annealing at 800–850°C and (870–920°C), air cooling, holding (550–600°C,2–5 h) air cooling are admissible

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Table 109 Annealing regimes of titanium alloys according to the current technical documentation. Table 109 (continued) . Alloy

Rods, forgings, stampings, Sheets, thin-walled thick-walled pipes, shapes, and pieces therefrom pipes, shapes, and pieces therefrom

Notes

Annealing temperature, °C VT23 VT16

750–800 680–790

VT22

740–760

VT22-I

720–750

VT35

760–780

VT32

780–800

VT15

790–810

750–800 770–790

Cooling within the furnace at a rate of 2–4°C/min to 550°C (in vacuum furnaces not higher than 500°C), then air cooling Heating up to 820–850°C, Admissible annealing: heating up to 700–780°C, holding, holding for 1–3 h, cooling cooling with furnace at a rate of within the furnace to 740–760°C, holding for 1–3 2–4°C/min to 450°C, then air h, air cooling; heating up to cooling 500–650°C, holding for 2–6 h, air cooling Heating up to 720–780°C, holding for 1–4 h, cooling within the furnace to 400°C – Recommended annealing in a protective atmosphere at a temperature of 760–780°C, cooling within the furnace to 600–550°C at a rate of 4–8°C/min, then cooling in the furnace – Recommended annealing in a protective atmosphere at a temperatures of 780–800°C, cooling with furnace to a temperature of 600–550°C at a rate of 4–8°C/min, then cooling in the furnace 790–810

To reduce or completely release residual stresses that emerge as a result of mechanical treatment, plastic deformation in a cold state or at temperatures below the recrystallization temperature, welding, and some other technological operations, partial annealing is recommended. Partial annealing temperatures are given in Table 110. Partial annealing is intended to decrease internal stresses by at least 50%.

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Table 110 Holding times at the temperatures of annealing. Maximal cross-section, mm

Holding time, min

Maximal cross-section, mm

Holding time, min

Up to 1.5

15

50.0–100

120

1.5–2.0

20

100–150

180

2.0–6.0

25

150–250

240

6.0–50.0

60

250–300

280

5.2.2 Hardening heat treatment (quenching and aging) Hardening heat treatment, which is of practical importance, is used for titanium alloys with Kβ > 0.2. The accepted regimes for quenching and aging of commercial titanium alloys are given in Table 111. Table 111 Hardening heat treatment regimes. Alloy

VT6S VT6 VT14 VT8, VT9 VT3-1 VT23 VT16 VT22, VT22-I VT35 VT32

Quenching temperature, °C 880–930 850–930 800–910 920–940 840–900 780–800 790–820 720–780 730–780 720–800

Aging regimes T, °C

τ, h

450–550 450–600 480–580 500–600 500–620 450–550 500–550 730–780 730–780 720–800

2–6 2–6 4–16 1–6 1–4 6–10 4–10 4–10 4–10 4–8

The capability of hardening heat treatment in precritical-composition (α+β)-titanium alloys (with Kβ < 1.0) depends totally on the value of Kβ. An important characteristic of hardening heat treatment of titanium alloys is their hardenability. The hardenability of precritical-composition alloys is significantly affected by the quenching temperature. The lower the quenching temperature, the higher the hardenability is, but the effect of hardening is lower. Figure 90 presents the dependence between Kβ of a titanium alloy and the hardenability at a thermal hardening (quenching and aging) for σB = 1200 MPa. The quenching media at hardening heat treatments could be air, water, oil, and other cooling media. The heating times in quenching should be set based on the data of Table 112.

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Table 112 Heating times for semiproducts and pieces. Diameter or thickness, mm

Heating time, min Min

Up to 3 10–20 20–30 30–50 50–70

5.3

5 10 15 20 25

Diameter or thickness, mm

Max 10 20 30 40 50

Heating time, min Min

70–100 100–130 130–160 160–200 200–250

30 40 50 60 80

Max 60 70 80 110 110

WELDING OF TITANIUM ALLOYS

One of the major methods of joining constructions from titanium alloys in mechanical engineering is welding. The effect of the thermal cycle of welding on metal is accompanied by the emergence of a special structural and phase state of a weld, which differs by a certain set of physicochemical properties from the base metal. One of the most widespread and efficient kinds of welding for titanium alloys, which makes it possible to produce various complex constructions is fusion welding. Each particular type of fusion welding – argon arc welding with and without additive, in one or several passes; electron beam welding; submerged welding, etc. – has its specific effect on metal. The effect is expressed in a change of the chemical and phase composition of metal in the welding zone (weld, transition zone, thermal effect zone). In turn, this leads to various physicomechanical properties of the welded joint in various regions. According to the existing views, all solid-solution α-, (α+β)-, and β-titanium alloys weld satisfactorily by fusion welding in a protective atmosphere. The difference in properties, structure, and emerging internal stresses of alloys with the different ratio of α- and β-phases (Kβ) is exhibited in the welded joint in the as-welded state. Alloys based on α-structure and with a small content of β-phase (Kβ < 0.250 have the welded joint, which by its properties and structure is close to the base metal. As the amount of β-phase increases, alloys with (α+β)-structure form a welded joint with a decreased plasticity, which is related to the possible formation of metastable α′- and ω-phases in the welded joint. In alloys based on β-structure, the plastic metastable β-phase is formed in the thermal cycle of welding, and welded joints have mechanical properties close to the quenched base metal. By means of the subsequent heat treatment (annealing or quenching and aging) the mechanical properties and phase composition of welded joints approach those of the base metal. One more important thing is the emergence of residual stresses in welded joints in various titanium alloys. The maximal residual stresses were found to emerge and be preserved in single-phase α- and β- or weakly heterogenized titanium alloys. In

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Hardenability, mm

240 200 160 120 80 40 0

0.2 0.4 0.6 0.8 1.0 Kβ

Figure 91 Dependence of hardenability on the β-phase stabilization coefficient for titanium alloys (Kβ).

(α+β)-alloys during the cooling of the welded joint the larger event is b→α transformation, which releases the residual stresses. Figure 91 schematically shows the dependence of the maximal residual tensile stresses σmax in argon-arc welded joints of 2-mm thick sheets from various alloys depending on the ratio of α- and β-phases in the structure. Residual stresses can be decreased or totally eliminated by heat treatment (annealing or quenching and aging). High residual stresses can be a cause of cracks in the weld if their effect is long-term. Therefore, specifying the time between the welding and heat treatment is recommended; depending on the alloy grade and the method of welding, this time can be within several days to 1–2 months. In welding of titanium alloys, especially high-strength, the amount of harmful impurities occurring in the welded joint is of importance, because in the course of welding the casting metal and the weld transition zone is additionally polluted, especially by gas impurities. The recommended admissible content of impurities should not exceed the limits specified in Table 113. Table 113 Admissible content of impurities in welded joints for some titanium alloys. Alloy

VT1-0 OT4-0 OT4-1 OT4 VT14 VT16 VT22 VT32

Content of impurities, %, no more than O

N

H

C

Si

Fe

0.12 0.15 0.15 0.15 0.15 0.15 0.20 0.15

0.04 0.05 0.05 0.05 0.05 0.05 0.05 0.05

0.010 0.012 0.012 0.012 0.015 0.015 0.015 0.015

0.07 0.10 0.10 0.10 0.10 0.10 0.10 0.10

0.10 0.15 0.15 0.15 0.15 0.15 0.15 0.15

0.20 0.30 0.30 0.30 0.30 0.25 – –

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The recommended annealing temperature of the welded joints for titanium alloys is as follows: Alloy VT1-00, VT1-0 OT4-0, OT4-1, OT4-1V OT4, OT4V VT5-1, VT6, VT6-S VT14 VT3-1 VT16, VT22,VT22-I VT35, VT32 VT23 VT8, VT9, VT18-U VT20

