Shape Memory and Superelastic Alloys: Technologies and Applications (Woodhead Publishing in Materials)

Shape memory and superelastic alloys

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Shape memory and superelastic alloys Technologies and applications

Edited by K. Yamauchi, I. Ohkata, K. Tsuchiya and S. Miyazaki

Oxford

Cambridge

Philadelphia

New Delhi

© Woodhead Publishing Limited, 2011

Published by Woodhead Publishing Limited, 80 High Street, Sawston, Cambridge CB22 3HJ, UK www.woodheadpublishing.com Woodhead Publishing, 1518 Walnut Street, Suite 1100, Philadelphia, PA 19102-3406, USA Woodhead Publishing India Private Limited, G-2, Vardaan House, 7/28 Ansari Road, Daryaganj, New Delhi – 110002, India www.woodheadpublishingindia.com First published 2011, Woodhead Publishing Limited © Woodhead Publishing Limited, 2011 The authors have asserted their moral rights. This book contains information obtained from authentic and highly regarded sources. Reprinted material is quoted with permission, and sources are indicated. Reasonable efforts have been made to publish reliable data and information, but the authors and the publisher cannot assume responsibility for the validity of all materials. Neither the authors nor the publisher, nor anyone else associated with this publication, shall be liable for any loss, damage or liability directly or indirectly caused or alleged to be caused by this book. Neither this book nor any part may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, microfilming and recording, or by any information storage or retrieval system, without permission in writing from Woodhead Publishing Limited. The consent of Woodhead Publishing Limited does not extend to copying for general distribution, for promotion, for creating new works, or for resale. Specific permission must be obtained in writing from Woodhead Publishing Limited for such copying. Trademark notice: Product or corporate names may be trademarks or registered trademarks, and are used only for identification and explanation, without intent to infringe. British Library Cataloguing in Publication Data A catalogue record for this book is available from the British Library. ISBN 978-1-84569-707-5 (print) ISBN 978-0-85709-262-5 (online) The publisher’s policy is to use permanent paper from mills that operate a sustainable forestry policy, and which has been manufactured from pulp which is processed using acid-free and elemental chlorine-free practices. Furthermore, the publisher ensures that the text paper and cover board used have met acceptable environmental accreditation standards. Typeset by Toppan Best-set Premedia Limited, Hong Kong Printed by TJI Digital, Padstow, Cornwall, UK

© Woodhead Publishing Limited, 2011

Contents

Contributor contact details Preface

Part I Properties and processing 1

Mechanisms and properties of shape memory effect and superelasticity in alloys and other materials: a practical guide K. Tsuchiya, National Institute for Materials Science, Japan

xi xv

1

3

1.1 1.2 1.3 1.4 1.5 1.6

Introduction Properties of shape memory alloys (SMAs) Fundamentals of shape memory alloys (SMAs) Thermodynamics of martensitic transformation Conclusions References

3 4 5 12 13 14

2

Basic characteristics of titanium–nickel (Ti–Ni)based and titanium–niobium (Ti–Nb)-based alloys S. Miyazaki and H. Y. Kim, University of Tsukuba, Japan

15

2.1 2.2 2.3 2.4 2.5

Introduction Titanium–nickel (Ti–Ni)-based alloys Titanium–niobium (Ti–Nb)-based alloys Conclusions References

15 16 29 40 41

3

Development and commercialization of titanium–nickel (Ti–Ni) and copper (Cu)-based shape memory alloys (SMAs) K. Yamauchi, Tohuku University, Japan

3.1 3.2

Introduction Research on titanium–nickel (Ti–Ni)-based shape memory alloys (SMAs)

43 43 43 v

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Contents

3.3

Research on copper (Cu)-based shape memory alloys (SMAs) Conclusions References

3.4 3.5 4

Industrial processing of titanium–nickel (Ti–Ni) shape memory alloys (SMAs) to achieve key properties T. Nakahata, Sumitomo Metal Industries Ltd, Japan

48 49 52

53

4.1 4.2 4.3 4.4 4.5

Introduction Melting process Working process Forming and shape memory treatment References

53 54 58 60 62

5

Design of shape memory alloy (SMA) coil springs for actuator applications T. Ishii, Sogo Spring Mfg Co. Ltd, Japan

63

5.1 5.2 5.3 5.4 5.5

Introduction Design of shape memory alloy (SMA) springs Design of shape memory alloy (SMA) actuators Manufacturing of shape memory alloy (SMA) springs Reference

63 63 68 69 76

6

Overview of the development of shape memory and superelastic alloy applications S. Takaoka Furukawa Electric Co. Ltd, Japan

77

6.1 6.2 6.3 6.4

Introduction History of the applications of titanium–nickel (Ti–Ni)based shape memory and superelastic (SE) alloys Other shape memory alloys (SMAs) Examples of the main applications of titanium–nickel (Ti–Ni)-based alloys

Part II Application technologies for shape memory alloys (SMAs) 7

7.1

77 79 81 82

85

Applications of shape memory alloys (SMAs) in electrical appliances T. Habu, Furukawa Techno Material Co. Ltd, Japan

87

Introduction

87

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7.2 7.3 7.4 7.5

Automatic desiccators Products utilizing shape memory alloys (SMAs) Electric current actuator Reference

87 88 94 99

8

Applications of shape memory alloys (SMAs) in hot water supplies A. Suzuki, Daido Steel Co. Ltd, Japan

100

8.1 8.2 8.3

Shower faucet with water temperature regulator Gas flow shielding device Bathtub adaptors

100 103 103

9

The use of shape memory alloys (SMAs) in construction and housing M. Ozawa, NEC TOKIN Corporation, Japan, A. Suzuki, Daido Steel Co. Ltd, Japan and T. Inaba, Nishimatu Construction Co. Ltd, Japan

110

9.1 9.2 9.3 9.4 9.5

Introduction Underground ventilator Static rock breaker Easy-release screw Acknowledgements

110 111 112 116 119

10

The use of shape memory alloys (SMAs) in automobiles and trains T. Kato, Piolax Inc., Japan

120

10.1 10.2 10.3 10.4 10.5 10.6

Introduction Shape memory alloys (SMAs) in automobiles Oil controller in Shinkansen Steam trap Conclusions References

120 120 121 122 124 124

11

The use of shape memory alloys (SMAs) in aerospace engineering T. Ikeda, Nagoya University, Japan

125

11.1 11.2 11.3

Introduction Development and properties of CryoFit (Aerofit, Inc.) Development and properties of Frangibolt (TiNi Aerospace, Inc.)

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125 126 128

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Contents

11.4

Development and properties of Pinpuller (TiNi Aerospace, Inc.) Development and properties of variable geometry chevrons (VGC) (The Boeing Company) Development and properties of hinge and deployment of lightweight flexible solar array (LFSA) on EO-1 (NASA and Lockheed Martin Astronautics) Development and properties of rotating arm for material adherence experiment (MAE) in Mars Pathfinder mission (NASA) References

11.5 11.6

11.7

11.8 12

12.1 12.2 12.3 12.4 12.5 12.6 12.7 12.8

Ferrous (Fe-based) shape memory alloys (SMAs): properties, processing and applications T. Maruyama, Awaji Materia Co. Ltd, Japan and H. Kubo, Kanto Polytechnic University, Japan Introduction Iron–manganese–silicon (Fe–Mn–Si) shape memory alloys (SMAs) Shape memory effect of iron–manganese–silicon (Fe–Mn–Si) alloy Mechanical properties of iron–manganese–silicon (Fe–Mn–Si) shape memory alloys (SMAs) Proper process for shape memory effect Applications of iron–manganese–silicon (Fe–Mn–Si) shape memory alloys (SMAs) Future trends References

Part III Application technologies for superelastic alloys 13

13.1 13.2

14

Applications of superelastic alloys in the telecommunications, industry T. Habu, Furukawa Techno Material Co. Ltd, Japan Introduction Products utilizing superelastic alloys in the telecommunications industry Applications of superelastic alloys in the clothing, sports and leisure industries T. Habu, Furukawa Techno Material Co. Ltd, Japan

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130 131

134

137 139

141

141 142 145 146 149 153 156 158

161

163 163 163

169

Contents 14.1 14.2

15

15.1 15.2 15.3 15.4 15.5 15.6. 15.7 15.8 15.9

Introduction Products utilizing superelastic alloys in the clothing, sports and leisure industries

ix 169 169

Medical applications of superelastic nickel–titanium (Ni–Ti) alloys I. Ohkata, Piolax Medical Devices Inc., Japan

176

Introduction Hallux valgus Orthodontic wire Guide wire Biliary stents Regional chemotherapy catheter Endoscopic guide wire Device for onychocryptosis correction References

176 176 178 179 183 187 191 195 196

Appendix: History of the Association of Shape Memory Alloys K. Shimizu, Osaka University, Japan

197

Index

201

© Woodhead Publishing Limited, 2011

Contributor contact details

(* = main contact)

Editors K. Yamauchi Innovation of New Biomaterial Engineering Center Tohoku University 1-1 Seiryo-machi, Aoba-ku Sendai Japan E-mail: [email protected]. ac.jp I. Ohkata Piolax Medical Devices Inc. 179 Kariba-cho Hodogaya-ku Yokohama 240-0025 Japan

K. Tsuchiya Hybrid Materials Center National Institute for Materials Science Sengen 1-2-1 Tsukuba Ibaraki 305-0047 Japan E-mail: [email protected] S. Miyazaki* Institute of Materials Science University of Tsukuba Tsukuba Ibaraki 305-8573 Japan E-mail: [email protected] [email protected]

E-mail: [email protected]

xi © Woodhead Publishing Limited, 2011

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Contributor contact details

Chapter 1

Chapter 4

K. Tsuchiya Hybrid Materials Center National Institute for Materials Science Sengen 1-2-1 Tsukuba Ibaraki 305-0047 Japan

T. Nakahata Sumitomo Metal Industries Ltd Shimaya 5-1-109, Konohana-ku Osaka 554-0024 Japan E-mail: nakahata-tkj@ sumitomometals.co.jp

E-mail: [email protected]

Chapter 5 Chapter 2 S. Miyazaki* and H. Y. Kim Institute of Materials Science University of Tsukuba Tsukuba Ibaraki 305-8573 Japan

T. Ishii Sogo Spring Mfg Co. Ltd 2-3-24, Yoshioka-Higashi Ayase-City Kanagawa 252-1125 Japan E-mail: takashi_ishii@sogospring. co.jp

E-mail: [email protected] [email protected]

Chapter 6 Chapter 3 K. Yamauchi Innovation of New Biomaterial Engineering Center Tohoku University 1-1 Seiryo-machi, Aoba-ku Sendai Japan

S. Takaoka Furukawa Electric Co. Ltd Special Metal Sales Div. Metal Company 5-1-8, Higashi-Yawata Hiratsuka-City Kanagawa 254-0016 Japan E-mail: [email protected]

E-mail: [email protected]. ac.jp

© Woodhead Publishing Limited, 2011

Contributor contact details

Chapter 7, 13 and 14 T. Habu Furukawa Techno Material Co. Ltd Engineering & Development Section 5-1-8, Higashi-Yawata, Hiratsuka-City Kanagawa 254-0016 Japan E-mail: [email protected]

Chapter 8

xiii

A. Suzuki Daido Steel Co. Ltd. 2-30 Daido-cho Minami-ku Nagoya 457-8545 Japan E-mail: [email protected] T. Inaba Nishimatu Construction Co. Ltd 2570 Shimotsuruma Yamato Kanagawa 242-8520 Japan E-mail: tsunomu_inaba@ nishimatsu.co.jp

A. Suzuki Daido Steel Co. Ltd 2-30 Daido-cho Minami-ku Nagoya 457-8545 Japan

Chapter 10

E-mail: [email protected]

Chapter 9 M. Ozawa* NEC TOKIN Corporation 7-1, Koriyama 6-Chome Taihaku-ku Sendai 982-8510 Japan E-mail: [email protected]

T. Kato Piolax Inc. 179 Kariba-cho Hodogaya-ku Yokohama 240-0025 Japan E-mail: [email protected]

Chapter 11 T. Ikeda Composite Engineering Research Center Graduate School of Engineering Nagoya University Furo-cho, Chikasa-ku Nagoya 464-8603 Japan E-mail: [email protected]

© Woodhead Publishing Limited, 2011

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Contributor contact details

Chapter 12

Chapter 15

T. Maruyama Awaji Materia Co. Ltd 2-3-13 Kanda-Ogawamachi Chiyoda-ku Tokyo 101-0052 Japan

I. Ohkata Piolax Medical Devices Inc. 179 Kariba-cho Hodogaya-ku Yokohama 240-0025 Japan

E-mail: t.maruyama@awaji-materia. co.jp

E-mail: [email protected]

H. Kubo Kanto Polytechnic University 612-1 Mitake Yokokura Oyama 323-0810 Japan

Appendix K. Shimizu Osaka University 1500-601, Ohitomi-chou Nishi-ku, Hamamatsu Shizuoka Japan E-mail: [email protected]

© Woodhead Publishing Limited, 2011

Preface

It has been a long time since shape memory alloys (SMAs) drew attention as functional materials possessing quite fascinating properties such as shape memory effect (SME) and superelasticity (SE), which are not possessed by ordinary metals. Nearly half a century has passed since the discovery of Ti–Ni SMA in 1963. After developing numerous applications of SMAs, some of them have grown up to be important world standard devices, though many of them ended up as just ideas or as transiently used applications disappearing from the market. This book has been written by members of the Association of Shape Memory Alloys (ASMA), who have worked on research and development of SMAs in addition to their practical use and industrialization. The book sums up the results of applications most of which have appeared on the market from the dawn of the research and development. In this book, we have tried to include as many examples as possible of applications with their key points of ideas, features and commercial performance. The book introduces not only successful applications but also unsuccessful ones which could not be commercialized simply because of bad timing. This book is naturally divided into three parts: research and development, fundamentals and production technologies (Part I), application technologies for SMAs (Part II) and application technologies for superelastic alloys (SEAs) (Part III). Part I covers mechanisms and properties of SME and SE for practical users (Chapter 1), basic characteristics of Ti–Ni based and Ti–Nb based SM/SE alloys (Chapter 2), development of alloy production technology (Chapter 3), industrial processing and device elements (Chapter 4), design of SMA actuators (Chapter 5) and an overview on the development of SM and SE applications (Chapter 6). Part II introduces SMA applications: SMAs in electrical applications (Chapter 7), SMAs in hotwater supply (Chapter 8), SMAs in construction and housing (Chapter 9), SMAs in automobiles and railways (Chapter 10), SMAs in aerospace engineering (Chapter 11) and Fe-based SMAs (Chapter 12). Part III focuses on xv © Woodhead Publishing Limited, 2011