Annealing temperature of weld, °C 550–680 620–700 700–800 750–850 800–830 840–870 870–920

Titanium alloys containing a minor amount of β-phase (up to Kβ = 2.5) are not sensitive to the cooling rate in air. Alloys with Kβ > 0.3 are hardenable and can be embrittled in long-term operation at elevated temperatures and stresses. Welded constructions from these alloys should be cooled slowly after annealing or by a stepwise regime with holdings in certain intervals. To avoid oxidation at annealing temperatures, it is recommended to subject welded constructions to heat treatment in furnaces with a protective atmosphere. In practice, welded sheet constructions from titanium are often subjected to partial annealing at lower temperatures. This annealing could be done in air without the risk of considerable oxidation of the surface. Partial annealing does not release the residual stresses completely, but stabilizes the α- and β-structures to a sufficient extent. Below, we present temperatures at which partial annealing in air should be done. Alloy VT1-00, VT1-0, OT4-0 OT4-1, OT4-1V, OT4 OT4-V VT5-1, VT14, VT16 VT23 VT3-1, VT6, VT6-S VT8, VT9, VT20 VT18-U VT22, VT22-I, VT35 VT32

Temperature of partial annealing of the weld, °C 450–520 520–585 530–620 550–620 580–650 620–680

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Duration of partial annealing is chosen to be within the limits of 1–4 h depending on the alloy, complexity of pieces or an item and value of internal stresses. In annealing of welded constructions in furnaces with protective atmosphere the duration of annealing for a more compete release of residual stresses can be increased. The extent of releasing residual stresses in welded joints of various alloys depending on the annealing temperature of a weld done by argon-arc automatic welding by non-consumable electrode is given in Table 114. Table 114 Residual stresses vs annealing temperatures for titanium alloys with different Kβ. Alloy

OT4, OT4-V VT14,VT16 VT22 VT32

Release of residual stresses, % of initial value, after annealing for 2 h at a temperature, °C 400

450

500

550

600

650

700

750

30 20 15 15

45 30 20 20

60 45 35 35

70 60 55 50

80 75 60 60

95 80 70 75

100 95 85 85

100 100 100 100

As it follows from the table, there is a tendency to preserve residual stresses in welded joints as the content of β-phase in the alloy increases. Still, it can be noted that irrespective of the type of alloy about 50% of residual stresses is released after annealing at 500–550°C, and practically the entire 100% after annealing at 700–750°C. Annealing to release residual stresses is recommended also in vacuum furnaces or in furnaces with a protective atmosphere. In this case, the annealing temperature can be slightly increased and more residual stresses can be released. The recommended temperatures of vacuum annealing of welded joints and constructions from titanium alloys are as follows (annealing for 1–2 h). Alloy VT1-0, OT4-0, OT4-1V, OT4, OT4-V VT6-S, VT6, VT14, VT16 VT3-1, VT8, VT9, VT22, VT35, VT32 VT5-1, VT20, VT18-U

Annealing temperature, °C 600–670 600–700 600–720 600–750

Wherever possible, after vacuum annealing it is recommended to subject the surface to shotblasting or vibration cold hardening. The purpose of these treatments is to increase fatigue and repeated static-load resistance of vacuum annealed pieces and constructions. Thermal hardening of welded joints of titanium alloys, which consists of quenching and aging (tempering) is used when the ultimate strength of an annealed welded joint is insufficient. It should be noted that the use of thermal hardening on the welded joint is more

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limited as compared with the base metal. The large-grain needle-like structure of cast metal is poorly susceptible to the thermal hardening, i.e., does not ensure a sufficient combination of strength and plasticity after quenching and aging. For welded joints, a “soft” thermal hardening is used, which acts to increase the ultimate strength by 10–20% as compared with that of the annealed state. Several technologies for thermal hardening of welded joints of titanium alloys have been developed in the recent years. These technologies somewhat broaden the possibilities of using heat treatments in welded constructions. A widely used procedure is when the base metal is hardened, and the weld is made thicker and is annealed by a local heat treatment. This technique yields a construction of equal strength both with respect to the base metal and to the welded joint with the sufficiently high performability. The recommended regimes of thermal hardening of welded joints (welded constructions) from various titanium alloys are given in Table 115. Table 115 Heat treatments of welded joints from titanium alloys. Alloy

Quenching temperature, °C

VT6S VT6 VT14 VT3-1 VT23 VT16 VT22, VT22-I VT35 VT32

Aging regime T, °C

850–900 850–900 850–900 850–900 780–800 790–820 720–780 730–780 720–800

τ, h

450–500 450–500 510–540 520–620 500–550 520–590 510–600 520–600 500–560

2–4 2–4 4–16 1–4 6–10 4–10 4–10 4–10 4–8

Holding times at a quenching temperature of welded joints after a given temperature is reached should be set within the following limits. Minimal cross-section, mm Holding time, min

1.5 5

1.6–2.0 7

2.1–4.0 10

4.1–10.0 25

10.0 60

Hypercritical-composition titanium alloys can be quenched at rather low cooling rates of less than 4–6°C/min. A technology was developed to enable thermal hardening of welded constructions in protective-atmosphere furnaces without their transfer to a quenching medium. In this process, large-size nonrigid welded constructions can be fabricated unoxidized and unwarped. For instance, welded high-strength constructions from alloy VT22 are treated as off-the-shelf, fixed in the building ways in special argon-arc furnaces, according to the regime: 830°C, 1 h; cooling in the furnace to 750°C; holding for 2 h; cooling within the furnace to 600°C; holding for 4 h. This heat treatment of welded items

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from alloy VT22 ensures UTS ≥ 1150 MPa, δ ≥ 6%, ψ ≥ 16%. This technology is even more efficient for welded constructions from alloys VT35 and VT32. Designer calculations for welded constructions should take into account the coefficient of weld weakening as compared with the base metal. The coefficient considers the imperfections of the casting structure of the weld, admissible welding defects (pores, decrease of casting-metal thickness, pollution of the weld with impurities, etc.). Below, we present approximate values of this coefficient for various titanium alloys in automatic argon-arc welding of up to 3-mm-thick as-annealed sheets without filler. Alloy

Recommended designer’s value of weld weakening, % to the base metal

VT1-0,OT4-0, OT4-1, OT4-1V, OT4, OT4-V, VT5-1 VT6, VT6-S, VT14, VT16 VT8-1, VT8, Vt9, VT3-1, VT20, VT22, VT23, VT35, VT32

100 95 90

In welding with filler, which, as a rule, is less strong than the base metal, the value of weld weakening can increase.

5.4 SURFACE ENGINEERING OF TITANIUM ALLOYS Long-term experience of using titanium alloys in mechanical engineering has shown that the state of metal surface has a significant effect on its physicomechanical and chemical properties. This effect is so large that it should be taken into account practically at all stages of fabrication. Titanium and titanium alloys are distinguished with a high chemical activity with atmospheric gases at elevated temperatures. An increased liability of adhesion both in friction and machining or other operations associated with the contact at elevated temperatures and pressure, low thermal conductivity of titanium contribute to the emergence of high residual tensile stresses on the surface during the machining, especially grinding without sufficient release of heat. This leads to a significant decrease of the fatigue characteristics and low-cycle endurance of metal. In this connection, it is expedient to consider several particular features of titanium alloys occurring in fabrication of pieces and constructions. 5.4.1 Interaction of titanium with atmospheric gases during heating First visual signs of oxidation (heat colors) are manifested in titanium alloys at temperatures of about 300–350°C. Herewith, no changes in the properties of metal

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Hµ, kgf/mm2

400

480 240 120 60

300

0

Hµ, kgf/mm2

1000 800 600

480 240 120 60 30 15

350

(a) 0.025 0.05 0.10 0.050 0 Distance from surface, mm

480 240

1100 1000 900 800

120

700

60 30 15

600 500 400

400 200

(c) 0

0.2

0.4

(b) 0.15

240 120 60 30 15

300 200

0 0.6 0.4 0.2 Distance from surface, mm

(d) 0.6

0.8

1.0

Figure 92 Hardness of titanium surface layers after heating by various regimes (numbers on the curves are heating times in min): (a) heating up to 700°C, (b) 800°C, (c) 900°C, (d) 1000°C.

are found. In practice, short-time heating of metal (30–60 min) at temperatures up to 500–600°C is admissible even for thin sheets without the obligatory removal of scales. This heat treatment is used in partial annealing of titanium alloys. The further increase of temperature and time of heating is accompanied by coarsening of scales and the onset of diffusion of oxygen and nitrogen into the surface layers of metal. At temperatures higher than 650–700°C, diffusion of atmospheric gases becomes rather significant. The increase in the content of gas impurities in the surface layer of metal is accompanied by the increase of hardness and can be revealed by measuring its value. Figure 92 presents the results of hardness measurements for the surface layers of technical-grade titanium VT1-0 after heating in air at various heating temperatures and times. The gas-polluted layer has the most negative effect on the fatigue strength characteristics that are sensitive to the quality of surface. Some mechanical properties (σB, σ0.2, δ, ψ) are not strongly affected by the occurrence of a gas-polluted layer, but the cyclic strength characteristics – fatigue and low-cycle endurance – are rather sensitive to cyclic loads. Table 116 presents

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the changes of the mechanical properties of annealed sheets from alloy VT14 after heating in air at various temperatures for 1 h. Table 116 Properties of 2 mm-thick sheets from annealed alloy VT14 as a function of heating temperature (τ = 1 h). Temperature, °C

Properties

Ultimate strength, MPa Yield strength, MPa Relative elongation based on 11.3 Γ 0 , % Bending angle, degrees at t = 1.5 thickness Crack sensitivity at bending impact, kgf/cm2 Fatigue limit based on 107 cycles, MPa, no less than

Initial

550

650

750

850

9940 9360 8.7

9910 9400 7.9

9960 9250 8.7

9804 9200 7.2

9300 8750 6.0

68 6.6 38

68 6.8 38

65 6.5 38

58 6.3 32

49 5.4 29

The gas-polluted layer has a similar effect on the properties of other titanium alloys. As it follows from the data of Table 116, heating at temperatures up to 650°C does not significantly affect such surface state-sensitive characteristics as bending tests, fatigue, resistance to repeated static loads, and others. Heating at higher temperatures fails to decrease the plasticity characteristics in tension and impact strength tests, for example, but significantly worsens fatigue and low-cycle endurance in repeated static loads. Therefore, when titanium alloys are heated higher than 650–700°C, it is necessary to remove scales and the gas-polluted layer to increase the service life of pieces and constructions. The most widespread methods for removing scales and the gas-polluted surface layer are shot- or sandblasting and chemical etching. The deeply gas-polluted layer formed in long-term heating at high temperatures is removed by machining. In practice, operation of pieces, especially from high-strength alloys (σB ≥ 1100 MPa), with a sufficiently deeply gas-polluted layer was observed to lead to their premature breakdown. Therefore, it is considered that the hardness of the surface layer of a piece should not be increased more than 15–20% as compared with its core.

5.4.2 Effect of mechanical treatment on titanium alloy castings High sensitivity of titanium to the state of surface requires advertency to its finish. Machining of titanium is largely similar to that of structural stainless steels. Low thermal conductivity of titanium and its increased liability to sticking to the cutting tool imposes a certain effect on its machinability. Thus, for instance, in grinding by abrasive disks titanium and titanium alloys are rather liable to burns, which is mainly due to their low heat conduction. Burns have a rather negative effect on the resistance of metal to repeated static and fatigue loads of ground pieces. This is explained by the possibility of formation of microcracks at the site of a burn or the

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emergence of high residual tensile stresses. Therefore, the machining operations should be strictly regulated. Machining should be done by carbide-tipped tools with increased wear resistance. The requirement of a strict regulation of the machining regimes is believed to increase the labor intensity of machining 2.0–2.5-fold as compared with common carbon steels. Table 117 presents the endurance limits for specimens from alloy VT3-1 (rod 16 mm in diameter) with different surface conditions at optimal regimes of lathing, grinding, and polishing of titanium alloys. Table 117 Endurance limits based on 2 . 107 cycles for alloy VT3-1 as a function of surface condition. Kind of machining Lathing Lathing Lathing Lathing + grinding Lathing + grinding Lathing + grinding + polishing Lathing + grinding + polishing

Surface finish class

σ–1, MPa

∇4 ∇5 ∇6 ∇7 ∇8 ∇9 ∇10

300 320 330 370 400 430 480

As it follows from the data presented in Table 117, the endurance characteristics continuously increase as the surface condition of the specimens improves. It should be taken into account that not only the surface condition but also the other machining parameters can have an effect on the cyclic endurance characteristics. Thus, for instance, the use of cooling liquids, especially at the final stage of machining, or formation of burns can greatly change the behavior of metal in cyclic strength tests. In this case, a determining factor is the value and character of residual stresses on the metal surface.

5.4.3 Surface hardening of titanium alloy items Surface hardening of titanium items and constructions is a rather efficient method to increase their quality and durability. The aim of surface hardening is, on the one hand, to increase the surface condition and, on the other, to optimally redistribute residual stresses on the surface of metal. Various surface hardening techniques can be used for treatment of pieces from titanium alloys. These are techniques that cause a significant plastic deformation of the surface layer without removing metal: vibration cold hardening, pneumatic cold hardening, hydraulic shotblast cold hardening, ball and roll burnishing and rolling. Other techniques that cause plastic deformation and provide removal of metal are also used: hydraulic sandblasting, sandblasting, vibration polishing.

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The choice of the hardening technique is determined in each particular case not only by the requirements of surface condition and optimization of surface residual stresses, but also by the design and process features of the hardened pieces. Sandblasting and hydraulic sandblasting, vibration polishing, vibration cold hardening, and shotblasting can be used for treatment of pieces of virtually any shape. Pneumodynamic cold hardening is recommended for pieces of comparatively simple shape. Roll burnishing is used for hardening the outer surfaces of bodies of rotation, etc. Various methods of hardening make it possible to obtain various degrees of surface finish on titanium alloys, including rather high ones. Herewith, the billet to be subjected to surface hardening, should be satisfy certain requirements. Pieces to be subjected to vibration cold hardening, pneumodynamic cold hardening, hydraulic sandblasting, sandblasting, and vibration polishing should have a surface finish not lower than ∇4; those to be subjected to burnishing and rolling, not lower than ∇6. Pneumodynamic and vibration treatments provide a surface finish of ∇6–∇ 7; hydraulic sandblasting, ∇6 at the initial surface finish of ∇5–∇6. However, the main purpose of surface hardening should be considered to be the development of contracting stresses on the surface of a piece, which would significantly increase both the service life and reliability at the action of cyclic loads (Table 118). Table 118 Effect of various surface hardening treatments on the low-cycle fatigue of alloy VT22 (smooth specimen). Hardening technique

Number of cycles to breakdown at σmax = 840 MPa

No hardening, grinding ∇7 Sandblasting treatment by corundum sand Hydrosandblasting by quartz sand Vibration cold hardening, pneudodynamic cold hardening

8000–25000 35000–780000 108600–288300 ≥600000

A similar effect of surface hardening is also observed on other titanium alloys. Table 119 presents the effect of roll burnishing on the endurance limit of rods from alloy VT22 16 mm in diameter with ultimate strength of 1200 MPa. The tests were done by pure bending with frequency n = 3000 cycles per minute. Table 119 Effect of roll burnishing on the endurance limit of alloy VT22. Shape of specimen Smooth Notched (αR = 1.6)