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Preface

SEA applications: SEAs in telecommunications, and other areas (Chapter 13), SEAs in clothing, sports and leisure (Chapter 14) and SEAs in medical applications (Chapter 15). In the appendix, the history and activity of ASMA, which promotes the development and researches of SMAs, is briefly described. Throughout the book, many photographs of application products utilizing SMA or SEA are shown for the readers to grasp the images of the applications. Since many of the photographs are not published in any journals or books and were obtained through personal connections of each author, in many cases no specific references are cited. The authors are greatly appreciative of the generosity of the owners for allowing use of the photographs in this book. The editors hope this book will be useful for readers to access precious information on SM/SE alloys and their applications as well as applying SMAs and SEAs for readers’ application development. They also express their heartfelt thanks to the team of distinguished contributors who have been working on SMAs and SEAs in companies and universities for many years. Finally, they express special thanks to the staff at Woodhead Publishing Limited for their assistance. K. Yamauchi I. Ohkata K. Tsuchiya S. Miyazaki

© Woodhead Publishing Limited, 2011

Appendix: History of the Association of Shape Memory Alloys K. SHIMIZU, Osaka University, Japan

The Association of Shape Memory Alloys, the editorial committee responsible for this book, was established in 1993 in order to promote further research and development in the areas of fundamentals and applications of shape memory alloys (SMAs). In this appendix, the circumstances of the establishment, as well as the recent activities, of the association will be introduced in brief. As is well known, in 1963, Dr Buehler’s group at the US Naval Ordinance Laboratory recorded the appearance of a unique phenomenon of the shape memory effect (SME) in a familiar Ti–Ni alloy, although a similar phenomenon had already been observed in unfamiliar Au–Cd and In–Tl alloys in 1951 and 1954, respectively. Because of its remarkable uniqueness, the SME was immediately investigated to discover potential applications in the manufacture of the machine parts of industrial products and even in living essentials, mainly in the United States and the Netherlands. However, the results of this investigation could not be developed for practical use. After a while, in about 1970, the Raychem Corporation in the United States developed a Cryofit coupling and an electrical pin-and-socket contact made of the Ti–Ni SMA and offered them for sale in large quantities; the former coupling was typically applied in the production of the aircraft hydraulic tubing of the US F-14 fighter plane. On the other hand, at about the same time, the fundamental mechanism of the SME was investigated and clarified in relation to a thermoelastic martensitic transformation in ordered alloys. During the investigation, researchers discovered another unique phenomenon, super elasticity (SE), which was closely related to the SME. Since then, the SME and SE phenomena have been found not only in Ti–Ni alloys but also in many noble metal base and other alloys. In 1975, the first international symposium on SMAs was held at Toronto, Canada, and many academic and technological researchers participated from all over the world. By 1980, the US patented right for the processing and technological applications of Ti–Ni SMAs had lapsed, and extensive research had developed globally concerning the fundamentals and applications of Ti–Ni base, noble metal base and other SMAs. In the circumstances mentioned above, two organizations were established in Japan to promote the development research into the fundamentals 197 © Woodhead Publishing Limited, 2011

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Shape memory and superelastic alloys

and applications of SMAs; one (a semi-governmental organization) was a ‘Committee for Promotion of Applications of SMAs’ formed in Osaka Science and Technology Center in 1982, and the other (a private organization) was a ‘Research Cooperative Union for Processing of SMAs’ formed by six manufacturing companies of SMAs in 1983. The former organization, ‘Committee for Promotion of Applications of SMAs’, consisted of members from typical universities, governmental research institutes and manufacturing companies, and had conducted extensive investigations into the existing circumstances of the development research for the processing and applications of SMAs and of the test characterization methods for those SMAs. The organization had further examined the subjects and systems required to promote this research and development. As a result, the first work of the organization was the standardization of the terminology relating to SMAs used in academic and technological fields and of the methods of measuring martensitic transformation temperatures. After a little while, this organization was succeeded by a ‘Committee for SMAs’, which was a subdivision of the Committee for Investigations on Standardization of Test Characterization Methods on New Materials used as Electric Power Source instead of Petroleum, which was specially established in the Osaka Science and Technology Center with governmental support as one of the national projects intended to activate research into the various kinds of functional materials that were newly developed about that time. The investigation work carried out in the ‘Committee for SMAs’ effectively took over from that of the previous ‘Committee for Promotion of Applications of SMAs’. Thus, six items relating to the standardization of terminologies and test characterization methods of SMAs were established via careful discussion in the Japan Industrial Standards (JIS) Committee, which were JIS H7001, JIS 7101, JIS H7103, JIS H7104, JIS H7105 and JIS H7106; the first two were based on the investigations of the previous committee. After the establishment of the six JISs, the ‘Committee for SMAs’ was closed in 1992. The latter organization, the ‘Research Cooperative Union for Processing of SMAs’, was formed by six manufacturing companies and it was fortunately able to obtain a governmental grant-in-aid to carry out improvement work on processing technologies for SMAs under the national support system for the development of industrial technology research. Three of the six manufacturing companies were those which produced Ti–Ni SMAs. In alphabetical order, these were: Daido Special Steels, Furukawa Electric Industry and Tohoku Kinzoku (now NEC TOKIN). The other three companies were manufacturers of Cu–base SMAs: Dowa Mining, Mitsubishi Metals and Sumitomo Special Metals. All the companies had investigated individually and/or cooperatively with governmental

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support for various topics relating to SMAs, such as the minimization of inclusions, control of martensitic transformation temperatures, improvement of SMA and SE characteristics due to the addition of a third and/or forth element and due to thermomechanical treatments, and so on. Thus, those companies could be successful in producing various products (lines, pipes, plates, thin foils and coil springs) from SMAs with better SME and/ or SE characteristics, and the products were supplied for application in the manufacture of various industrial machine parts and medical interposition devices in the human body. The switch of a dry box, the sensor flap of an air conditioner, a medical stent and many others, as have been introduced in this book. This private organization had been acting in close cooperation with the former semi-governmental organization, contributing to the establishment of the above-mentioned six JISs for the SMAs. It was closed in 1993, having obtained the expected results to some extent although, with very few exceptions, noble metal-based SMAs could not be supplied for practical uses because of their being to some degree unsuitable for such applications. The above two organizations were closed as mentioned, but some members of those organizations had promptly advocated the establishment of another new organization. Ti–Ni SMAs had attracted an increasing amount of attention, not only among professional workers but also among ordinary people worldwide, and research and development into the fundamentals and applications of SMAs were required to promote them more extensively and strongly than before. Thus, the Association of Shape Memory Alloys (ASMA) was established in October 1993, as mentioned at the beginning. At the start, the ASMA was constituted of several individual members and of six supporting members from industrial companies. These were, in alphabetical order: Daido Special Steels, Furukawa Electric Industry, Kato Spring (now Piolax), Mitsubishi Material, NEC TOKIN and Sogo Spring. The ASMA has expanded little by little, and it now consists of 39 individual members and of nine supporting members from industrial companies. The individual members join the association voluntarily from universities and other research institutes, and the supporting members typically come from nine companies in the manufacturing industries, coil spring makers and those working with applications of Ti–Ni and ferrous SMAs. The ASMA is operated under a board of trustees and a general meeting, the former being constituted of one president, one secretary-general, eight trustees, one inspector and one counselor. The objective of the ASMA is to promote progress and development in the science and technology of SMAs and to contribute to the development of related industries. In order to achieve this objective, the following projects have been enforced:

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• • •

investigation and research on SMAs and their trusts; holding of lectures and research meetings on SMAs; publication of journals and other documents on the activities of the ASMA; • connection and cooperation with other related academic and technological organizations in domestic and foreign countries; • other necessary projects in order to achieve the objective of the ASMA. These projects, as well as the annual budget for them, are planned and implemented via discussion among the board of trustees and the general meeting. After its establishment in 1993, the ASMA has actively carried out various projects, such as the holding of short courses and symposiums on SMAs every year, the publication of a newsletter and of summary booklets of patent reports and new models for practical uses, financial support to several domestic and international meetings on SMAs, the holding of the Japan–China Bilateral Symposium on SMAs (1997), the organization of the International Conference on Shape Memory and Superelastic Technology (2007) and many others. The ASMA has also produced a standardization work on the Ti–Ni SMA wire itself, JIS H7107, in addition to some corrections and supplements to the other six JIS previously established. As has been mentioned above, the ASMA is now continuing its activity steadily, although some projects have been inevitably reduced due to the economic depression, and has greatly contributed to the progress and development of the science and technology of SMAs and to the development of SMAs industries in Japan and also throughout the world.

© Woodhead Publishing Limited, 2011

1 Mechanisms and properties of shape memory effect and superelasticity in alloys and other materials: a practical guide K. TSUCHIYA, National Institute For Materials Science, Japan

Abstract: This chapter describes fundamental knowledge about shape memory and superelastic alloys which may be useful to potential users of these alloys. Basic characteristics and properties of various shape memory/superelastic alloys are described in the first and second sections. The mechanisms of the shape memory effect and superelasticity are then explained, followed by a section on thermodynamics which is intended for more proficient readers. Key words: shape memory effect, superelasticity, stress–strain curve, martensitic transformation, martensite, austenite, R phase.

1.1

Introduction

As one of the most prominent functional metallic materials, shape memory alloys (SMAs) are widely used in a range of appliances, from coffee maker thermostats to glasses frames. They have also found an increasing number of applications in the rapidly progressing field of minimally invasive surgery, specifically in the production of medical devices such as stents, guide wires, and filtration devices (Morgan, 2004). It is the shape memory effect (SME) and superelasticity (SE), characteristics unique to SMAs, that make them suitable to these applications. SME and SE are illustrated in the form of stress–strain curves in Fig. 1.1. In SME, a previously deformed alloy can be made to recover its original shape simply by heating (Fig. 1.1(a)); while in SE, the alloy can be bent or stretched to a great extent, but returns to its original shape once the load is released (Fig. 1.1(b)). In the case of SME, shape recovery takes place at a particular temperature, and thus a single piece of material functions both as a sensor and as an actuator. This is the reason why SMAs are often referred to as smart or intelligent materials. Such materials are useful in the production of simple, compact and reliable actuator devices. SMA actuators will be discussed in greater detail in Chapter 5. SE is suited to applications which require the use of a material with large recoverable deformation. For example, in the case of TiNi wires, it is 3 © Woodhead Publishing Limited, 2011

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Shape memory and superelastic alloys

Stress

(b)

Stress

(a)

Loading

Loading

Unloading

Heating

Unloading

Strain

Strain

1.1 Stress–strain curves describing (a) shape memory effect and (b) superelasticity.

typically possible to recover approximately 8% strain, which is about 800 times larger than conventional elastic strain (Hooke’s law) in metals. Another important characteristic of superelasticity is its non-linear stress– strain response. This chapter describes the fundamental properties of SMAs in order to assist the reader in understanding the working principles of a wide variety of SMA applications (such as those described in Parts II and III) as well as to aid and motivate them towards researching and inventing novel applications for these materials.

1.2

Properties of shape memory alloys (SMAs)

Figure 1.2 consists of a series of tensile stress–strain curves for TiNi, obtained at different temperatures (Miyazaki et al., 1981). It is clear, at a glance, that the properties of SMAs are strongly temperature dependent. At low temperatures, the stress (or load) required to deform the sample is relatively low. Once the load has been removed, the deformation persists, just like a plastic deformation in conventional metals, such as steels or aluminum, but vanishes after the sample has been heated, as indicated by the broken arrows (SME). Above 232.5 K, the deformation stress starts to increase with temperature, and the deformation vanishes upon the removal of the load, even without heating (SE). At higher temperatures, the residual strain appears and the stress–strain curves are more or less similar to those of conventional metals. Stress–strain behavior is not the only property of SMAs to be affected by temperature. Various other properties, such as elastic modulus, electrical resistivity, and specific heats, are also strongly temperature dependent. The reason for such complex behavior is that the sample changes state as the temperature increases, and thus its deformation mechanism is different for various temperature ranges.

© Woodhead Publishing Limited, 2011

A practical guide

5

(a) 77.0 K 300 (b) 153.0 K

(c) 164.1 K

200

(d) 171.2 K

100 0 200

0

(e) 182.7 K

0 (f) 193.3 K

100

0 (h) 213.5 K

(g) 203.0 K

0 0

0

0

Stress (MPa)

400

(l) 251.0 K (k) 241.0 K

300

(j) 232.5 K (i) 223.7 K

200 100 0

0

0

0 (o) 276.5 K

600

(n) 273.2 K (m) 263.4 K

400

(p) 283.7 K

200

Ms = 190 K At = 221 K 0

0

2

4

0

2

4

0

2

4

0

2

4

Strain (%)

1.2 Tensile stress–strain curves of Ti–50.6Ni obtained at different temperatures (Ms = martensitic transformation start temperature, Af = reverse transformation finish temperature, (Miyazaki et al., 1981).

1.3

Fundamentals of shape memory alloys (SMAs)

1.3.1 Martensitic transformation Both the SME and SE share a common origin, known as martensitic transformation. Martensitic transformation is a particular class of phase transformation. Most metallic materials and ceramics are crystalline materials in

© Woodhead Publishing Limited, 2011

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Shape memory and superelastic alloys

which the constituent atoms are regularly arranged in three dimensions, forming a particular crystal structure representing each phase. The structure of crystalline materials is often altered in response to a change in the external environment, such as a change in temperature, pressure, stress, etc. Phase transformation in crystalline materials can be classified into two categories; one is diffusional transformation and the other, diffusionless or displacive transformation. In the case of diffusional transformation, atoms leave one crystal structure to form another structure by diffusion. For this reason, a high temperature is generally necessary to ensure that the mobility of atoms is high, otherwise the transformation is too sluggish. Meanwhile, it is possible for the atoms to alter the crystal structure without leaving the original crystal by their coordinated displacive movements. This does not require long range diffusion of atoms and takes place in a relatively short time. Martensitic transformation belongs to this second category of diffusionless transformation and is characterized by well-coordinated shear dominant atomic displacement, as shown in Fig. 1.3. It can be seen that even though the displacement of each atom is much less than its interatomic distance, the process of transformation creates a large shear strain. Another important implication of shear dominant transformation is that the volume change during the transformation can be very small. Starting from the high temperature phase, which is typically a cubic phase such as a body centered cubic (bcc) or face centered cubic (fcc) structure, the crystal structure is cooled to below martensitic transformation temperatures and transforms into a product phase with lower crystallographic symmetry. The high temperature phase is called the parent phase or austenite, and the product of martensitic transformation is called the martensite phase or martensite. This phase transformation can be detected by various measurements, such as electrical resistivity measurements or calorimetric measurements. Figure

Cooling

Heating

Austenite

Martensite

1.3 Schematic illustration of change in unit cell shape on martensitic transformation.

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A practical guide

7

1.4 contains an example of a differential scanning calorimetry (DSC) measurement for a Ti–50.2 mol% Ni. On cooling from a temperature at which the austenite is stable, the alloy transforms to a martensite phase via an exothermic reaction. Heating the sample again leads to a reverse transformation to austenite with an endothermic reaction. The martensitic transformation occurs within a certain temperature interval. In order to bracket these temperature ranges, it is practical to use four transformation temperatures, which consist of martensitic transformation start temperature (Ms), martensitic transformation finish temperature (Mf), reverse transformation start temperature (As) and reverse transformation finish temperature (Af). Peak temperatures of forward and reverse transformation peaks can be also useful. The corresponding changes in atomic structures during the transformation are illustrated in Fig. 1.5. A plate of martensite starts to form in austenite at Ms; the area of martensite increases on cooling and the whole sample becomes martensite at Mf. At the temperature range between Ms and Mf, the sample constitutes a mixture of martensite and austenite. It should be noted that in a fully martensitic state (below Af) the sample is composed of martensite crystals with two different orientations: opposite shear directions which are twin-related with each other. This morphology enables the martensite crystals to mutually cancel out the shear