State of surface

Endurance limit based on N = 107, MPa

Grinding, ∇8 Roll burnishing Lathing, ∇8 Roll burnishing

450 530 350 450

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References 1. Sokolikov, K.I. and Moiseyev, V.N. (1958) Hot rolling of technical-grade titanium and some of its alloys. In Titanium and its Alloys. Moscow: USSR Academy of Sciences Publishers, pp. 162–179 (in Russian). 2. Moiseyev, V.N. and Luzhnikov, L.P. (1960) Diffusion of gases into titanium during its heating in the air and the effect of diffusing gases on the mechanical and process properties of sheet titanium. In Titanium and its Alloys, Issue III. Moscow: USSR Academy of Sciences Publishers, pp. 17–22 (in Russian). 3. Glazunov, S.G., Moiseyev, V.N., and Mikhailov, B.M. (1964) Alloy VT15 clad with non-alloyed titanium. In Applications of Titanium Alloys. Moscow: Department of Scientific and Technical Information, All-Russian Institute of Aircraft Materials, pp. 62–89 (in Russian). 4. Glazunov, S.G., Khorev, A.I., and Moiseyev, V.N. (1964) Effect of plastic deformation on the structure and mechanical properties of welded joints of titanium alloys VT14, VT15, and VT16. Svarochn. Proizvodstvo 10: 27–28 (in Russian). 5. Glazunov, S.G., Moiseyev, V.N., and Mikhailov, B.M. (1966) Titanium-clad high-temperature titanium alloys. Tsvetn. Metally 5: 80–82 (in Russian). 6. Evstronova, E.I., Kalugin, V.F. et al. (1969) Effect of deformation and heat treatment on the properties of sheets from alloy VT14. Production of titanium alloys. Moscow: All-Russian Institute of Light Alloys, No 5, pp. 35–42 (in Russian). 7. Moiseyev, V.N., Sholokhova, L.V., and Terent’eva, L.N. (1969) Thermomechanical treatment of titanium alloy OT4. Production of titanium alloys. Moscow: All-Russian Institute of Light Alloys, No 5, pp. 152–155 (in Russian). 8. Moiseyev, V.N. and Znamenskaya, E.V. (1971) Effect of fine structure on the mechanical properties of high-strength titanium alloy VT14. Tekhnol. Lyogkikh Splavov 2: 58–62 (in Russian). 9. Matveyenko, A.F., Moiseyev, V.N., and Kalugin, V.F. (1972) Effect of plastic deformation and heat treatment on the properties of sheets from alloy VT16. In Titanium Alloys. Moscow: Department of Scientific and Technical Information, All-Russian Institute of Aircraft Materials, p. 211 (in Russian). 10. Moiseyev, V.N., Znamenskaya, E.V., and Povarov, I.A. (1975) Formation of structure and properties of structural titanium alloys in isothermal forging. Tekhnol. Lyogkikh Splavov 6: 61–66 (in Russian). 11. Prokhodtseva, L.F., Drozdovsky, B.A., Moiseyev, V.N. et al. (1974) Anisotropy of the mechanical properties of sheets from titanium–aluminum–manganese alloys. Problemy Prochnosti 5: 81 (in Russian). 12. Rodionov, V.L. et al. (1975) Studies of the optimal modes of deformation in the β-region and of heat treatment of forgings from titanium alloy VT22. In Production of Titanium Alloys. Moscow: All-Russian Institute of Light Alloys, No 7, pp.164–169 (in Russian). 13. Voiteshenko, L.G., Moiseyev, V.N., Povarov, I.A. et al. (1975) Precise-shape isothermal forging from structural titanium alloys. In Hot Forging. Moscow: Research Institute of Aviation Technology, pp. 35–38 (in Russian). 14. Moiseyev, V.N., Povarov, I.A., Voiteshenko, L.G. et al. (1975) Formation of structure and properties of titanium alloys in isothermal deformation. In Hot Forging under Isothermal Conditions. Moscow: Research Institute of Aviation Technology, pp. 30–35 (in Russian). 15. Rodionov, V.L., Matveyenko, A.F., and Kleymenov, V.Ya. (1975) Studies of the optimal modes of deformation in the β-region and heat treatment of forgings from titanium alloy VT22. In Production of Titanium Alloys. Moscow: All-Russian Institute of Light Alloys, No 7, pp. 164–169 (in Russian). 16. Matveyenko, A.F., Moiseyev, V.N., Kalugin, V.F. et al. (1977) Effect of plastic deformation and heat treatment on the properties of sheets from alloy VT15. In Titanium Alloys. Moscow: Department of Scientific and Technical Information, All-Russian Institute of Aircraft Materials, pp. 181–189 (in Russian).

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17. Prokhodtseva, L.V., Moiseyev, V.N., Drozdovsky, B.A. et al. (1980) Effect of hot deformation modes on the structure and breakdown characteristics of alloy VT22 with ultimate strength of 120 kgf/mm2. Tekhnol. Lyogkikh Splavov 8: 57–60 (in Russian). 18. Moiseyev, V.N., Povarov, I.A., and Kaplin, Yu.I. 91984) Structure and properties of titanium alloys after isothermal deformation at small rates. Metalloved. Term. Obrabotka Metallov 5: 43–46 (in Russian). 19. Glazunov, S.G., Moiseyev, V.N., and Tarasenko, G.N. (1961) Sheet thermally strengthened titanium alloys. In Applications of Titanium Alloys. Moscow: Department of Scientific and Technical Information, All-Russian Institute of Aircraft Materials, pp. 14–26 (in Russian). 20. Glazunov, S.G. and Moiseyev, V.N. (1961) Heat treatment, structure and properties of alloy VT14. In Titanium in Industry. Moscow: Oborongiz, pp. 232–244 (in Russian). 21. Moiseyev, V.N. and Geras’kova, L.V. (1965) Change of structure and properties of titanium alloys depending on heat treatment. Metalloved. Term. Obrabotka Metallov 5: 3–9 (in Russian). 22. Moiseyev, V.N. (1966) Heat treatment and properties of titanium alloys with molybdenum and aluminum. In Applications of Titanium Alloys. Moscow: Department of Scientific and Technical Information, All-Russian Institute of Aircraft Materials, pp. 5–21 (in Russian). 23. Moiseyev, V.N. (1965) Change of structure and properties of a+β-titanium alloys depending on heat treatment. In New Research into Titanium Alloys. Moscow: Nauka, pp. 198–205 (in Russian). 24. Moiseyev, V.N. and Sholokhova, L.V. (1967) Strengthening heat treatment of alloy OT4. In Applications of Titanium Alloys, Part 1. Moscow: Department of Scientific and Technical Information, All-Russian Institute of Aircraft Materials, pp. 51–57 (in Russian). 25. Moiseyev, V.N. (1967) Thermal production process in enterprises of the industry. Voronezh, pp. 23–30 (in Russian). 26. Moiseyev, V.N. and Terent’eva, L.N. (1977) Air-hardening titanium alloy. Tekhnol. Lyogkikh Splavov 4: 46–51 (in Russian). 27. Moiseyev, V.N. (1977) Prospects of the development of strengthening heat treatment of titanium alloys. Metalloved. Term. Obrabotka Metallov 10: 63–68 (in Russian). 28. Moiseyev, V.N., Znamenskaya, E.V., and Tarasenko, G.N. (1977) Effect of structure and heat treatment on the properties of high-strength titanium alloys. Metalloved. Term. Obrabotka Metallov 5: 38–42 (in Russian). 29. Spektr, Ya.I., Moiseyev, V.N., and Smirnov, A.M. (1977) Strengthening heat treatment of items and constructions from hypercritical-composition titanium alloys under vacuum and argon. In Technology of Aircraft Production. Moscow: Research Institute of Aviation Technology, pp. 114–117 (in Russian). 30. Antipov, A.I., Moiseyev, V.N., and Moder, N.I. (1996) Strengthening of titanium alloy VT35 in aging. Metalloved. Term. Obrabotka Metallov 12: 2–5 (in Russian). 31. Bratukhin, A.G., Ivanov, Yu.L., Mar’in, B.N. et al. (1967) Forging, welding, soldering, and heat treatment. Moscow: Mashinostroenie, 595 pp. with illustrations (in Russian). 32. Tretyakov, F.E. (1967) Fusion welding of titanium and its alloys. Moscow: Mashinostroenie, p. 143 (in Russian). 33. Shorshorov, M.Kh. (1965) Physical metallurgy of welding of titanium alloys and steel. Moscow: Nauka (in Russian). 34. Grabin, V.F. (1975) Basics of physical metallurgy and heat treatment of welded joints from titanium alloys. Kiev: Naukova Dumka (in Russian). 35. Gurevich, S.M., Kulikov, F.R., Zamkov, V.N. et al. (1975) Welding of high-strength titanium alloys. Moscow: Mashinostroenie (in Russian). 36. Moiseyev, V.N., Kulikov, F.R., and Kirillov, Yu.G. (1978) Welded joints of titanium alloys. Moscow: Metallurgiya (in Russian). 37. Kulikov, F.R. et al. (1969) Properties of welded joints of high-strength titanium alloy VT22. In Production of Titanium Alloys. Moscow: All-Russian Institute of Light Alloys,

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No 5, pp. 35–42 (in Russian). 38. Moiseyev, V.N. and Kulikov, F.R. (1973) On the weldability of titanium alloys. Aviatsion. Promyshlennost 12: 70–73 (in Russian). 39. Kirillov, Yu.G., Kulikov, F.R., Moiseyev, V.N. et al. (1977) Properties of welded joints of high-strength titanium alloy VT22. In Titanium Alloys. Moscow: Department of Scientific and Technical Information, All-Russian Institute of Aircraft Materials, pp. 159–170 (in Russian). 40. Moiseyev, V.N., Kulikov, F.R., Kirillov, Yu.G. et al. (1978) Welded joints of titanium alloys (structure and properties). Moscow: Metallurgiya (in Russian). 41. Moiseyev, V.N. (1968) Sheet titanium alloys for forged/welded constructions. Aviatsion. Promyshlennost 8 (Appendix): 4–8 (in Russian). 42. Glazunov, S.G., Moiseyev, V.N., and Danilov, Yu.S. (1965) Deformed welded titanium alloys. Moscow: Sov. Entsiklopediya, pp. 330–333 (in Russian). 43. Kulikov, F.R., Kirillov, Yu.G., Moiseyev, V.N. et al. (1977) Properties of welded joints of high-strength titanium alloy VT22. In Titanium Alloys. Moscow: Department of Scientific and Technical Information, All-Russian Institute of Aircraft Materials, pp. 159–170 (in Russian). 44. Kulikov, F.R., Kirillov, Yu.G., Vas’kin, Yu.V. et al. (1972) Welding of large-size vital components from alloy VT22 of large thicknesses. Aviatsion. Promyshlennost 2: 48–52 (in Russian). 45. Moiseyev, V.N. and Kulikov, F.R. (1973) On the weldability of titanium alloys. Aviatsion. Promyshlennost 12: 70–73 (in Russian). 46. Gorshkov, A.I., Moiseyev, V.N., Filatova, T.V. et al. (1979) Arc welding of alloy VT16. Aviatsion. Promyshlennost 10: 61–64 (in Russian). 47. Moiseyev, V.N., Kirillov, Yu.G., and Zakharov, Yu.I. (1983) Heat treatment optimization of welded joints of high-strength titanium alloy VT22. Proc. II All-Russian Conf. on Welding of Nonferrous Metals. Tashkent, October, pp. 341–347 (in Russian). 48. Moiseyev, V.N. and Luzhnikov, L.P. (1960) Diffusion of gases into air-heated titanium or effect of diffusing gases on the mechanical and process properties of sheet titanium. In Titanium and its Alloys, Issue 3. Moscow: USSR Academy of Sciences, pp. 17–22 (in Russian). 49. Glazunov, S.G., Khorev, A.I., and Moiseyev, V.N. (1964) Construction strength of thermally strengthened titanium alloys. Metalloved. Term. Obrabotka Metallov 6: 2–5 (in Russian). 50. Glazunov, S.G., Moiseyev, V.N., and Mikhailov, B.M. (1964) Alloy VT15 clad with nonalloyed titanium. In Applications of Titanium Alloys. Moscow: Department of Scientific and Technical Information, All-Russian Institute of Aircraft Materials, pp. 62–69 (in Russian). 51. Prokhodtseva, L.V., Drozdovsky, B.A., Moiseyev, V.N. et al. (1974) Anisotropy of the mechanical properties of sheets from titanium–aluminum–manganese alloys. Problemy Prochnosti 5: 81 (in Russian). 52. Moiseyev, V.N., Znamenskaya, E.V., and Tarasenko, G.N. (1975) Assessment of the serviceability of titanium alloys with ultimate strength over 150 kgf/mm2. In Production of Titanium Alloys. Moscow: Department of Scientific and Technical Information, All-Russian Institute of Light Alloys, No 7, pp. 64–72 (in Russian). 53. Moiseyev, V.N., Ponomarev, A.P., Sholokhova, L.V. et al. (1977) Effect of the state of surface on the characteristics of serviceability of sheets from titanium alloys OT4 and VT14. In Titanium Alloys. Moscow: Department of Scientific and Technical Information, All-Russian Institute of Aircraft Materials, pp. 307–310 (in Russian). 54. Moiseyev, V.N. and Spektr Ya.I. (1979) Strengthening heat treatment of components and constructions from hypercritical-composition titanium alloys in a protective atmosphere. Aviatsion. Promyshlennost 5: 47 (in Russian). 55. Moiseyev, V.N. and Spektr, Ya.I. (1983) Strengthening heat treatment of titanium alloys in furnaces with protective atmosphere. Metalloved. Term. Obrabotka Metallov 10: 23–26 (in Russian).

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56. Moiseyev, V.N., Sysoyeva, N.V., and Ishun’kova, T.V. (1995) Granular metallurgy of high-strength titanium alloys. Metalloved. Term. Obrabotka Metallov 6: 28–30 (in Russian). 57. Sulimova, A.M. and Evstigneyev, M.I. (1974) Quality of the surface layer and fatigue strength of items from high-temperature and titanium alloys. Moscow: Mashinostroenie (in Russian). 58. Solonina, O.P. and Glazunov, S.G. (1976) High-temperature titanium alloys. Moscow: Metallurgiya, pp. 368–387 (in Russian).

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Applications of Titanium and Titanium Alloys

A combination of high specific strength, heat resistance, corrosion resistance, and other positive properties open broad vistas for applications of titanium and its alloys in various industries. The largest consumer of titanium alloys is the aerospace industry. In the recent decade, titanium and its alloys have been increasingly more used in chemical machine building, shipbuilding, the auto industry, in equipment for the oil and gas industry, the food industry, medicine, and civil engineering. Titanium applications in Russian industries are schematically shown in Fig. 93. The latest data for the United States are presented in Table 120. In aviation, the important characteristics of titanium alloys are mainly their high specific strength and heat resistance; a major attraction for other industries is their high corrosion resistance.

Aerospace

Shipbuilding

Other industries

Figure 93 Applications of titanium in Russian industries.

Table 120 Latest data for U.S. titanium applications. Application Commercial aircraft jet engines Military aircraft jet engines Commercial airframes Military airframes Space rocketry Helicopters and armaments Other industries

Percentage 31 20 15 10 7 1 16

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Of great importance for commercial applications of titanium alloys is the cost of titanium semiproducts.