Ti–50.2at%Ni 0.3 M* = 289.0 K 0.2

Heat flow, Q/W g–1

15.84 J/g 0.1

0.0

Mf = 277.4 K

Ms = 298.8 K

As = 313.3 K

Af = 338.6 K

–0.1 –16.79 J/g –0.2

–0.3 150

A* = 327.9 K 200

250

300

350

400

450

500

Temperature, T/K

1.4 Differential scanning calorimetry (DSC) measurement of martensitic transformation temperatures.

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Shape memory and superelastic alloys P Ms

MB

MB

Mf

MA

MA

P MB

MB Af

MA

As

MA

P Martensite (self-accommodated structure)

Austenite

1.5 Schematic illustration of atomic arrangements during martensitic transformation. P: parent phase, M: martensite phase, subscripts A and B stand for the martensite crystals with different shear directions.

deformation and thereby minimize the deformation of the sample, a process which is referred to as self-accommodation. Those martensite crystals that form from a single austenite but have a different orientation are called variant. There can be up to 24 different orientations when the martensite is monoclinic, as in the case of the B19′ phase in TiNi (Kudoh et al., 1985). When the sample is heated, the transformation from martensite to austenite occurs in reversed manner . It can also be seen from Fig. 1.4 that the transformation temperatures are different for forward and reverse transformation. This difference is called transformation hysteresis. The origin of hysteresis is related to the mobility of the austenite/martensite interface. Sometimes a martensite can be transformed into another martensite with a different crystallographic structure. Figure 1.6 shows the DSC curves for such multi-step martensitic transformation for Ti–50.6 mol% Ni. In this case, the austenite transforms first to the R phase and then to the B19′ phase on further cooling. It should be noted that on heating, the B19′ phase transforms to austenite without transforming into the R phase. This is due to the fact that there is a much larger transformation hysteresis in the B19′ to B2 (austenite) transformation than in the R to B2 transformation. The R phase has a much smaller transformation strain (∼1%) than that which occurs in B19′ (∼8%) (Stachowiak and McCormick, 1988) and often appears in a cold-worked sample or in a sample containing precipitates. A stress field, such as those produced by dislocation structures, tends to suppress the B19′ transformation and stabilizes The R phase. In this section, the martensitic transformation induced by temperature was described. However, the same transformation can be also induced by stress. This will be explained in Section 1.3.3.

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A practical guide

9

Endotherm.

Exotherm.

M* = 233 K R* = 302 K

A* = 308 K 150

200

250 300 350 400 Temperature, T/K

450

500

1.6 DSC curves showing multi-step martensitic transformation. Heating above Af

Austenite

Martensite

Deformation

1.7 Mechanism of the SME.

1.3.2 SME Figure 1.7 illustrates the mechanism of the SME. In this case, a piece of SMA (for example, a wire) is in martensite phase at room temperature and its martensitic transformation temperature is sufficiently above room temperature. The wire can be easily bent since the martensite phase can be easily deformed by twinning or detwinning. The deformation can be seen as a change in the fraction of the variants. If you heat the bent wire to a temperature above Af, the martensite transforms back to austenite and the sample regains its original straight shape. This is possible because all of the variants were originally formed from a single austenite crystal. On cooling

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Shape memory and superelastic alloys

to room temperature the wire re-transforms to martensite, but its shape does not change due to the self-accommodated structures. This is the mechanism of the SME. The strain recovered by the reverse transformation can be as great as ∼8 % whilst the recovery stress can be as large as several hundred MPa in the case of TiNi. This leads to an energy density as high as 107 J/m3 which is three orders of magnitude higher than those of other actuator materials, such as piezo-ceramics or magnetostrictive materials. It should be emphasized that the shape change caused by the SME occurs only upon heating, and this is referred to as the one-way SME. In order to obtain a shape change upon cooling, it is necessary to control the arrangement of the variants that form on cooling from Af to room temperature. This can be achieved, at least to some extent, with the introduction of a stress field, either through the fine precipitation Ti3Ni4 phase (Kainuma et al., 1986), which is semi-coherent to the austenite, or through the introduction of dislocation structures.

1.3.3 SE In the case of the SME, the SMA is deformed when it is in the martensite phase, which is deformation given at a temperature below Mf. In contrast, in the case of SE, the alloy is deformed at a temperature above Af. The mechanism of SE is illustrated schematically in Fig. 1.8. When the austenite phase is subjected to a stress , it transforms into martensite. In this case, the variant that forms upon the application of stress is that which gives the maximum strain in the given stress direction. Once the imposed stress is released the sample transforms back into the austenite and the strain also vanishes. The stress required to induce the transformation increases linearly with temperature, as illustrated in Fig. 1.9. The gradients of the lines are given by the Clausius–Clapeyron equation: dσ ΔH a / m =− dT εT0

1.1

where σ is transformation inducing stress, T0 is equilibrium temperature and ΔH is transformation enthalpy. For TiNi, the gradient is approximately 5.5 MPa/K for the B19′ phase and approximately 13 MPa/K for the R phase (Stachowiak and McCormick, 1988). This implies that the transformation temperature varies with load, an important factor to consider in the design of SMA actuators. It is apparent that the temperature at which superelasticity is obtained is limited to a certain range. If the temperature is below Af, the stress-induced martensite does not revert to austenite. Thus there remains some residual

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A practical guide s

p

11

q A B

s

r

0

e

0 →p, s →0

q →r

A, B P M-A P

M-A P

1.8 Mechanism of superelasticity. The upper part of the figure shows a typical superelastic stress (σ)-strain(ε) curve. The three lower figures schematically illustrate the atomic arrangements in the different states of the sample marked as p, q, r, s, A and B in the stress-strain curve. s s

Yield stress

T > Af sf ⎛ ds ⎛ DH ⎜ ⎜ =– ⎝ dT ⎝ eT

~150 MPa sr

Ms

~30 K

Af

Td

T

em

e

1.9 Temperature dependence of transformation stress.

strain which vanishes on heating above Af due to the SME. If the deformation temperature is too high, then the transformation stress exceeds the yield stress of the material and the alloy deforms plastically. Thus, in order to widen the window of a material’s superelastic temperature, one should

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Shape memory and superelastic alloys

increase the yield stress of the austenite, an effect which is usually achieved by work hardening and precipitation hardening.

1.4

Thermodynamics of martensitic transformation

This section describes the basic thermodynamic aspects of martensitic transformation. Those not familiar with thermodynamics may wish to consult an introductory textbook, such as Porter and Eastering (1981). Since many of the essential parameters, such as martensitic transformation temperatures and transformation stress, are determined by the difference in free energy between the martensite phase and the parent phase, we will consider the Gibbs free energy of the two phases. The molar Gibbs free energies of two phases, G, are given by: Ga = Ha − TSa

1.2

Gm = Hm − TSm

1.3

and

where H is enthalpy, S is entropy and T is temperature. Superscripts a and m stand for the austenite phase and the martensite phase, respectively. The enthalpy and entropy are strongly dependent upon the chemical composition of the alloy. At a certain temperature, the difference in free energy is given by: ΔGa/m = (Ha − Hm) − T(Sa − Sm) = ΔHa/m − TΔSa/m

1.4

The thermodynamic equilibrium temperature between the austenite and martensite, T0, is the temperature at which the free energy of the two phases is equal (ΔGa/m = 0) and is given by: T0 =

ΔH a / m ΔS a / m

1.5

In practice, the formation of the martensite requires some under-cooling, ΔT, since the process requires extra energy, Ge, due to the formation of the a/m interface and the elastic strain energy. Hence Ms is lower than T0 temperature, and is given by: Ms = T0 − ΔT =

ΔH a / m − Ge ΔS a / m

1.6

Similarly, in the case of reverse transformation on heating, Af, is given by: Af = T0 + ΔT =

ΔH a / m + Ge ΔS a / m

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1.7

A practical guide

13

Therefore, from the experimental values of Ms and Af, the equilibrium temperature can be determined as: T0 =

Ms + Af 2

1.8

In the case of stress-induced transformation (superelasticity), the effect of the applied uniaxial stress, s, and the strain, e, should be taken into account as an additional term in eq. 1.4: ΔGa/m = ΔHa/m − TΔSa/m − se

1.9

At an equilibrium state under a given temperature, equilibrium stress can be given by:

σ =−

ΔH a / m − TΔS a / m ε

1.10

The differentiation of eq. 1.10 with respect to T leads to the Clausius– Clapeyron equation (1.1). A more detailed analysis of the Clausius– Clapeyron equation can be found in Ahlers (1986).

1.5

Conclusions

In this chapter, the fundamental properties of the SME and SE were described. Although most of the currently used SMAs are TiNi, this particular alloy comes with some disadvantages. For example, its shape recovery temperature is limited to about 100 °C, although we might expect it to acquire more extensive applications if Af could be increased to above 150 °C. Some high-temperature SMAs have been developed, but most of them contain noble metal elements or rare earth elements, and therefore are not cost effective. Also the motion of SMA actuator elements is often sluggish, owing to the fact that it is limited by the thermal conductivity of the alloy. Ferromagnetic SMAs are able to operate at a much higher speed through the use of a magnetic field instead of temperature (Sozinov et al., 2002; Ullakko et al., 1997). However, all of the materials mentioned above are still at various stages of development. Those who want to obtain more detailed and precise knowledge about SMAs should consult Ahlers (1986), Nishiyama (1978), Otsuka and Wayman (1998) and Wayman (1992). The conference proceedings of the International Conferences on Martensitic Transformation (ICOMAT) (Ko, et al., 2006), the European Symposium on Martensitic Transformation (ESOMAT) (Eggeler and Kostorz, 2008) and Shape Memory and Superelastic Technology (SMST)

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Shape memory and superelastic alloys

(Miyazaki, 2008) are useful sources for those seeking the most up to date information on the research and applications of SMAs.

1.6

References

Ahlers, M., Martensite and equilibrium phases in Cu–Zn and Cu–Zn–Al alloys, Prog. Mater. Sci., 1986, 30, 135–186. Eggeler G. and Kostorz G. (eds), Proceedings of the 7th European Symposium on Martensitic Transformations (ESOMAT 2006), Mater. Sci. Eng., 2008, A481–482. Kainuma, R., Matsumoto, M., Honma, T., The mechanism of the all-round shape memory effect in a Ni-rich TiNi Alloy, in: ICOMAT-86, JIM, Nara, Japan, 1986, pp. 717–722. Ko T., Hsu Y. , Zhao L.C. and Kostorz G. (eds), Proceedings of the International Conference on Martensitic Transformations, Mater. Sci. Eng., 2006, A438–440. Kudoh, Y., Tokonami, M., Miyazaki, S., Otsuka, K., Crystal structure of the martensite in Ti-49.2 at.% Ni alloy analyzed by the single crystal X-ray diffraction method, Acta Metall., 1985, 33, 2049–2056. Miyazaki S. (ed.), Proceedings of the International Conference on Shape Memory and Superelastic Technologies (SMST-2007), ASM International, 2008. Miyazaki, S., Otsuka, K., Suzuki, Y., Transformation pseudoelasticity and deformation behavior in a Ti–50.6 at% alloy, Scrip. Metall., 1981, 15, 287–292. Morgan, N. B., Medical shape memory alloy application the market and its products, Mater. Sci. Eng., 2004, A378, 16–23. Nishiyama, Z., Martensitic Transformation, Academic Press, 1978. Otsuka, K., Wayman, C. M. (eds), Shape Memory Materials, Cambridge University Press, 1998. Porter, D. A., Eastering, K. E., Phase Transformations in Metals and Alloys, Van Nostrand Reinhold, 1981. Sozinov, A., Likhachev, A. A., Lanska, N., Ullakko, K., Giant Magnetic-field-induced strain in NiMnGa seven-layered martensite phase, Appl. Phys. Lett., 2002, 80, 1746–1748. Stachowiak, G. B., McCormick, P. G., Shape memory behaviour associated with the R and martensitic transformations in a NiTi alloy, Acta Metall., 1988, 36, 291–297. Ullakko, K., Huang, J. K., Kokorin, V. V., Handley, R. C. O., Magnetically controlled shape memory effect in Ni2MnGa intermetallics, Scripta Mater., 1997, 36, 1133–1138. Wayman, C. M., Shape memory and related phenomena. Prog. Mater. Sci., 1992, 36, 203–224.

© Woodhead Publishing Limited, 2011

2 Basic characteristics of titanium–nickel (Ti–Ni)based and titanium–niobium (Ti–Nb)-based alloys S. MIYAZAKI and H. Y. KIM, University of Tsukuba, Japan

Abstract: Development of titanium–nickel (Ti–Ni)-based and titanium– niobium (Ti-Nb)-based shape-memory/superelastic alloys is surveyed. Their basic characteristics are reviewed: e.g., the crystal structures of the parent and martensite phases, the recoverable strain associated with the martensitic transformation, orientation dependence of deformation behavior and cyclic deformation behavior. Key words: shape memory alloy, superelastic alloy, titanium–nickel (Ti– Ni), nickel–titanium (Ni–Ti), Ti alloy, biomaterial, shape memory effect, superelasticity, martensitic transformation.

2.1

Introduction

Shape memory effect (SME) and superelasticity (SE) are associated with the crystallographically reversible nature of the martensitic transformation which appears in shape memory alloys (SMAs). Such crystallographically reversible martensitic transformation was especially named ‘thermoelastic martensitic transformation’. The martensitic transformation itself is not a new phenomenon, having first been found long ago in a steel which was heat-treated at a high temperature followed by rapid quenching: the martensitic transformation in most iron and steels is not thermoelastic, hence the SME does not appear. It has been found that many alloys including some special ferrous alloys show SME and SE (Miyazaki and Otsuka, 1989). Among them, the Ti–Ni alloys have been successfully developed as practical materials for many applications. The Ti–Ni alloys have been under investigation since the first report on SME in a Ti–Ni alloy in 1963 (Buehler et al., 1963). However, the Ti–Ni alloys had presented many difficult problems with many puzzling phenomena for about 20 years until 1982, when the basic understanding was established on the relationship between the microstructure and the corresponding deformation behavior such as SME and SE (Miyazaki et al., 1982; Miyazaki, 1990). Since then, many puzzling phenomena have been clarified: e.g., the microstructures which cause the rhombohedral phase (R-phase) transformation to appear (Miyazaki and Otsuka, 1984, 1986), the orientation dependence of shape memory and superelastic behavior observed in 15 © Woodhead Publishing Limited, 2011

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Shape memory and superelastic alloys

single crystals (Takei et al., 1983; Miyazaki et al., 1984; Miyazaki and Wayman, 1988; Miyazaki et al., 1988), the temperature dependence of deformation and fatigue behavior (Miyazaki and Otsuka, 1986; Miyazaki et al., 1986; Miyazaki, 1990), the shape memory mechanism (Miyazaki et al., 1989a, b), etc. The Ti–Ni alloys have been successfully applied as biomaterials in such devices as orthodontic arch wires, guide wires and stents in addition to many engineering applications as shown in the following chapters. Ti–Ni alloys are also considered among the most attractive candidates for orthopedic implants. However, it has been pointed out that pure Ni is a toxic element and causes Ni hypersensitivity. Although the Ti–Ni alloys are considered to be safe in the human body based on experience and scientific consideration, in order to solve the psychological problem of the risk of Ni hypersensitivity, Ni-free Ti-based shape memory and superelastic alloys have been recently developed, e.g., Ti–Nb–Sn (Takahashi et al., 2002), Ti–Nb–Al (Fukui et al., 2004), Ti–Nb–Ta (H. Y. Kim et al., 2006a, b), Ti–Nb–Zr (H.Y. Kim et al., 2005a; J.I. Kim et al.,2006), Ti–Nb–O (Kim et al., 2005b), Ti–Nb–Pt (Kim et al., 2007), Ti–Mo–Ga (Kim et al., 2004a), Ta–Mo–Sn (Maeshima and Nishida, 2004) and Ti–(8–10)Mo–4Nb–2V–3Al (mass%) (Zhou et al., 2004). It has been reported that Ti–Nb binary alloys exhibit SME and SE at room temperature, and their superelastic properties can be considerably improved by thermomechanical treatment (H. Y. Kim et al., 2004b, 2006c, d). It has been also reported that superelastic properties of Ti-Nb alloys can be improved by the addition of alloying elements such Zr, Ta, Pt and O (H. Y. Kim et al., 2006b, 2007; J. I. Kim et al., 2005a, b, 2006). The Ni-free Ti-based alloys have not been used for applications, but will be used for medical applications in the future. In this chapter, the basic characteristics such as the martensitic transformation and shape memory properties of both the Ti–Ni alloys and Ti–Nb alloys are to be reviewed based on the present authors’ works. The effects of alloying elements and thermomechanical treatment on shape memory properties are also to be mentioned. Much of Section 2.2 on Ti–Ni-based alloys is reprinted from Miyazaki et al. (2009), with permission of Cambridge University Press, which is greatly appreciated.

2.2

Titanium–nickel (Ti–Ni)-based alloys

2.2.1 Phase diagram An equilibrium phase diagram of the Ti–Ni system is shown in Fig. 2.1, which describes a middle composition region including an equiatomic composition Ti–Ni. Full information of the equilibrium phase diagram can be found in Murray (1987). In this chapter, Ti–Ni includes nearly equiatomic

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Basic characteristics of Ti–Ni and Ti–Nb-based alloys

17

2000

Temperature (K)

1800

L

1653 K

1583 K

1600

1391 K

1400 1257 K

Ti–Ni

1000

TiNi3

Ti2Ni

1200

903 K 30

40

50 60 Ni content (at.%)

70

80

2.1 An equilibrium phase diagram of the Ti–Ni system.

compositions and locates around the equiatomic composition region, while Ti2Ni and TiNi3 intermetallic compounds locate at 33.3 at.% Ni and 75 at.% Ni, respectively. These three alloys are equilibrium phases. There is another phase Ti3Ni4, which is not an equilibrium phase but is important since it affects both the transformation temperature and shape memory behavior (Miyazaki, 1990). The Ti–Ni single phase region terminates at 903 K as shown in Fig. 2.1; however, the region seems to extend to around room temperature in a narrow Ni-content width according to empirical information.

2.2.2 Crystallography of martensitic transformation The parent phase of the Ti–Ni has a CsCl-type B2 superlattice, while the martensite phase is three-dimensionally close packed (monoclinic or B19′) as shown in Fig. 2.2. The Ti–Ni alloy also shows another phase transformation prior to the martensitic transformation according to heat treatment and alloy composition. This transformation (rhombohedral phase or R-phase transformation) can be formed by elongating along any one of the <111> directions of the B2 structure as shown in Fig. 2.3 and is characterized by a small lattice distortion when compared with that of the martensitic transformation. The R-phase transformation usually appears prior to the martensitic transformation when the martensitic transformation start temperature Ms is further lowered by some means other than the R-phase transformation start temperature TR. There are many factors effective to depress Ms as follows (Miyazaki and Otsuka, 1986):

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Shape memory and superelastic alloys b c′

a

c

c′

b′ b′ a′

a0 B2

β a′

Lattice correspondence

M-phase

2.2 Crystal structures of the parent (B2) and martensite (B19′) phases and the lattice correspondence between the two phases.

[110]B2 a¢ [100] R

–

[111]B2 c¢ [001] R c [001]B2

c [001]B2

a [100]B2 a [100]B2 b [010]B2

– –

[112]B2 b¢ [010] R

b [010]B2

(a) B2

(b) R-phase

2.3 Crystal structure of the R-phase which is formed by elongation along one of <111> directions of B2 lattice.

• • • • •

increasing Ni-content; aging at intermediate temperatures; annealing at temperatures below the recrystallization temperature after cold working; thermal cycling; substitution of a third element.

Among these factors, all but the first are effective at revealing the R-phase transformation. The martensitic transformation occurs in such a way that the interface between the martensite variant and parent phase becomes an undistorted and unrotated plane (invariant plane or habit plane) in order to minimize the strain energy. In order to form such a martensite variant (habit-plane variant), it is necessary to introduce a lattice invariant shear such as twins,

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Basic characteristics of Ti–Ni and Ti–Nb-based alloys

19

dislocations or stacking faults. The lattice invariant shear is generally twinning, which is reversible, in the shape memory alloys. Crystallographic characteristics of martensitic transformations are now well understood by the phenomenological crystallographic theory (Wechsler et al., 1953; Bowles and Mackenzie, 1954; Lieberman et al., 1955). This theory describes that the transformation consists of the following three operational processes: (1) a lattice deformation B creating the martensite structure from the parent phase, (2) a lattice invariant shear P2 (twinning, slip, or faulting) and (3) a lattice rotation R. Thus, the total strain (or the shape strain) associated with the transformation is written in the following matrix form: P1 = RP2B

2.1

This theory requires that the shape strain produced by the martensitic transformation is described by an invariant plane strain, i.e., a plane of no distortion and no rotation, which is macroscopically homogeneous and consists of a shear strain parallel to the habit plane and a volume change (an expansion or contraction normal to the habit plane). Thus, the shape strain can also be represented in the following way: P1 = I + m1d1p′1

2.2

where I is the (3x3) identity matrix, m1 the magnitude of the shape strain, d1 a unit column vector in the direction of the shape strain, and p′1 a unit row vector in the direction normal to the invariant plane. If we know the lattice parameters of the parent and martensite phases, a lattice correspondence between the two phases and a lattice invariant shear, the matrix p′1 can be determined by solving Eq. (2.1) under invariant plane strain condition. Then, all crystallographic parameters such as P1, m1, d1 and orientation relationship are determined. The lattice invariant shear of the Ti–Ni is the <011>M Type II twinning (Knowles and Smith, 1981; Matsumoto et al., 1987). There are generally 6, 12 or 24 martensite variants with each shape strain P1. Each variant requires formation of other variants to minimize the net strain of the groupe of the variants. This is called self-accommodation, hence the whole specimen shows no macroscopic shape change except surface relief corresponding to each variant by the martensitic transformation upon cooling.

2.2.3 Transformation strain The strain induced by the martensitic transformation shows strong orientation dependence in Ti–Ni alloys (Miyazaki et al., 1984, 1986; Miyazaki and Wayman, 1988). It is conventionally assumed that the most favorable martensite variant grows to induce the maximum recoverable transformation

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i strain e iM in each grain: e M can be calculated by using the lattice constants of the parent phase and martensite phase. The lattice constants of the parent and martensite phases of a Ti–Ni alloy are as follows: a0 = 0.3013 nm for the parent phase and a = 0.2889 nm, b = 0.4150 nm, c = 0.4619 nm and b = 96.923 degrees for the martensite phase, respectively. A calculation process for transformation strain follows. Using the lattice constants of the parent phase and martensite phase, the transformation strain produced by lattice distortion due to the martensitic transformation can be calculated. If it is assumed that the most favorable martensite variant grows to induce the maximum transformation strain in each grain, the lattice distortion matrix T′ is expressed in the coordinates of the martensite as follows using the lattice constants of the parent phase (a0) and those of the martensite phase (a, b, c, b):

⎡a ⎢a ⎢ 0 ⎢ T′ = ⎢ 0 ⎢ ⎢ ⎢0 ⎣

0 b 2a0 0

c′γ ⎤ ⎥ 2a0 ⎥ ⎥ 0 ⎥ ⎥ c′ ⎥ ⎥ 2a0 ⎦

2.3

where c′ = c sinb and g = 1/tanb. Then, the lattice distortion matrix T which is expressed in the coordinates of the parent phase can be obtained as follows: T = RT′Rt

2.4

where R is the coordinate transformation matrix from the martensite to the parent phase and Rt is the transpose of R. R corresponding to the most favorable martensite variant is expressed as follows: ⎡ −1 0 ⎢ ⎢ 1 R=⎢0 ⎢ 2 ⎢ ⎢0 − 1 ⎢⎣ 2

0 ⎤⎥ ⎥ 1 ⎥ − 2⎥ ⎥ 1 ⎥ 2 ⎥⎦

2.5

Since any vector x in the coordinates of the parent phase is transformed to x′ due to the martensite transformation using the following equation x′ = Tx

2.6

the maximum transformation strain e in each grain can be calculated as follows: i M

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Basic characteristics of Ti–Ni and Ti–Nb-based alloys

ε Mi =

x′ − x x

21 2.7

i Figure 2.4 shows the calculated result of the transformation strain e M expressed by contour lines for each direction in a [001] − [011] − [ 111] standard stereographic triangle. For example, the transformation strains along [001], [011], [ 111] and [ 311] are 3.0%, 8.4%, 9.9% and 10.7%, respectively. By applying the similar calculation for the R-phase transformation, the transformation strain e Ri at a temperature 35 K lower than TR is as shown in Fig. 2.5. The result indicates that the strain is the maximum along [ 111] and that along [001] is the minimum nearly equal to zero. The strain decreases with decreasing temperature from TR, because the rhombohedral angle of the R-phase lattice shows temperature dependence. i By averaging e M for representative 36 orientations which locate periodically in a stereographic standard triangle, the transformation strain for a polycrystal can be estimated as follows if there is no specific texture and the axis density distributes uniformly (Tan and Miyazaki, 1997):

⎛ i ⎞ ⎜⎝ ∑ ε M ⎟⎠ i =1 0 εM = 36 36

2.8

If there is texture, the axis density I i is not uniform in each inverse pole figure so that it is necessary to consider I i in the calculation of the transformation strain as follows (Miyazaki et al., 2000):

–

111 Calculated strain (%) 10.5

8.0 7.0 6.0 5.0 4.0 3.0 001

10.0 9.0 011

2.4 Orientation dependence of calculated strain induced by the martensitic transformation.

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Shape memory and superelastic alloys –

111 Calculated strain (%) 0.9

0.8 0.5

0.7

0.4 0.3 0.2 0.1

0.6

001

011 T = (TR–35) K

2.5 Orientation dependence of calculated strain induced by the R-phase transformation.

⎛ i i⎞ ⎜⎝ ∑ ε M I ⎟⎠ i =1 εM = 36 36

2.9

2.2.4 Transformation temperature The martensitic transformation start temperature Ms is shown in Fig. 2.6 as a function of Ni content. In the composition range of the Ti–Ni, Ms decreases with increasing Ni content above 49.7 at.% Ni, while they are constant below 49.7 at.% Ni. The reverse martensitic transformation temperature Af is above 30 K higher than Ms in all specimens in the composition region. The reason for the constant Ms in the Ni content region less than 49.7 at.% can be ascribed to the constant Ni content in the Ti–Ni phase, because the Ti2Ni appears in the Ni content region as shown in Fig. 2.6, keeping the Ni content of the Ti–Ni at 49.7 at.%.

2.2.5 Deformation behavior The deformation behavior of SMAs is strongly temperature sensitive, because the deformation is associated with the martensitic transformation: this is different from plastic deformation by slip which occurs in conventional metals and alloys. Schematic stress–strain curves of a Ti–Ni alloy obtained at various temperatures (T) are shown in Fig. 2.7. In the temperature range of T < Mf, the specimen is fully transformed before applying

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Basic characteristics of Ti–Ni and Ti–Nb-based alloys

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370 350

Temperature (K)

330 310 290 270 250 230 210 190

Ms

170 48.5

49.0

49.5 50.0 50.5 Ni content (at.%)

51.0

51.5

2.6 Ni content dependence of Ms temperature.

(a) T<Mf

(b) Mf
(c) Ms
Stress

σM

Heating (d) As
(e) Af
(f) Ts
σM

Strain

2.7 Schematic stress–strain curves at various temperatures in a Ti–Ni alloy.

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Shape memory and superelastic alloys

stress so that the elastic deformation takes place in the martensite phase at first as shown in Fig. 2.7(a), where many martensite variants selfaccommodate each other before loading. Upon further loading, twin planes in the martensite phase move to create an apparent plastic deformation. Therefore, the yield stress in Fig. 2.7(a) corresponds to the critical stress for twinning deformation in the martensite phase. In the temperature range Mf < T < Ms, the parent and martensite phases coexist so that yielding occurs due to twinning in the martensite phase and/or stress-induced martensitic transformation in the parent phase. Both the yield stresses by twinning and stress-induced transformation in Fig. 2.7(b) are lowest in this temperature range, because the former decreases with increasing temperature and the latter decreases with decreasing temperature until this temperature region. The stress–strain curves in Figs. 2.7(a) and (b) are essentially the same, except the yield stress is a little lower in Fig. 2.7(b) than in Fig. 2.7(a). In the temperature range of Ms < T < As, the parent phase elastically deforms at first and yielding occurs due to the stress-induced martensitic transformation. Therefore, the yield stress linearly increases with increasing temperature, satisfying the Clausius–Clapeyron relationship. The stress-induced martensite phase remains after unloading, because the temperature is below As. The shape of the stress–strain curve of Fig. 2.7(c) is similar to those of Fig. 2.7(a) and (b). In the temperature range As < T < Af, the deformation induced by the stress-induced martensitic transformation recovers partially upon unloading as shown in Fig. 2.7(d), resulting in partial superelasticity and partial shape memory effect by following heating. In the temperature range Af < T < Ts, perfect superelasticity appears as shown in Fig. 2.7(e), where Ts stands for the critical temperature above which the martensitic transformation does not take place and deformation occurs by slip. If T is above Ts, plastic deformation occurs as in conventional metals and alloys as shown in Fig. 2.7(f). The effect of temperature on the critical stress for inducing martensite (sM) and the critical stress for slip (ss) is shown in Fig. 2.8. The line for sM shows a positive temperature dependence, while the line for ss shows a negative temperature dependence, resulting in an intersection at Ts. The stress for the rearrangement of martensite variants due to the movement of twin planes is shown by a dashed line in the temperature range below Ms. The slope of the dashed line shows a negative temperature dependence as well as the solid line for slip deformation, because both the deformation modes, slip and twinning, are thermal activation processes. Deformation paths corresponding to those shown in Figs 2.7(a)–(f) are shown in Fig. 2.8. Ti–Ni alloys show successive stages of transformation in the stress–strain curve. The deformation is associated with both the R-phase and the martensite in Ti–Ni alloys which include a high density of dislocations and/or fine Ti3Ni4 precipitates. Therefore, the deformation behavior is sensitive to