6.1 TITANIUM ALLOYS IN THE AIRCRAFT INDUSTRY The major requirements to the materials for the aircraft industry are their characteristics of specific strength and heat resistance, fatigue resistance, crack resistance, and sufficient corrosion resistance. Of great importance is the processibility of titanium alloys in manufacturing aircraft items and components – their plasticity in deformation, weldability, and machinability. The efficiency of commercial titanium alloys in the aircraft industry with respect to steels and aluminum alloys is compared in Table 121. Table 121 Characteristics of the most common structural steels, aluminum, and titanium alloys. Steels and alloys

Aluminum alloys Titanium alloys Steels

AK4-1 D16T V95 OT4 VT6 VT22 30KhGSA EI643 VNS-2 VNS-5

Density, g/cm3

2.80 2.87 2.85 4.55 4.45 4.55 7.85 7.81 7.76 7.82

Ultimate tensile strength, σB, MPa 420 450 520 800 900 1100 1100 1300 1250 1450

Specific static strength, cm×105

Endurance limit, σ–1, MPa based on 2×107 cycles

Specific endurance of smooth specimens, MPa, cm3/g

15.0 16.2 18.2 17.5 20.0 24.2 14.0 23.0 16.0 18.5

135 150 165 420 520 620 600 750 620 720

48 54 58 92 116 134 77 96 80 92

Titanium alloys noticeably surpass other structural materials by specific characteristics, especially fatigue strength. Substitution of titanium alloys for steels and aluminum alloys reduces the weight of aircraft constructions and gives a higher weight efficiency, which is especially important in aircraft industry. Titanium alloys are an indispensable material in supersonic aircraft, when aluminum alloys are not usable due to low heat resistance, and steels are too heavy. Figure 94 presents a steady-state distribution of temperatures along the contour of an aircraft flying at an altitude of 20 km at a speed equal to three sound velocities. The temperature of the skin in this case reaches 240–315°C. In these conditions, titanium alloys are the preferred material.

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316 260

302

246 288

302

197

246°C

Figure 94 Steady temperatures on the surface of an aircraft at the flight speed of three sound velocities (altitude, 20 km).

Relative change of weight of construction, %

80 1

60 40

2

20

3

0 −20

4

5 −40 20 93 204 316 427 538 T,°C

Figure 95 Change of weight of aircraft constructions from different materials as a function of operational temperature (as compared with the weight of the construction from aluminum alloy V95 intended for operation at 20°C): 1, high-alloyed steels (σB = 1230 MPa); 2, corrosion-resistant steels (σB = 1550 MPa); 3, steel 30KhGSNA (σB = 1610 MPa); 4, hot-die steels (σB = 1970 MPa); 5, titanium-based alloys (σB = 1260 MPa).

Figure 95 shows how the weight of flying vehicles manufactured from various materials can be changed depending on the operational temperature. The weight of constructions from aluminum alloy 7075 (of the type of V95) operated at room temperature is given for comparison. The data show that the titanium-alloy construction has the smallest weight up to a temperature of about 430°C. By the scale of use of titanium alloys, aircraft could be divided into two groups: subsonic and supersonic. The major structural material of the airframe in subsonic aircraft is aluminum, which accounts for 65% (by weight) of the airframe. Steel and titanium are about 20%, and the rest are mainly nonmetals. In the near future, the use of titanium in subsonic and transport aircraft is expected to increase up to 15% of the airframe weight. For supersonic aircraft, especially flying at speeds higher than Mach 2, aluminum as a basic material rapidly loses its significance and the share of titanium increases up to 40–95% of the airframe weight.

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Figure 96 Welded aircraft constructions fabricated from titanium alloy with σB ≥ 70 kgf/mm2.

Figure 97

Machining of a large-size titanium piece.

In Russian passenger aircraft – IL-76, IL-86, IL-96, TU-204, YAK-42, AN-124, AN-74, etc. – titanium is within the limits of 4–8% of the airframe weight, and in the supersonic TU-144 it is up to 12%. Titanium alloys are used in the airframe for such components and constructions as the skin; wing frame; components of fasteners, chassis, wing mechanization; pylons, hydraulic cylinders, various aggregates, etc. Titanium alloys are efficiently used in helicopters, mainly for components of main rotor and drive, and also the control system. Main rotor hubs, tail rotor hubs, pivots, clamps, bodies of axle hinges, and blade tips are the major items produced from titanium alloys. The use of titanium in helicopter manufacturing reduces the weight of the components by 35–40% as compared with steel.

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Figure 98 Landing gear beam from alloy VT22 for IL76 airliner. Dimensions, 1200×1400 mm.

Figure 100

199

Figure 99 Nose landing gear unit for IL76 airliner. Weight, 130 kg (diameter, 360 mm; height, 900 mm).

A welded aircraft pipeline unit from titanium alloy OT4 (σB ≥ 700 MPa).

In aircraft and helicopter, titanium can substitute for almost all steel items. The weight gain can be up to 20–35%. As compared with steel, a drawback of titanium is its lower elasticity modulus and lower friction properties. Some applications of titanium alloys in Russian aircraft industry are shown in Figs. 96–100. Since the 1970s, Russian manufacturers have widely used a titanium alloy VT22 (Ti–5% Al–5% Mo–5% V–1% Fe–1% Cr, σB ≥ 1100 MPa) for highload components and units. Until recently, Western aircraft industries have been oriented to a mid-strength titanium alloy Ti–6% Al–4% V with σB = 900–950 MPa. Boeing recently began to use in passenger aircraft high-strength titanium alloys of the type of Ti–3% Al–10% V–2% Fe comparable by its weight efficiency to VT22.

6.2 TITANIUM IN ENGINE MANUFACTURING Engine manufacturing is one of the first and most promising applications of titanium. By the 1970s, titanium used in Russian aircraft-engine manufacturing reached 50% of the metal produced. It should be noted that in the recent years the consumption of

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Figure 102 An aircraft engine stator unit fabricated by welding from sheets of titanium alloy OT4 with σB ≥ 70 kgf/mm2.

Figure 101 An aircraft engine compressor piece from alloy VT3-1.

Decrease of weight, %

25 20

1

15

2

10 3 5 0

3

Figure 103 Economy of weight achieved in the case of using titanium for engines: 1, large-size; 2, medium-size; 3, small-size.

titanium for military aircraft engines has dropped against the background of an increase in passenger aircraft engines. This increase can be explained by the greater use of gas turbine engines for passenger aircraft and the accumulated experience of titanium production and application, increased level of properties, and reliability of titanium alloys. It is especially profitable, from the point of view of weight gain, to use titanium in a new variety of gas turbine engines, namely, in turbofan engines, as they are the most advanced for passenger airliners and large transport aircraft. Production of these engines in recent years has been considerably increased and demonstrates a tendency of a further increase. A relative decrease of titanium consumption for military aircraft can be explained not only by smaller-scale production, but also by the fact that military aircraft require engines developing supersonic speeds, i.e., turbojet engines which use less titanium.

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Titanium alloys are used mainly in fan and compressor assemblies – disks, blades, guides, spacer rings, engine body, various engine-body parts, air collectors, and other items (Figs. 101 and 102). Together with heat-resistant titanium alloys operated at temperatures of up to 500–550°C or even 600°C, structural titanium alloys are used, which can be successfully operated at temperatures of up to 300–350°C. The use of structural titanium alloys, especially for large-size fan disks and blades, is preferable as they have a better ratio of strength, plasticity, and crack resistance as compared with heat-resistant titanium alloys. According to the data from various sources, the use of titanium alloys in aircraft engines instead of steel reduces the weight of components by 30–40%. The economy in weight in this case will depend on purpose and design. Figure 103 presents a graph of the economy in weight from the use of titanium alloys depending on the purpose of the engine (aircraft speed) and its size.