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Basic characteristics of Ti–Ni and Ti–Nb-based alloys

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sM

M Stress

ss

B2

(a)

(b) Mf

(c)

Ms

As

(d)

(e) Af

(f) Ts

Temperature

2.8 Critical stresses for inducing the martensitic transformation (sM) and for slip deformation (sS) shown as a function of test temperature in the specimen of Fig. 2.6 in a Ti–Ni alloy.

test temperature; it is clasified into six categories according to the relative relationship between test temperature and transformation temperatures as schematically shown in Fig. 2.9. In range 1 (T < Mf), only one stage associated with the rearrangement of martensite variants appears as shown in Fig. 2.9(a). In range 2 (Mf < T < Ms), both the R-phase and the martensite coexist, revealing two stages associated with the rearrangement of the R-phase and martensite variants as shown in Fig. 2.9(b). In the figure, the stress–strain curve associated with the R-phase is drawn by a solid line, while that associated with the martensite is by a broken line; the dashed line shows the shape recovery associated with the two reverse transformations upon heating. In range 3 (Ms < T < Af), the specimen is in a fully R-phase state prior to loading, and hence deformation first proceeds by the rearrangement of the R-phase variants to a favorable one as shown in Fig. 2.9(c). Upon further loading the martensite is stress-induced in the second stage. In range 4 (Af < T < TR), the superelasticity associated with the forward and reverse martensitic transformations appears, although a part of the deformation is still associated with the rearrangement of the R-phase variants. In range 5 (TR < T < Tx), the R-phase is also stress-induced, exhibiting twostage superelasticity. The critical stresses for inducing both the R-phase and martensite phase satisfy the Clausius–Clapeyron relationship as shown in Fig. 2.10, where the critical stresses are plotted against test temperature. Since the slope of the stress–temperature relation for the R-phase is steeper than that for the martensite, both lines cross each other at a temperature

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26

Shape memory and superelastic alloys (a) T<Mf

(b) Mf
(c) Ms
sM sM

Stress

sT

(d) Af
eT

(e) TR
(f) TX
sM

sT

sR sTR

Strain

2.9 Schematic typical stress–strain curves at specific temperatures in a Ti–Ni alloy which exhibits both R-phase (rhombohedral phase) and martensite phase (monoclinic phase) transformations.

Tx. Thus, the deformation associated with the R-phase does not appear in range 6 (Tx < T) as shown in Fig. 2.10(f). The steepness of the Clausius– Clapeyron relationship for the R-phase transformation mainly originates from the small transformation strain associated with the R-phase transformation, i.e., only a tenth of that associated with the martensitic transformation (Miyazaki and Wayman, 1988; Miyazaki et al., 1988).

2.2.6 Stress cycling effect Since superelasticity is frequently used in practical applications, degradation in superelastic behavior by stress cycling is of critical concern. It is necessary to evaluate the degradation of superelasticity and use a method to suppress the degradation. Two mechanisms are available to raise the critical stress for suppressing the degradation; one is precipitation hardening and another is hardening due to high density of thermally rearranged dislocations (Miyazaki et al., 1982; Miyazaki, 1990). The former can be realized by aging Ni-rich Ti–Ni alloys at 673 K for 3.6 ks after solution treatment at 1273 K; the latter by annealing at 673 K for 3.6 ks following

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Basic characteristics of Ti–Ni and Ti–Nb-based alloys

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sM

Stress

M sT B2 R (f) (a)

(b) Mf

(c)

Ms

(d) Af

(e) TR

TX

Temperature

2.10 Schematic phase diagram of Ti–Ni alloy in a temperature–stress coordinate.

preceeding cold work. Figure 2.11 shows the effect of thermomechanical treatment on the change of the stress–strain curve by cyclic deformation; (a) and (b) are the cases for the former, and (c) is for the latter. The plateau in the first stress–strain curve changes to a gradually increasing slope by cyclic deformation in (a) and (b), while the plateau is retained even after 100 cycles in (c), though it is not clear what this difference comes from. Since the aging and annealing temperatures are the same, both mechanisms are available simultaneously in an Ni-rich specimen annealed at 673 K without preceding solution-treatment. Figure 2.11(d) shows that case. The change in the shape of stress–strain curve is small compared with the other cases (a)–(c), showing that the combined effect is more effective for stabilizing the superelasticity characteristics against cyclic deformation. Although the change in the superelastic behavior is rapid in the initial cycling, the change around 100 cycles is hardly appreciable. Thus, it is anticipated that more stable superelasticity is achieved by cyclic training prior to application. The result of this is shown in Fig. 2.11(e), where N′ denotes the number of cycles after preceding 100 cyclic deformations at a little higher stress level. The stress–strain curve shows hardly any change even after 100 cycles.

2.2.7 Fatigue life Although the deformation mode of the Ti–Ni alloy seems to be reversible macroscopically, the Ti–Ni alloy also shows fatigue failure. Since the Ti–Ni alloy is a functional material characterized by SE and SME, the fatigue

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Shape memory and superelastic alloys (a) Ti–50.6 at.% Ni 1273 K → 673 K N=1

N=2

N=10

N=50

N=100

400 200 2% 0 600

0 0 0 (b) Ti–51.6 at.% Ni 1273 K → 673 K N=1 N=2 N=10

0 N=50

N=100

400 200 2% 0 600 Stress (MPa)

28

0 0 0 (c) Ti–49.8 at.% Ni 673 K N=1 N=2 N=10

0 N=50

N=100

400 200 2% 0 600

0 0 0 (d) Ti–50.6 at.% Ni 673 K N=1 N=2 N=10

0 N=50

N=100

400 200 2% 0 600

0

0

0

(e) Ti–50.6 at.% Ni 673 K N¢=1 N¢=2 N¢=10

0 N¢=50

N¢=100

400 200 2% 0

0

0

0

0

Strain (%)

2.11 Effect of cyclic deformation on the stress–strain curves of Ti–Ni alloys subjected to various heat treatments.

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Basic characteristics of Ti–Ni and Ti–Nb-based alloys

29

Tensile stress (MPa)

800 600 400 200 0 101

Ti–50.8 at.% Ni 1273 K 1 h 673 K 1 h 1273 K 1 h → 673 K 1 h Cycled at 293 K 102

103

104

105

106

Nf

2.12 Effect of heat treatment on the fatigue life of the Ti–50.8 at.% Ni alloy which was tested at 293 K.

mode subjected to such a material in use is commonly a loading–unloading cycling without or with heating after each unloading. Figure 2.12 shows such data obtained in a tension–unloading fatigue mode in the Ti–50.8 at.% Ni alloy. The data are represented by two straight lines. The deformation mode for each region is different form the others; the short life region is for cyclic stress-induced transformation, while the long-life region is for cyclic elastic deformation. However, as an exception, the solution-treated specimen (1273 K 1 h) does not reveal SE, because the stress-induced transformation is accompanied by a large amount of slip deformation in this case. When concerning with SE, the shorter life region is more concerned for the other two types of specimens, which are subjected to age-treatment after solutiontreatment (1273 K 1 h – 673 K 1 h) and annealing at an intermediate temperature after cold work (673 K 1 h), respectively. The fatigue life for the latter is about ten times longer than that for the former; the former includes fine Ti3Ni4 precipitates as microstructure, while the latter includes not only the precipitates but also a high density of dislocations which were introduced during cold working. Therefore, it is clear that the internal structure with both precipitates and dislocations is effective to improve the fatigue life. When SME is concerned, the stress for deforming martensite is lower than that for inducing the martensitic transformation in SE, hence longer fatigue life is expected for actuator applications utilizing SME.

2.3

Titanium–niobium (Ti–Nb)-based alloys

2.3.1 Crystallography and transformation temperature Quenching from b (bcc) phase leads to a martensitic transformation from the b phase to either a′ (hexagonal) phase or a ″ (orthorhombic) phase in

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Shape memory and superelastic alloys

Ti–Nb-based alloys. Shape memory effect in Ti–Nb-based alloys is associated with the reversible transformation between the b and a″ phases. Figure 2.13 shows a schematic illustration exhibiting the lattice correspondence among the b, a″ and a phases. The a″ orthorhombic martensite has been considered as a distortion of the hexagonal structure of a-Ti; the a′ and c′ axes of the orthorhombic cell correspond to the a and c axes of the hexagonal cell and the b′ of the orthorhombic cell corresponds to 3a. The lattice parameters of a-Ti are a′ = 0.295 nm and c′ = 0.468 nm which result in b/a′ = 1.732 and c/a′ = 1.586. On the other hand, both b/a and c/a of the b phase are 2 . Figure 2.14 shows the Nb content dependence of b′/a′ and c′/a′ of the a″-orthorhombic martensite phase. Both of the b′/a′ and c′/a′ decrease linearly with increasing Nb content. The lower compositional limit of a″-orthorhombic martensite phase is estimated to be 6 at.% Nb because the b′/a′ and c′/a′ become the same values for the a-Ti. On the other hand, the upper compositional limit of a″-orthorhombic martensite phase is estimated to be 32 at.% Nb by a linear extrapolation, where b′/a′ and c′/a′ become 2 . This result indicates that the a″-orthorhombic martensite phase is a compromise phase between the b and a phases (Duerig et al., 1982). The transformation strain produced by the lattice distortion due to the martensitic transformation is determined by the lattice constant of the parent b phase and those of the orthorhombic a″ martensite phase. The transformation strain varies according to crystal orientation. Figure 2.15 β phase

α″ phase

α phase

c¢

c a

a¢

b

b a

b [1100]β

b¢ –

[010]α″

b [011]β [011]β

a

b¢ a¢

a

–

c

–

[100]α″

[100]β [001]α″

–

[1120]α

[0001]α

2.13 The lattice correspondence among b, a″ and a phases.

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Basic characteristics of Ti–Ni and Ti–Nb-based alloys

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1.8 b′/a′ c′/a′

b/a of α-Ti

b′/a′ and c′/a′ ratio

1.7

c/a of α-Ti

1.6

1.5

b/a and c/a of β-Ti 1.4

0

5

10

15 20 25 Nb content (at.%)

30

35

2.14 Nb content dependence of b′/a′ and c′/a′ of the a″-orthorhombic martensite phase.

–

111 (1.5%) 2.1% 2.6% 3.1% 3.6%

4.1% 001 (2.1%)

011 (4.2%)

2.15 Orientation dependence of the calculated transformation during the martensite transformation from the b to a″ in the Ti–22Nb alloy.

shows the orientation dependence of the calculated transformation strain associated with the martensitic transformation from the b to a ′′ phase for a Ti–22Nb alloy (H. Y. Kim et al., 2006c). It is seen that the largest transformation strain of 4.2% is obtained along the [011]b direction, which corresponds to the [010]a″ direction. Figure 2.16 shows the Nb content dependence of transformation strains for three representative orientations,

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Shape memory and superelastic alloys

Transformation strain (%)

10 [011] direction [001] direction – [111] direction

8

6

4

2

0 10

15

20 Nb content (at.%)

25

30

2.16 Nb content dependence of the transformation strain for Ti–Nb binary alloys.

600

Ms Temperature (K)

500

400

300

200

100 10

Kim et al., 2004b Matsumoto et al., 2005 Brown et al., 1964 Baker, 1971 15

20 Nb content (at.%)

25

30

2.17 Nb content dependence of MS for Ti–Nb binary alloys.

[011]b, [001]b and [ 111]β . It is seen that the transformation strains decrease linearly with increasing Nb content. Figure 2.17 shows the Nb content dependence of the martensitic transformation start temperature Ms for Ti–Nb binary alloys. It is seen that Ms decreases by 40 K with 1 at.% increase of Nb content for Ti–(20–28)Nb alloys. Ms becomes lower than room temperature when the Nb content increases more than 25.5 at.%. It has been reported that Ti–(22–25)Nb

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Basic characteristics of Ti–Ni and Ti–Nb-based alloys

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alloys exhibit shape memory effect (Kim et al., 2004b). Superelastic behavior is observed in the Ti–(26–27)Nb alloys at room temperature (Kim et al., 2004b). It is seen that the maximum transformation strain is less than 3% for alloys exhibiting superelasticity at room temperature. This intrinsic small transformation strain is one of drawbacks of Ti–Nb superelastic alloys.

2.3.2 Shape memory and superelastic properties Figure 2.18 shows a series of stress–strain curves obtained at various temperatures for the Ti–(26–28)Nb alloys (H. Y. Kim et al., 2006c). Broken lines with an arrow indicate shape recovery by heating. The shape memory effect is observed for the Ti–26Nb alloy deformed at temperatures between 193 and 273 K. Superelastic behavior is seen at 293 K, although the shape recovery was incomplete. For the Ti–27Nb alloy, superelastic behavior is observed at temperatures between 193 and 293 K. The Ti–28Nb alloy exhibited good superelastic behavior at 193 K. The residual strain increased with increasing temperature. It is also seen that the apparent yield stress, which corresponds to the stress for the inducing martensitic transformation, increases with increasing temperature. This is due to the fact that the parent phase becomes more stable with increasing temperature when the temperature is above Ms, thus a higher stress is required for inducing the martensitic transformation at a higher temperature, which is in accordance with the

400

193 K

233 K

253 K

273 K

293 K

200

Ti–26Nb

Stress (MPa)

0 400 200

Ti–27Nb

0 600 400 200

Ti–28Nb

0 Strain (%)

2%

2.18 Stress–strain curves obtained upon loading and unloading at various temperatures for the Ti–(26–28) at.% Nb alloys.

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Shape memory and superelastic alloys

Stress (MPa)

Clausius–Clapeyron relationship. Shape recovery was hardly observed when the test temperature was higher than 273 K, implying that the stress to induce the martensite becomes higher than the critical stress for slip. Figure 2.19(a) shows the stress–strain curves obtained by strain increment cyclic loading–unloading tensile tests for the Ti–27 at.% Nb alloy subjected to solution treatment after cold rolling with the reduction of 99% in thickness. The specimen was loaded along the rolling direction. At the first cycle, tensile stress was applied until the strain reached about 1.5%, and then the stress was removed. The measurement was repeated by increasing the maximum strain by 0.5% by loading with the same specimen. It is seen that the Ti–27Nb alloy exhibits a two-stage yielding. The first yield stress corresponds to the critical stress for inducing martensitic transformation, while the second stage yielding is due to the plastic deformation. At the first cycle, the specimen exhibits good superelasticity. However, with increasing applied tensile strain, superelastic behavior became incomplete and the remained plastic strain increased. The recovery strain e r upon unloading consists of an elastic strain eel and a superelastic strain e se, the

600

(a) Ti–27Nb

(b) Ti–22Nb–7Ta

400 200

0 e se

e el

er

er Stress (MPa)

600 (c) Ti–22Nb–6Zr

(d) Ti–19Nb–2Pt

400 200

0 er

er Strain (%)

2%

2.19 Stress–strain curves obtained by strain increment cyclic loading and unloading tensile tests in (a) T1–27Nb, (b) Ti–22Nb–7Ta, (c) Ti–22Nb–6Zr and (d) Ti–19Nb–2Pt.

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Basic characteristics of Ti–Ni and Ti–Nb-based alloys

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latter strain being due to the reverse transformation. The maximum e r of 2.0% was obtained in the Ti–27Nb alloy. A strong recrystallization texture of {112} <110> is developed by solution treatment in severely cold-rolled b-Ti alloys (Inamura et al., 2004; H. Y. Kim et al., 2006a). Figure 2.20 shows the inverse pole figure for the rolling direction of the Ti–27Nb specimen solution treated at 1173 K for 1.8 ks. The inverse pole figure exhibits very strong axis density at [011], implying that the large superelastic strain close to the transformation strain along the [011]b direction is expected along the rolling direction. The transformation strain along the [011]b direction, eM[011], of the Ti–27Nb alloy is 2.5% as can be seen in Fig. 2.16. However, it is noted that the maximum ese of 1.2% was obtained in a solution- treated specimen as shown in Fig. 2.19(a), which is less than half of the expected transformation strain. It is considered that the small ese of the Ti–27Nb is due to not only small transformation strain but also low critical stress for slip.

2.3.3 Effect of alloying element Figures 2.19(b)–(d) show stress–strain curves of Ti–Nb-based ternary alloys exhibiting superelasticity at room temperature. As shown in Fig. 2.21, Ms decreases by about 30 K or 35 K with 1 at.% increase of Ta or Zr content in the Ti–22Nb alloy, respectively, indicating that Ti–27Nb, Ti–22Nb–7Ta and Ti–22Nb–6Zr alloys exhibit similar Ms. Here it is noted that Zr lowers Ms although it has been considered as a neutral element for a/b transformation temperature. Ms decreases considerably by the addition of Pt. It has been 111

001

011 2

4

6

8 10 12 14 16 18

2.20 The inverse pole figure for the rolling direction of the Ti–27Nb specimen solution treated at 1173 K for 1.8 ks.

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Shape memory and superelastic alloys 500 Ti–22Nb–xZr Ti–22Nb–xTa

450

Ms temperature (K)

400 350 300 250 200 150 100

0

2

4 6 Alloy content (at.%)

8

10

2.21 Effect of Zr and Ta addition on the martensitic transformation temperature Ms in the Ti–22Nb alloy.

reported that the effect of 1 at.% Pt in decreasing transformation temperature is equivalent to 4 at.% Nb, indicating that Ms decreases by about 160 K with an increase of 1 at.% Pt (Kim et al., 2007). This implies that Ms of the Ti–19Nb–2Pt alloy also similar to that of the Ti–27Nb alloy. It is seen that the Ti–22Nb–7Ta alloy exhibits superelastic behavior similar to the Ti–27Nb alloy. The maximum e r of 1.9% was obtained in the Ti–22Nb–7Ta alloy. On the other hand, the Ti–22Nb–6Zr and Ti–19Nb–2Pt alloys exhibit good superelasticity with a larger recovery strain. The maximum er of 3.5% and 3.0% was obtained in the Ti–22Nb–6Zr and Ti–19Nb–2Pt alloys, respectively. It is supposed that a larger recovery strain of the Ti–22Nb–6Zr and Ti–19Nb–2Pt alloys is due to a large transformation strain because it is clear that the elastic strain eel of the Ti–22Nb–6Zr alloy is smaller than those of Ti–27Nb and Ti–22Nb–7Ta alloys as shown in Fig. 2.19. Figure 2.22 shows the effect of Ta or Zr addition on the transformation strain eM[011]. The transformation strain decreases by 0.13% or 0.28% with 1 at.% increase of Zr or Ta content, respectively. It is noted that the transformation strain eM[011] decreases by 0.34% with 1 at.% increase of Nb content as shown in Fig. 2.16. When comparing different composition alloys which reveal similar Ms, for example, Ti–27Nb, Ti–22Nb–6Zr and Ti–22Nb– 7Ta alloys, it is clear that the addition of Zr is most effective in increasing the transformation strain. This indicates that the addition of Zr as a substitute of Nb is effective to increase the transformation strain with keeping

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Basic characteristics of Ti–Ni and Ti–Nb-based alloys

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6 Ti–22Nb–xZr Ti–22Nb–xTa

5

εM[011] (%)

4

3

2

1

0

0

2

4 6 Alloy content (at.%)

8

10

2.22 Effect of Zr and Ta addition on the transformation strain along the [011]b direction in the Ti–22Nb alloy.

(a) Ti–22Nb–1O

(b) Ti–22Nb–1N

(c) Ti–20Nb–4Zr–2Ta–0.6N

Stress (MPa)

800 600 400 200 0 Strain (%)

2%

2.23 Stress-strain curves obtained by strain increment cyclic loading and unloading tensile tests in (a) Ti–22Nb–1O, (b) Ti–22Nb–1N and (c) Ti–20Nb–4Zr–2Ta–0.6N.

Ms similar. It has been reported that the addition of Pt as a substitute of Nb is also effective at increasing the transformation strain of Ti–Nb-based superelastic alloys, because Pt is four times more effective for decreasing Ms than Nb, while Pt is only three times more effective for decreasing transformation strain than Nb. It is also noted that the addition of Ta is not effective at increasing the transformation strain. Addition of interstitial elements is effective at improving superelastic properties. Figure 2.23 shows the stress–strain curves of Ti–22Nb–1O,

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Shape memory and superelastic alloys

Ti–22Nb–1N and Ti–20Nb–4Zr–2Ta–0.6N alloys obtained at room temperature. All the specimens exhibit superelastic behavior at room temperature. The Ti–22Nb alloy exhibits shape memory effect at room temperature and the Ms of the Ti–22Nb alloy is about 430 K, indicating that O and N reduce the transformation temperature remarkably. Figure 2.23 also shows that the addition of interstitial elements is also effective to increase the critical stress for slip, which is the stress for the second stage yielding, when compared with the result of Ti–27Nb as shown in Fig. 2.19(a). It is supposed that the increase of the critical stress for slip by the addition of O or N is due to the solution hardening effect because no precipitates were observed in the solution-treated specimens. The maximum er of 2.7% and 3.9% were obtained in the Ti–22Nb–1O and Ti–22Nb–1N alloys, respectively. It is noted that the Ti–22Nb–1N alloy exhibits a higher critical stress for slip than the Ti–22Nb–1O alloy, resulting in more stable superelasticity with a larger recovery strain. The Ti–20Nb–4Zr–2Ta–0.6N alloy reveals almost perfect superelasticity when an applied strain is less than 3.5% as shown in Fig. 2.23(c). The large superelastic recovery strain of 4.0% and high critical stress for slip of 660 MPa were obtained in the Ti–20Nb–4Zr–2Ta–0.6N alloy, which is due to the combined effects of Zr and N. As a result, it is suggested that Ti–Nb–Zr-based alloys with small amount of interstitial elements can be good candidates for biomedical superelastic alloys.

2.3.4 Effect of heat treatment condition The low critical stress for slip is one of major drawbacks of Ti–Nb alloys. As mentioned above, interstitial elements such as O and N are effective to increase critical stress for slip due to the solid solution hardening effect. The critical stress for slip of the Ti–Nb-based alloys can also be improved by thermo-mechanical treatment. It has been reported that low temperature annealing after severe cold working improves the superelastic properties. Figure 2.24 shows the stress–strain curves of the Ti–22Nb–6Zr alloy obtained upon cyclic loading and unloading tensile tests (J. I. Kim et al., 2006). The specimens were elongated up to 2.5% strain followed by unloading at each cycle. All the specimens show good superelasticity. In particular the specimen annealed at 823 K exhibits superior superelastic behavior with a narrow stress hysteresis. This is caused by the increase in the stress for inducing martensites and the critical stress for slip due to the presence of fine a precipitates and fine subgrain structure for the specimen annealed at 823 K. It has been reported that fine and dense w precipitates are effective to increase the critical stress for slip and stabilize superelasticity (H. Y. Kim et al., 2006c, d). Figure 2.25 shows stress–strain curves obtained by cyclic loading-unloading tensile tests for the Ti–26 at.% Nb alloy. Superelastic

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Basic characteristics of Ti–Ni and Ti–Nb-based alloys 400

(a) 823 K

1st

2nd

3rd

5th

7th

39 10th

300 200 100

Stress (MPa)

1% 0 (b) 873 K

1st

2nd

3rd

5th

7th

10th

200 100 1% 0 (c) 973 K

1st

2nd

3rd

5th

7th

10th

200 100 1% 0 Strain (%)

2.24 Stress–strain curves obtained by repeated loading to the maximum strain of 2.5% followed by unloading at room temperature in the Ti–22Nb–6Zr annealed at (a) 823 K, (b) 873 K and (c) 973 K.

behavior was observed in the solution-treated specimen at the first cycle. With increasing tensile strain, the superelastic behavior became incomplete. The residual strains were almost completely recovered by heating at the second and third cycles. On the other hand, the specimen aged at 573 K for 3.6 ks after the solution treatment exhibits almost perfect superelasticity at the first and second cycles. The superelastically recovered strain increased with increasing applied strain although the retained plastic strain increased. It is obvious that both of the first and second yield stress increased by aging at 573 K for 3.6 ks. It is also clear that low temperature annealing also increases the critical stress for slip and stabilize superelastic behavior as can be seen by comparing the stress–strain curves in Figs 2.25(a) and (c). Furthermore, it is seen that the subsequently aged specimen exhibits excellent superelasticity due to the combined effect of work hardening and age hardening. Perfect superelasticity was observed until the 4th cycle as shown in Fig. 2.25(d).

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Stress (MPa)

600

(a) 1173 K/1.8 ks

600

400

400

200

200

0

0

600

(b) 1173 K/1.8 ks + 573 K/3.6 ks

(d) 873 K/0.6 ks + 573 K/3.6 ks

(c) 873 K/0.6 ks

600

400

400

200

200

0

0 Strain (%)

2%

2.25 Stress–strain curves obtained by cyclic loading–unloading tensile tests for the Ti–26 at.% Nb alloy heat treated at various conditions after cold rolling with a reduction of 99% in thickness.

Superelastic behavior was obtained even though the maximum stress reached 660 MPa upon loading in the specimen annealed at 873 K for 0.6 ks followed by aging treatment at 573 K for 3.6 ks. As a result, it can be concluded that excellent superelasticity can be achieved by annealing at an intermediate temperature lower than the recrystallization temperature followed by aging at a lower temperature in Ti–Nb alloys.

2.4

Conclusions

The Ti–Ni alloys have been investigated extensively for 30 years after the establishment of basic understanding on the relationship among the microstructure, transformation behavior and SME/SE. Many applications have been successfully developed in both engineering and medical fields. In particular, SE has been used for medical applications such as stents, guide wires and orthodontic arch wires. On the other hand, the systematic investigation of the Ni-free Ti-based SME/SE alloys started only about 10 years ago. The development and basic research of the Ti-based alloys (Chai et al., 2009; Tahara et al., 2009; Inamura et al., 2010a, b) will further accelerate the medical applications of SM/SE alloys in the near future. Commercially available Ti–Ni-based alloys can reveal the reverse transformation temperature Af below 385 K in a solution-treated condition. For practical applications for actuators using shape memory effect, Ti–Ni alloys are heat-treated at an intermediate temperature range in order to form stable microstructures consisting of dislocations and Ti3Ni4 precipitates. Thus, the transformation temperature becomes lower than 385 K: most engineering actuator applications have been used below 363 K. However, since there are many

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Basic characteristics of Ti–Ni and Ti–Nb-based alloys

41

electric appliances which require higher transformation temperatures between 363 and 473 K, development and basic research of such high temperature shape memory alloys (Buenconsejo et al., 2009a, b) will further expand engineering applications in the near future.

2.5

References

Baker C (1971), Metal Sci. J., 5, 92. Bowles J S and Mackenzie J K (1954), Acta Metall., 2, 129, 138, 224. Brown, Area, Clark D, Eastabroole J and Jepson K S (1964), Nature, 201, 91. Buehler W J, Gilfrich J V and Weiley K C (1963), J. Appl. Phys., 34, 1467. Buenconsejo P J, Kim H Y, Hosoda H and Miyazaki S (2009a), Acta Materialia., 57, 1068. Buenconsejo P J, Kim H Y and Miyazaki S (2009b), Acta Materialia., 57, 2509. Chai Y W, Kim H Y, Hosoda H and Miyazaki S (2009), Acta Materialia., 57, 4054. Duerig T W, Albrecht, Richter J D and Fischer P (1982), Acta Metall., 30, 2161. Fukui Y, Inamura T, Hosoda H, Wakashima K and Miyazaki S (2004), Mater. Trans., 45, 1077. Inamura T, Fukui Y, Hosoda H, Wakashima K and Miyazaki S (2004), Mater. Trans., 45, 1083. Inamura T, Hosoda H, Kim H K and Miyazaki S (2010a), Phios. Mag., 90, 3475. Inamura T, Yamamoto Y, Hosoda H, Kim H Y and Miyazaki S (2010b), Acta Materialia., 58, 2535. Kim H Y, Ohmatsu Y, Kim J I, Hosoda H and Miyazaki S (2004a), Mater. Trans., 45, 1090. Kim H Y, Satoru H, Kim J I, Hosoda H and Miyazaki S (2004b), Mater. Trans., 45, 2443. Kim H Y, Sasaki T, Okutsu K, Kim J I, Inamura T, Hosoda H and Miyazaki S (2006a), Acta Mater., 54, 423. Kim H Y, Hashimoto S, Kim J I, Inamura T, Hosoda H and Miyazaki S (2006b), Mater. Sci. Eng. A., 417, 120. Kim H Y, Ikehara Y, Kim J I, Hosoda H and Miyazaki S (2006c), Acta Mater., 54, 2419. Kim H Y, Kim J I, Inamura T, Hosoda H and Miyazaki S (2006d), Mater. Sci. Eng. A., 438–440, 839. Kim H Y, Oshika N, Kim J I, Inamura T, Hosoda H and Miyazaki S (2007), Mater. Trans., 48, 400. Kim J I, Kim H Y, Inamura T, Hosoda H and Miyazaki S (2005a), Mater. Sci. Eng. A., 403, 334. Kim J I, Kim H Y, Hosoda H and Miyazaki S (2005b), Mater. Trans., 46, 852. Kim J I, Kim H K, Inamura T, Hosoda H and Miyazaki S (2006), Mater. Trans., 47, 505. Knowles K M and Smith D A (1981), Acta Metall., 29, 101. Lieberman D S, Wechsler M S and Read T A (1955), J. Appl. Phys., 26, 473. Maeshima T and Nishida M (2004), Mater. Trans., 45, 1096. Matsumoto O, Miyazaki S, Otsuka K and Tamura H (1987), Acta Metall., 35, 2137. Matsumoto H, Watanabe S, Hanada S (2005), Mater. Trans., 46, 1070.