6.3 TITANIUM IN ROCKET MANUFACTURING The rocket industry is, as the aircraft industry, a considerable titanium consumer. Titanium alloys were widely used in the Russian piloted spacecraft Vostok and Soyuz, in the unpiloted Luna, Mars, and Venera, and in later space systems Energiya and the orbiter Buran. The use of titanium in these spacecraft is efficient with respect to the weight gain, especially in multistage rockets. The major applications of titanium are solid-fuel and liquid-propellant rocket engines, skin, black-powder engine casings, tubular constructions of stage sections, various aggregates, in particular, high-pressure gas vessels, fasteners, etc. A requirement for titanium alloys in these items is high specific strength, and in some cases low cold shortness, high elasticity of vapors in deep vacuum, and other properties. Rocket production makes use of almost all structural titanium alloys. A feature of using titanium alloys in the rocket industry is that they can be used with higher strength than in the aircraft industry. As a rule, this is due to shorter operation times of rocket constructions as compared with aircraft constructions. Some applications of titanium alloys in rocket manufacturing are given in Figs. 104 and 105. Figure 104 presents a correction solid engine from titanium alloy VT22

Figure 104 Correction solid engine from alloy VT22.

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Figure 105 Torsion assembly for the Lunokhod moon robot from titanium alloy VT22.

(σB = 1300 MPa), and Fig. 105 shows a torsion bar of the Lunokhod moon robot from the same alloy. Alloy VT23 was widely used in spars and frames in the orbiter Buran.

6.4 TITANIUM IN SHIPBUILDING The Russian shipbuilding industry is also a major titanium consumer. Here titanium alloys are used mainly as materials resistant to corrosion in a marine environment. The items manufactured from them include ships’ skins, propellers, heat exchangers, and other equipment. Low- and mid-strength titanium alloys are used, which are well weldable by all types of welding and have a satisfactory plasticity. There is a special range of titanium alloys for shipbuilding, which includes alloys PT-7M, PT-3V, etc., with the ultimate strength, as a rule, not exceeding the guaranteed limit of 800 MPa.

6.5 TITANIUM IN THE CHEMICAL ENGINEERING INDUSTRY AND OTHER FIELDS In Russian industries, titanium alloys are used in chemical engineering, heavy engineering, power engineering, and transport engineering, in the light and food industries, medicine, and in the manufacturing of home appliances. Standard equipment from titanium alloys is mainly manufactured by chemical engineering plants. The range of these items is rather broad and includes various valves, ball and reverse cocks, pumps, tanks, columns, and special-purpose devices. Various tanks are widely used in many industries for storing, mixing, crystallization, and heat treatment. Most often, titanium alloys are used for construction of towers; adsorbers; extensions of bubbling, plate-type, and fractionating columns operated in contact with acidic media (pH 3.5); acid vapors; acetic and hydrochloric acids. Heat-exchange equipment is widely used in many industries for heating, boiling, evaporating, condensing, and cooling various aggressive media. Working aggressive media can be liquid, paste, solid, gaseous, and vapor. Coil, helical, shell-and-tube, double-pipe, etc., types of heat exchangers are used. Titanium alloys are the most acceptable material to increase corrosion resistance and to keep the walls sufficiently thin for the purpose of efficient heat exchange.

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Besides, titanium alloys are less susceptible to erosion and precipitation on their surface, which increases the heat exchange coefficient in operation of heat exchange equipment. Automatic titanium filter presses FPAKM, produced commercially, are intended for filtration of aggressive media with a temperature of 278–353°C, with the content of suspended particles from 5 up to 600 g/cm3, forming a precipitate with a large hydraulic pressure. Components of filter presses in contact with aggressive media are manufactured from titanium alloys. Many plants of chemical engineering, nonferrous metallurgy, and other industries produce titanium equipment for their own needs. The use of titanium is widespread in many items with rotating components – centrifugal machines, separators, driers, compressors, etc. The expediency of using titanium alloys is determined by technical and economical considerations. In some cases, titanium alloys with high specific strength should be used, for example, in production of long working blades for steam turbines (1000–1200 mm). The use of titanium alloys for the working blades of less than 1000 mm, usually manufactured from steel, reduced the load on the stressed rotor of the low-pressure turbine cylinder and increases the reliability of the construction on the whole. The long-term operation of titanium blades in the turbines of the St. Petersburg metallurgical plant showed good performance of this material. Mass production of turbines with the titanium blades for the last stages has also been organized in other Russian turbine manufacturing plants. To increase the power rating of car engines, the weight of the components of the reciprocating systems should be reduced without the loss of strength. The use of high-strength heat-resistant titanium alloys successfully solves this problem. In the automobile-making industry, titanium alloys should primarily be used for manufacturing such components as connecting rods, inlet and outlet valves, valve rockers, and exhaust mufflers. Their use for connecting rods reduces the load on the connecting rod ball bearings by 30%, which considerably increases their reliability and life. The experience of using titanium alloys shows that they are most expedient in high-load components: mid-strength alloys are recommended for load-carrying structures of automobiles; mid- and high-strength alloys, for the undercarriage; mid-strength and heat-resistant, for the engine components. It is known that titanium alloys possess a high cold resistance – their mechanical properties at low temperatures do not change significantly, which makes them applicable in borehole perforators operated in mines in the North. Titanium alloys are especially efficient in cryogenic engineering. Titanium has been intensively used in the recent years by light industry and food-machine building plants to produce various machines and equipment from titanium: painting and finishing equipment, packaging equipment, centrifuges with titanium separators, kitchenware. By the volume of production, the leading place is occupied by the metallurgical industry, which manufactures and operates equipment from titanium in the production of nickel, cobalt, and copper.

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Figure 106

The monument in honor of the first Earth satellite in Moscow.

The most aggressive conditions of hydrometallurgical production of nickel and cobalt are characteristic of electrolysis and chlorine–cobalt shops and sections of nickel salts production. A list of titanium equipment produced includes various filters, autoclaves, heat exchangers, evaporation apparatuses, templates, chlorine ejectors, and tanks. Severonickel Plant and Norilsk Mine-and-Mill Complex introduced integrated automation of hydrometallurgical processes based on the development of a complex of titanium-alloy process equipment and components and items of automated systems using titanium. The use of titanium equipment in copper production is determined by the high corrosion resistance of titanium alloys in most media of copper electrolyte and sulfuric acid productions. The Mednogorsk copper–sulfur combine widely uses equipment made from titanium instead of lead, which was earlier used for complex shapes of acid pipelines, components of pumps, electric filters. Titanium electrodes in wet electric filters serve for 8–12 years, and lead electrodes, for 3–4 years. Titanium alloys are also widely used for equipment in production of lead, zinc, tungsten, antimony, precious metals, aluminum, in the processing of nonferrous metals and in ferrous metallurgy. Besides the above fields of titanium applications, we should also mention medicine, sports equipment, architecture, and civil engineering. One of the valuable properties of titanium is its biological compatibility with live cell tissue. Therefore, various titanium implants in surgery and dentistry are

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rather promising. However, not all titanium alloys are applicable for these purposes. For instance, such a widespread alloy as VT6 (Ti–6% Al–4% V) is not recommended; preference is given to titanium alloys containing no aluminum. Even more interesting materials for implants are chemical compounds – titanium aluminides and nickelides. A rapidly expanding field of titanium use is sports equipment – bicycles and mountain accessories. Over 2000 tons of titanium was used for heads of golf clubs in the United States in 1996. One of the consumers of titanium can be monumental-type architecture. In Russia, for instance, at least two large monuments are made from titanium – in honor of the first Earth satellite and the first cosmonaut in Moscow (Fig. 106). Titanium is successfully used as armor material, e.g., for the physical protection of individuals. The titanium alloy Ti–6% Al–4% V is 30–45% more efficient by weight than rolled armored steel or aluminum. A obstacle for wide application of titanium alloys in some fields is their high cost.