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Miyazaki S (1990), in: Engineering Aspects of Shape Memory Alloys, edited by Duerig T W, et al., Butterworth-Heinemann, 394. Miyazaki S and Otsuka K (1984), Phios. Mag., A, 50, 393. Miyazaki S and Otsuka K (1986), Met. Trans., A, 17, 53. Miyazaki S and Otsuka K (1989), ISLJ Int., 29, 353. Miyazaki S and Wayman C M (1988), Acta Met all., 36, 181. Miyazaki S, Ohmi Y, Otsuka K and Suzuki Y (1982), J. de Phys., 43, Suppl. 12, C4–255. Miyazaki S, Kimura S, Otsuka K and Suzuki Y (1984), Scripta Met., 18, 883. Miyazaki S, Kimura S and Otsuka K (1988), Phios. Mag. A, 57, 467. Miyazaki S, Otsuka K and Wayman C M (1989a), Acta Met all., 37, 1873. Miyazaki S, Otsuka K and Wayman C M (1989b), Acta Met all., 37, 1885. Miyazaki S, Igo Y and Otsuka K (1986), Acta Met all., 34, 2045. Miyazaki S, No V H, Kitamura K, Anak K and Hosoda H (2000), Int. J. Plasticity, 16, 1135. Miyazaki S, Fu Y Q and Huang W M (2009) ed., Thin Film Shape Memory Alloys, Cambridge University Press. Murray J L (1987) ed., Phase Diagram of Binary Titanium Alloys, Monograph Series on Alloy Phase Diagrams, Materials Park, Ohio ASM International. Tahara M, Kim H Y, Hosoda H and Miyazaki S (2009), Acta Materialia, 57, 2461. Takahashi E, Sakurai T, Watanabe S, Masahashi N and Hanada S (2002), Mater. Trans., 43, 2978. Takei F, Miura T, Miyazaki S, Kimura S, Otsuka K and Suzuki Y (1983), Scripta Met., 17, 987. Tan S M and Miyazaki S (1997), Acta Materialia, 46, 2729. Wechsler M S, Lieberman D S and Read T A (1953), Trans. AIME, 197, 1503. Zhou T, Aindow M, Alpay S P, Blackburn M J and Wu M H (2004), Scripta Mater., 50, 343.

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3 Development and commercialization of titanium–nickel (Ti–Ni) and copper (Cu)-based shape memory alloys (SMAs) K. YAMAUCHI, Tohoku University, Japan

Abstract: The Research Association of Shape Memory Alloy, which was established in 1983 funded by a national program of the Japanese government, has stimulated research efforts to develop fundamental technologies for the commercialization of shape memory alloys (SMAs) in Japan. This chapter describes the production technology for the commercialization of Ti–Ni and Cu-based SMAs. Key words: national program, ASMA, titanium–nickel (Ti–Ni)-based shape memory alloys (SMAs), copper (Cu)-based shape memory alloys (SMAs).

3.1

Introduction

The first ASMA, Research Association of Shape Memory Alloy, was organized by six shape memory alloy (SMA) companies funded by a national program of the Japanese government in 1983. The six companies were Furukawa Electric Co. Ltd, NEC TOKIN Corporation, Sumitomo Metal Industries Ltd, Mitsubishi Materials Corporation, Dowa Mining Co. Ltd and Daido Steel Co. Ltd. The Research Association conducted research and development on SMAs for three years, which include not only industrial fabrication technologies of SMAs, such as melting, casting, hot- and coldworking, heat treatment, forming and special processes, but also software technologies for device design. This chapter introduces the production technology for the commercialization of Ti–Ni and Cu-based SMAs which has been developed by the organizing companies of ASMA.1,2,3

3.2

Research on titanium–nickel (Ti–Ni)-based shape memory alloys (SMAs)

3.2.1 In situ composition control furnace A composition control furnace was designed and developed for the precise control of the transformation temperature of ingots. Figure 3.1 shows an example of an in situ composition control furnace. In the furnace, the 43 © Woodhead Publishing Limited, 2011

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Shape memory and superelastic alloys

3.1 In situ composition control furnace; sampling and operation system for adding alloying element.

molten alloy is picked up from the melt and the transformation temperature of the sample is quickly analyzed and alloying elements are added to adjust the transformation temperature. The key points of this development are (1) sampling from the melt, (2) quick measurement of transformation temperature and (3) adding alloy elements. The sample of molten alloy is picked up by inserting a carbon rod into the melt and attaching the molten alloy in the tip. The measurement of the transformation temperature is conducted by high-speed differential scanning calorimetry (DSC), where analysis method and sweeping speed were improved. This DSC can determine the transformation temperature with an accuracy of ±1 °C within a few minutes. Equipment for adding alloying elements is shown in Fig. 3.2. This furnace can control the transformation temperature to within ±2 °C of the setting value as shown in Fig. 3.3.

3.2.2 Effect of alloying elements on shape memory properties To improve shape memory properties, the effect of addition of various third elements on the transformation temperature and shape memory properties was investigated. A Ni–Ti–Cr alloy was found to have transformation tem-

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Development and commercialization

45

Deviation from setting temperature (°C)

3.2 Equipment for adding alloying elements.

+2

+1

0

–1

–2 40

60

80

Af temperature (°C)

3.3 Distribution of transformation temperature in the alloy prepared by the in situ composition control furnace (Af, austenite finish temperature).

perature of −10∼20 °C and show good hot and cold workability and to be suitable for the low temperature SMA. It was also found that a Ni–Ti–V alloy is suitable for the low temperature superelastic alloy, and its transformation behavior was clarified.

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Shape memory and superelastic alloys

3.2.3 Precision casting A precision casting machine using an Ar pressure casting method was designed and developed. A schematic of a precision casting machine is shown in Fig. 3.4. Pipe shape products were successfully fabricated using the casting machine as shown in Fig. 3.5.

3.2.4 Hot drawing Compared with hot-working, cold-working of Ni–Ti alloys is far more difficult. In the cold-working process, a lot of work is necessary because the maximum reduction for one pass is only 10∼15%. To reduce fabrication cost, it is favorable to process by hot-working to as near the final shape as pos-

Crucible Reserve

Mechanical booster

Ar Mold

Oil-sealed rotary pump

3.4 Schematic of precision casting machine.

3.5 Casting pipe of Ni–Ti alloy using a graphite mold.

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Development and commercialization

47

sible. In the case of wire products, hot-working is usually carried out using a bar rolling mill but the minimum diameter of wire fabricated in this process is 5 mm. The hot-drawing technique using resistance heating by directly conducting current to the wire was developed to reduce the diameter to 2 mm. This hot-drawing system is shown in Figs 3.6 and 3.7. The specifications of the hot-drawing system are as follows: •

Conducting method: directly conducting of solder bath dipping wire and die.

3.6 Hot-drawing machine.

3.7 Dies in hot-drawing machine.

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Shape memory and superelastic alloys

3.8 Appearance of thin sheet prepared by the continuous casting and rolling method.

• •

Fabricating speed: 5∼20 m/min. Final diameter: 2 mm.

In the case of over-heating (heating current = 900 A), the wire caused creep fracture because of insufficient cooling of wire after hot drawing. The problems to be solved were lubricating (molasses) and electrode (the contact is bad because the wire moves during current heating). The wire moves due to shape recovery by heating, and it causes contact failure and sparking. The fabrication of ϕ 8.5–8 mm, ϕ 8–6 mm and ϕ 6–5 mm were possible in current of 600, 400 and 300 A, respectively.

3.2.5 Fabrication of thin sheet using the continuous casting and rolling method The melt made from a Ni–Ti alloy by induction melting is ejected through a nozzle to the mill roll by Ar gas. The melt is cooled by the roll and fabricated to the setting thickness. The optimal condition to obtain good quality of sheet was investigated. The optimal nozzle material, temperature of melt, revolution speed of roll, ejection pressure of Ar and roll gap to obtain a sheet of 0.2 mm in thickness are silica, 1350 °C, 200 rpm, 30 Pa and 0.2 mm, respectively. Figure 3.8 shows an example of the thin sheet fabricated using the continuous casting and rolling method.

3.3

Research on copper (Cu)-based shape memory alloys (SMAs)

3.3.1 Prospect for practical application An advantage of the Cu-based SMAs compared with Ni–Ti SMAs is their low cost. For Cu–Al–Be and Cu–Zn–Al alloys, the shape memory proper-

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Development and commercialization 1.1 k g

100

/mm 2

90 Change in displacement (%)

49

2.2 80

kg/ mm

1.9 2

kg /m

m2

70 60 50 40

Spring dimensions wire diameter 3.3 mm spring diameter 26.3 mm number of working coils 4 strain 1% Test condition max. 100 °C min. 0 °C 100

200

400

1000

2000 4000

10000

Number of thermal cycles

3.9 Thermal cycle properties in Cu–6Al–0.4Be–15Zn coil spring (martensitestart temperature, Ms = 34 °C, austenite start temperature, As = 33 °C).

ties needed in practical use were clarified. However, unfortunately they cannot not be substituted for the Ni–Ti shape memory alloys because of their bad thermal cycle properties (Fig. 3.9) and low yield stress (Fig. 3.10). The original practical applications for Cu-based alloys need to be developed.

3.3.2 Fabrication of pipe by press molding To establish the fabrication method for pipe coupling, equipment for fabricating pipes was designed and developed. The possibility of fabricating SMA pipes was investigated by a press forming technique using Cu–Zn–Al alloys, and optimal fabrication conditions were established. The samples used in this test were prepared by melting, casting, forging, rolling and boring (Fig. 3.11).

3.4

Conclusions

The research project based on The Research Association of Shape Memory Alloy in Japan ended successfully. Developmental efforts of the six union companies, Furukawa Electric Co. Ltd, NEC TOKIN Corporation,

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σ·Ms stress (kgf/mm2)

40

30

20

10

0

–20

0 (Ms)

20

40

60

80

100

Tensile testing temp., DT (= test temp–Ms)

3.10 Temperature dependence of yield stress in Cu–6Al–1.6Si–16Zn wire. (ϕ 4 mm, Ms = 4.6 °C).

(a)

(b) M–39

M–39

No. 90

No. 83

No. 80

No. 90

No. 83

No. 80

3.11 Appearance of press molding pipe of Cu–26.0Zn–4.1Al alloy. (a) Side view and (b) top view. Branch size: 2 mm in thickness, ϕ 35 mm∼ϕ 53 mm in diameter; mold’s size: cone angle of die: 30 °; diameter of die: ϕ19.6 mm; diameter of punch: ϕ 15.6 mm; mold’s temperature: die temperature: 45 °C; punch temperature: room temperature; branch temperature: 450 °C; punch speed: 10 mm/min.

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Improvement of two-way shape memory property Cost-reducing by special fabrication method

Improvement of properties of SMA springs

Improvement of fatigue life

• Applying precision casting machine using pressure casting method to products with complicated shape and reducing fabrication cost of the products to below 40% of that for cutting method. • Producing a pipe coupling by powder metallurgy and reducing the fabrication cost to 40% of that for cutting method • Establishment of technologies for hot drawing. • Establishment of fabrication technology for thin sheet using continuous casting and rolling method and the powder rolling method.

• Applied in industrial manufacture.

• Establishment of a technology to control chemical composition in melting furnace and controlling the transformation temperature of ingot to within ±2.5 °C. • Establishment of melting method using calsia (CaO) crucible. • Clarification of relation between shape memory properties and fatigue life in shape memory spring and development of shape memory devices with fatigue life over 106. • Clarification of the effect of alloying elements on fatigue of Ni–Ti alloys. • Formulation of the relation between the characteristics of shape memory spring and the load and stroke. Establishment of designing method to control the property to within ±20% of setting value. • Increasing a recovery force at low temperature to 50% of that obtained at high temperature.

Precise control of transformation temperature

• Established. • Established.

• Fundamental technology was established

• Fundamental technology was established.

• Accomplished.

• Ni–Ti–Cr and Ni–Ti–V alloys were put to practical use. • Established.

• A possibility for the practical application was demonstrated. • The fatigue properties were clarified.

Achievement

Objective

Technology

Table 3.1 The objective and status of achievement for R&D: Ni–Ti based alloy

52

Shape memory and superelastic alloys

Table 3.2 The objective and status of achievement for R&D: Cu based alloy Term

Objective

Achievement

Precise control of transformation temperature

• Controlling the transformation temperature of ingot to within ±5°C in Cu–Zn–Al and Cu–Al–Be alloys. • Establishment of melting and casting methods to prepare a homogeneous ingot without segregation. • Clarification of the effect of alloying elements on fatigue life and development of shape memory devices with fatigue life of over 104. • Clarification of the effect of stress on corrosion resistance in constituent phases. • Controlling the load and behavior stroke to within ±20% of setting value. • Producing a coupling and investigation of the effect of test temperature and size on generated force. • Achievement of the shape recovery of over 80% in 104 cycles.

• Established.

Improvement of fatigue life

Improvement of properties of SMA springs

Improvement of two-way shape memory property Cost-reducing by special fabrication method

• Conducting fabricating test of pipe in the several elements adding alloy and determination of the optimal fabricating condition.

• Established.

• Completed.

• Completed.

• Achieved.

• Completed.

• Achieved.

• Completed.

Sumitomo Metal Industries Ltd, Mitsubishi Materials Corporation, Dowa Mining Co. Ltd and Daido Steel Co. Ltd and the organizing company Furukawa Electric Co. Ltd established fundamental technologies for commercialization of SMA. The objective and achievement of the project are summarized in Tables 3.1 and 3.2.

3.5

References

1. The first ASMA 1983th R & D Report (1984). 2. The first ASMA 1984th R & D Report (1985). 3. The first ASMA 1985th R & D Report (1986).

© Woodhead Publishing Limited, 2011

4 Industrial processing of titanium–nickel (Ti–Ni) shape memory alloys (SMAs) to achieve key properties T. NAKAHATA, Sumitomo Metal Industries Ltd, Japan

Abstract: In this chapter, industrial processing technologies and methods to control the properties of titanium–nickel (Ti–Ni) shape memory alloys (SMAs) are described. Industrial processing of Ti–Ni consists of four basic steps: melting, hot and cold working, forming and shape memory treatment. It is known that each process is important to obtain the desired properties of Ti–Ni. Because impurity concentration strongly affects martensitic transformation temperature and durability, it is important to choose an appropriate melting method. Shape memory treatment is a characteristic process in the fabrication of Ti–Ni device elements, and it gives the device element the shape that should be recovered. Moreover, the condition of shape memory treatment affects shape recovery temperature, elastic modulus and durability. Thus, optimization of shape memory treatment is one of the most important aspects of fabrication of Ti–Ni device elements. Key words: processing, melting, forming, shape memory treatment.