References 1. Bratukhin, A.G., Ivanov, Yu.L., Marvin, B.N. et al. (1999) Modern aircraft industry technologies. Moscow: Mashinostroenie (in Russian). 2. Bratukhin, A.G., Kolachev, B.A., Sadkov, V.V. et al. (1995) Production process for titanium aircraft constructions. Moscow: Mashinostroenie (in Russian). 3. Novozhilov, E.V., Mishin, V.I., Kulikov, F.R. et al. (1996) Particular features of designing and experience of manufacturing vital components of heavy aircraft from high-strength titanium alloy. In Processing of Light and Special-purpose Alloys. Moscow: All-Russian Institute of Light Alloys, pp. 43–55 (in Russian). 4. Moiseyev, V.N. (2001) Titanium and titanium alloys. In Nonferrous Metals and Alloys (Encyclopedia). Moscow: Mashinostroenie, Vol. II-3, pp. 272–353 (in Russian). 5. Kolachev, B.A. et al. (2001) Titanium alloys in aircraft engines and aerospace components. Moscow: Moscow Aviation Institute (in Russian). 6. Moiseyev, V.N. (2001) High-strength titanium alloy for manufacturing fixing components by cold deformation. Metalloved. Term. Obrabotka Metallov 2: 8–32 (in Russian). 7. Moiseyev, V.N. (2000) High-strength titanium alloys for large-size items of aircraft engine. Metalloved. Term. Obrabotka Metallov 2: 34, 35 (in Russian). 8. Solonina, O.P. and Glazunov, S.G. (1976) High-temperature titanium alloys. Moscow: Metallurgiya, pp. 424–440 (in Russian). 9. Glazunov, S.G. and Moiseyev, V.N. (1976) Structural titanium alloys. Moscow: Metallurgiya, pp. 354–361 (in Russian). 10. Chechulin, B.B., Ushakov, S.S., Razuvayev, I.N. et al. (1977) Titanium alloys in mechanical engineering. Leningrad: Mashinostroenie, p. 248 (in Russian). 11. Klimov, V.T. and Sadkov, V.V. (1998) Titanium alloys in passenger aircraft constructions. Titan 1(10): 10–15 (in Russian). 12. Balabuyev, P.V. (1998) Titanium alloys in Antonov aircraft components. Titan 1(10): 15–19 (in Russian). 13. Moiseyev, V.N. (2002) Role of the All-Russian Institute of Aviation Materials in the

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development of the first aircraft from titanium. In Aviation Materials. Moscow: MISIS–VIAM, pp.100–110 (in Russian). 14. Khorev, A.I., Moiseyev, V.N. et al. (1962) Preparation of solid-rolled 335-mm diameter pipes from titanium alloys VT14 and VT15. In Applications of Titanium Alloys. Moscow: Department of Scientific and Technical Information, All-Russian Institute of Aircraft Materials, pp. 130–136 (in Russian). 15. Petrakov, A.F., Lavochkina, L., Moiseyev, V.N. et al. (1962) Pressure vessels from high-strength titanium alloy VT14. Aviatsion. Promyshlennost 9: 60–61 (in Russian). 16. Moiseyev, V.N. and Shor, E.S. (1964) Properties and applications of high-strength titanium alloys VT14, VT15, and VT16. Moscow: State Institute of Sci. & Techn. Information, pp. 1–8 (in Russian). 17. Terekhov, E.I, Glazunov, S.G., Moiseyev, V.N. et al. (1968) Spherical tanks from high-strength titanium alloys. Aviatsion. Promyshlennost 3: 26, 27 (in Russian). 18. Glazunov, S.G., Chinenov, Khorev, A.I. et al. (1968) Titanium alloys for drill pipes. In Applications of Titanium Alloys, Part 2. Moscow: Department of Scientific and Technical Information, All-Russian Institute of Aircraft Materials, pp. 253–260 (in Russian). 19. Glazunov, S.G., Moiseyev, V.N., Matveyenko, A.F. et al. (1969) Large-size aircraft components from alloys VT14 and VT22. Aviatsion. Promyshlennost 5: 3–6 (in Russian). 20. Sivets, V.N., Glazunov, S.G., Cherkasov, N.N. et al. (1973) Experience of using titanium alloys in aircraft industry. Aviatsion. Promyshlennost 11: 53–56 (in Russian). 21. Glazunov, S.G., Moiseyev, V.N., and Cherkasov, N.N. (1984) Titanium and its alloys in aviation. Aviatsion. Promyshlennost 1 (Appendix): 9–14 (in Russian). 22. Moiseyev, V.N. and Terent’eva, L.N. (1984) Titanium alloys for aircraft fixing components. In Aviation Materials, Titanium Alloys. Moscow: Department of Scientific and Technical Information, All-Russian Institute of Aircraft Materials, pp. 93–99 (in Russian). 23. Bratukhin, A.G., Anoshkin, N.F., and Moiseyev, V.N. (1993) Use of titanium alloys in aircraft constructions. Titan 1: 77–81 (in Russian). 24. Moiseyev, V.N. (2000) High-strength titanium alloys for large-size components of an aircraft motor. Metalloved. Term. Obrabotka Metallov 2: 34–36 (in Russian). 25. Moiseyev, V.N. (2001) High-strength titanium alloy VT16 for manufacturing fixing components by cold deformation. Metalloved. Term. Obrabotka Metallov 2: 28–32 (in Russian). 26. Moiseyev, V.N. (2002) High-strength titanium alloys in aircraft industry. Tekhnol. Lyogkikh Splavov 4: 115–121 (in Russian). 27. Moiseyev, V.N. (2002) Half a century of Russian titanium. Natsionaln. Metallurgiya 3: 25–29 (in Russian). 28. Bratukhin, A.G. (1998) Weldable titanium alloys in Russian aircraft industry. Titan 1: 3–10 (in Russian). 29. Mikheyev, S.V., Akin’shin, V.I., Bayev, A.S. et al. (1998) Experience of using titanium alloys in Kamov helicopters. Titan 10: 20–23 (in Russian). 30. Bratukhin, A.G. et al. (1996) Use of titanium alloys in constructions of passenger and heavy transport aircraft. Titan 1: 52–59 (in Russian).

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Subject Index Alloys for cast shapes 171 Alloy OT4 67–69 Alloy OT4-0 58–60 Alloy OT4-1 64–66 Alloy OT4-V 69–71 Alloy PT-3V 71–73 Alloy PT-7M 60–62 Alloy ST5 82–84 Alloy ST6 98–100 Alloy VT1-0 55–58 Alloy VT1-00 52–54 Alloy VT3-1 121–126 Alloy VT5-1 73–77 Alloy VT6 84–86 Alloy VT6S 77–80 Alloy VT8 126–130 Alloy VT8-1 138–141 Alloy VT9 131–134 Alloy VT14 86–91 Alloy VT15 96–98 Alloy VT16 91–96 Alloy VT20 80–82 Alloy VT22 100–103 Alloy VT22I 103–105 Alloy VT23 105–107 Alloy VT25I 135–138 Alloy VT32 107–110 Alloy VT35 110–112 Alloy VT36 145–147 Alloy VT37 112–114 Annealing 176–180 Chemical compounds 17–20 Classification by strength 50–51 Classification by structure 48–49

Classification of high-temperature alloys 120–121 Corrosion-resistant alloys 156–157 Effect of impurities 5–8 Effect of mechanical treatment 188–189 Enhancement of high-temperature strength 36–42 Enhancement of mechanical strength 29–35 Eutectoid-based alloys 162 Formation of structure 71–174 Functional-purpose alloys 150–156 Hardening heat treatment 180–181 Heat treatment 174–176 High-ductility alloys 51–52 High-strength alloys 84–118 High-temperature alloys 119–120 Interaction with elements 9–13 Martensite-type alloys 121 Medium-strength alloys 67–83 Oxidation in heating 186 Phase transformations 25–28 Solid α- and β-solutions 13–17 Stability of solid α- and β-solutions 22–25 Structural alloys 46–51 Surface engineering 186 Technological properties of alloys 171–174 Titanium aluminides 157–159 Titanium nickelide 160–161 Welding 181–186