4.1

Introduction

Titanium–nickel (Ti–Ni) alloys are widely used as the more practical shape memory or superelastic alloys due to their excellence of shape recoverability, durability and corrosion resistance. Ti–Ni alloys also exhibit excellent workability compared with other shape memory alloys (SMAs) although cold workability is poor when compared with conventional metallic materials such as steel. As shown in Fig. 4.1, the fabrication process of the Ti–Ni alloys consists of the several steps of melting, hot and cold working, forming and shape memory treatment. The fabrication process is similar to those used for other metallic materials, but there are some differences. Firstly, to draw wires, soft annealing must be repeated until the required dimensions are attained since the cold workability of the material is poor. Secondly, shape memory treatment (heat treatment) is required in the final stage of the process. It is known that each processing step affects the characteristics of the Ti–Ni alloys such as transformation temperatures, durability and elastic modulus. Industrial processing techniques for Ti–Ni SMAs 53 © Woodhead Publishing Limited, 2011

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Hot forging Ingot Melting

Hot rolled coil

Die

Hot roilling

Wire drawing Annealing

Forming of spring coil

Shape memory treatment

Shape memory treatment (straight shape)

4.1 Schematic view of TiNi fabrication process.

and the methods for controlling their properties are described in this chapter.

4.2

Melting process

4.2.1 Composition As discussed in the previous chapter, whether or not an SMA exhibits shape memory or superelasticity depends on the difference between the operation temperature and the transformation temperature of an alloy. Consequently, the transformation temperature is one of the most important properties of an SMA. Figure 4.2 shows the relation between the Ni content and transformation temperature in Ti–Ni alloys (Hosoda and Miyazaki, 2001). In the nickel-rich compositions with nickel concentrations over 50%, the transformation temperature decreases monotonically with increasing concentration of nickel.

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150

Temperature, T/°C

100

50

0

–50

–100 48.5

Af As

49

49.5

50

50.5

51

51.5

Ni, at.%

4.2 Relation between TiNi composition and transformation temperature (Af, austenite finish temperature; As, austenite start temperature) (Hosoda and Miyazaki, 2001).

Thus, the transformation temperature of Ti–Ni alloys is controlled mainly by adjusting nickel concentration. For example, a Ti–50.5% Ni alloy is used to make springs for water mixing valves that control hot water temperature; these springs have a transformation temperature (austenite finish temperature or Af) of about 40 °C. To obtain superelasticity, 51.0 at.% Ni is used, which results in an Af of less than zero. Since the composition range of the single Ti–Ni B2 phase is very narrow below 700 °C (Murray, 1996), in the nickel-rich composition with nickel concentrations over 50%, nickel-rich phases such as Ti3Ni4 and Ti2Ni3 can precipitate, depending on the shape memory treatment conditions (Nishida et al., 1986). The precipitation of the nickel-rich phase causes the transformation temperature to increase since the nickel concentration in the matrix decreases, and the desired transformation temperature can be difficult to obtain. In such a case, as shown in Fig. 4.3 (Yamauchi, 1996), a ternary element such as chromium or vanadium may be useful to lower the transformation temperature of the Ti–Ni alloys. To improve the durability of Ti–Ni at higher temperatures, copper is sometimes added. The change in the transformation temperature by adding copper is relatively small compared with other elements, as shown in Fig. 4.3.

4.2.2 Melting techniques As mentioned earlier, the transformation temperature depends on the concentration of nickel. The dependence is so strong that only a 0.1% change

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Shape memory and superelastic alloys 100 80 Temperature, Ms /°C

60 40 20 0

B Cu AI V Cr Mn

–20 –40 –60 –80 –100 0

2

4

6

8

10

12

14

16

18

20

X at.%

4.3 Ms temperature of TiNi100–xXx alloy (Ms martensite start temperature) (Yamauchi, 1996).

in nickel may result in a change of more than 10 °C. Since titanium can form stable chemical compounds with oxygen or nitrogen, it is necessary to suppress the formation of oxides or nitrides. Thus, the melting must be done in vacuum or inert gas to attain the desired transformation temperature. Note that oxides, if formed during the melting process, would likely create a source of failure, resulting in the degradation of the cold workability and durability of the alloys. Thus, atmospheric control during the melting process is also important for the purpose of reducing inclusions. Vacuum induction melting (VIM) or vacuum arc remelting (VAR) is typically used in industry; VIM being the most widely used method to melt TiNi. A schematic of VIM is shown in Fig. 4.4. In VIM, the material is melted by high frequency induction heating in vacuum or inert gas. The uniformity of the resulting material is excellent since it is completely solved, stirred and then molded. A possible drawback is contamination with impurities from the crucible. Note that although alumina and magnesia are commonly used as crucibles for melting steel products, these crucibles cause the oxidation of molten Ti–Ni since they are thermodynamically less stable than titanium oxide. A graphite or calcia (calcium oxide) crucible should be used instead for melting Ti–Ni. Graphite reacts with Ti–Ni although it is more stable than alumina and magnesia (Hamada et al., 1983), and it is necessary to melt Ti–Ni as quickly as possible at lower temperatures. Calcia is more stable than titania (titanium oxide) and is almost ideal for melting Ti–Ni. However, since calcia easily reacts with water to generate calcium hydroxide, which can supply oxygen to Ti–Ni, pretreatments such as prebaking the crucible are essential. If a large

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Industrial processing of Ti–Ni SMAs to achieve key properties

Batch feeder

57

Temperature sensor Melt sampler Vacuum chamber

Crucible

Vacuum pump

High frequency coil

Inert gases

Mold

4.4 Schematic view of vacuum induction melting (VIM).

Vacuum system TiNi electrode

Water cooled copper mold TiNi ingot

4.5 Schematic view of vacuum arc re-melting (VAR).

amount of calcium hydroxide is produced, it becomes difficult to maintain the shape of the crucible; crucibles cannot be used for long periods of time. On the other hand, an advantage of VAR is the absence of contamination with impurities from the crucible when melting Ti–Ni. Figure 4.5 shows a schematic view of VAR. A titanium sponge and electrolytic nickel grains are pressed to make a Ti–Ni electrode, and the discharge between the electrode and the water-cooled copper crucible melts the electrode, starting

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from the tip. In VAR process, since the copper crucible is water cooled, there is a low possibility of contamination due to the contact with the crucible. Table 4.1 compares VIM and VAR with respect to the impurity content of oxygen, nitrogen and carbon in Ti–Ni (Aiba, 2001). In the case of VIM, the melting of several tons of Ti–Ni would be difficult because of the limited crucible size, whereas there is no restriction on crucible size in the case of VAR. A possible problem of VAR is compositional fluctuation due to the partial melting of materials. Today, VIM appears to be used more frequently in industry, although the large processing capacity of VAR might attract more attention if the demand for the Ti–Ni alloys increases.

4.2.3 Control of ingot composition To control the transformation temperature within, for example, plus or minus 5 °C, it is necessary to control the nickel concentration with an accuracy of 0.1%. Since it is difficult to control nickel content around 50 at.% with such accuracy, manufacturers normally use the transformation temperature as a physical control indicator instead of nickel concentration. Differential scanning calorimetry (DSC) is used to measure the transformation temperature. A Ti–Ni single phase is required to measure the transformation temperature as a reference of nickel concentration. The composition range of Ti–Ni is widest at about 1100 °C (Murray, 1996), and thus heat treatment and quenching at such high temperatures is necessary for single phasing. Japanese Industrial Standards specify a heat treatment temperature of 900 °C for DSC measurement of Ti–Ni samples (JIS H7101).

4.3

Working process

4.3.1 Hot processing The workability of the Ti–Ni alloys is excellent compared with other SMAs, although it is inferior to that of steel; workability is especially good at high

Table 4.1 Impurity concentration in TiNi depending on melting techniques (Aiba, 2001) Impurity concentration (ppm) Melting techniques

O

N

C

VAR VIM with graphite crucible VIM with calcia crucible

400 400 400

50 50 50

<10 300 <10

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temperatures over 700 °C, where the Ti–Ni alloy becomes more stretchable due to its reduced strength as shown in Fig. 4.6. However, if excessive titanium is present locally, for example, due to segregation, a liquid phase appears at temperatures over 950 °C (Murray, 1996) and cracks may form during processing; thus it is advisable to use a processing temperature of 700 to 950 °C. In particular, since working speed is relatively fast in hot rolling, attention must be paid to avoid unregulated, local temperature increases due to the heat of processing. Although ductility and workability of Ti–Ni alloys are sufficient for hot working in this temperature range, fast surface oxidation makes it difficult to fabricate thin wires, which have relatively large specific surface areas. To draw wires with diameters greater than 4 mm, the most widely used method is hot processing, such as hot forging and hot rolling. To draw thinner wires, cold working is typically used in order to avoid surface oxidation.

4.3.2 Cold wire drawing As shown in Fig. 4.7, in Ti–Ni wires, cold working of over 50% would cause work hardening, which increases yield stress and tends to reduce ductility (Yoshida, 1990). To produce thin wires, it is necessary to repeat cycles of cold drawing and soft annealing, which raises the price of Ti–Ni products. In general, soft annealing is carried out between 700 and 800 °C. It is generally thought that the cold working ratio after the last annealing process affects the durability of superelasticity or shape memory properties. If the wire is fully annealed, the plastic deformation occurs easily during the repeated cycles of elongation and contraction due to the low critical stress of dislocation movement. The accumulation of plastic deformation results in the decrease in the recovery force. To prevent the degradation, it

1000

Stress, σ/MPa

800 600 400 200 0 0

200

400 600 800 Temperature, T/°C

1000

4.6 Tensile strength of TiNi at high temperatures.

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1200

60

Shape memory and superelastic alloys 2500 As annealed (f 3) Drawn wire (processing rate 13%) Drawn wire (processing rate 56%)

Stress, σ/MPa

2000 1500 1000 500 0 0

10

20

30 40 50 Strain, ε/%

60

70

80

4.7 Stress–strain diagram of Ti–55 wt%Ni wire (Yoshida, 1990).

Inert gases

Heating zone

Cooling zone

4.8 Continuous furnace for shape memory heat treatment (straight shape).

is necessary to strengthen the wire by work hardening. Work hardening improves durability; however, severe cold working makes the subsequent shape forming difficult. Thus, the amount of final cold working ratio is adjusted in the range of 20–50%, depending on the intended use or shape of the products.

4.4

Forming and shape memory treatment

The main goal of shape memory treatment is to make a specific shape to be memorised. After the product is formed into the desired shape and restrained, it is annealed for several minutes to one hour at a temperature between 300 and 500 °C. In cold-worked Ti–Ni alloy wire, as shown in Fig. 4.8, the linear shape is memorised by annealing the wire for several minutes, after it has been tensioned by pulling. Another typical Ti–Ni product is a spring coil, the forming process of which will be discussed in the next chapter. Formability into a desired shape generally improves as the tem-

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61

perature of shape memory treatment increased. However, recrystallisation may occur at temperatures over 520 °C, about half the melting temperature of Ti–Ni. Note that recrystallisation reduces the work hardening effect of cold working and induces plastic deformation during the use of products, possibly resulting in the degradation of cyclic durability. It has been reported that if an annealing temperature between 350 and 450 °C is used for shape memory treatment, the results of cyclic durability tests do not appreciably vary with annealing temperature (Tobushi et al., 1998). Another important objective of shape memory treatment is the control of transformation temperature. Figure 4.9 shows the effect of shape memory treatment temperature on the temperature dependence of the shear modulus of a Ti–50.6 at.% Ni alloy. It is seen that the transformation temperature increases as the shape memory treatment temperature is decreased. The reason is thought to be as follows. As discussed in Section 4.2.1, the single-phase region becomes narrower as the temperature decreases; thus, when excess nickel is present, if the shape memory treatment temperature is low, a second phase of nickel-rich compounds such as Ti3Ni4 is formed, and as a result, the nickel concentration decreases in the matrix. The austenite start temperature (As), martensite finish temperature (Mf), martensite start temperature (Ms), and Af are affected differently by the temperature used for shape memory treatment. Thus, the temperature dependence of shear modulus varies with the shape memory treatment temperature. As the shape memory treatment temperature decreases, the temperature dependence becomes weaker, and the difference in force generation becomes smaller, suggesting that a higher shape memory treatment

Modules of rigidity, G/GPa

25 450 °C

500 °C 20 15

400 °C 10 5 0 10

20

30

40

50

60

70

Temperature, T/°C

4.9 Effect of shape memory treatment temperature on the temperature dependence of shear modulus of TiNi (50.6% Ni).

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temperature is adequate for on/off actuators, and a lower shape memory treatment temperature is suitable for proportionally controlled actuators. It is summarized that the transformation temperature of the final products of Ti–Ni alloys can be controlled by not only composition but also heat treatment: a rough value is first established from the composition ratio during melting, and fine control is accomplished through the shape memory treatment process.

4.5

References

Aiba M (2001), Keijoukiokugoukin no ouyoutennkai (in Japanease), Tokyo, CMC Hamada T, Uratani F, Hanatate Y and Miyagi M (1983), ‘On the properties of the TiNi shape memory alloys by the high-frequency induction vacuum heating method’ (in Japanese), Technical report of technology research, Institute of Osaka prefecture, No. 82, 20–24 Hosoda H and Miyazaki S (2001), Keijoukiokugoukin no ouyoutennkai (in Japanease), Tokyo, CMC Murray J L (1996), Binary Alloy Phase Diagrams, Ohio, ASTM Nishida M, Wayman C M and Honma T (1986), ‘Precipitation processes in nearequiatomic TiNi shape memory alloys’, Met. Trans. A, 17A, 1505–1515 Tobushi H, Nakahara T, Shimeno Y and Hashimoto T (1998), ‘Low-cycle fatigue of TiNi shape memory alloy and formulation of fatigue life’, J Eng. Mater. and Technol., 122, 186–191 Yamauchi K (1996), ‘Shape memory alloy having the highest shape recovery temperatrure’ (in Japanese), Materia Japan, 35(11), 1195–1198 Yoshida Y (1990), ‘Drawability and diametrical recovery of Ni–Ti shape memory alloy wires (in Japanese)’, Journal of the JSTP vol. 31 no. 355, 1015–1022

© Woodhead Publishing Limited, 2011

5 Design of shape memory alloy (SMA) coil springs for actuator applications T. ISHII, Sogo Spring Mfg Co. Ltd, Japan

Abstract: Titanium–nickel (TiNi) shape memory alloys (SMAs) are used in the form of coil springs in most products because the coil springs generate a large stroke and a high recovery force. Another advantage of using the SMA springs is that they can function as actuators as well as sensors. The SMA spring operates as a two-way actuator by the combination of a bias spring. This chapter introduces the design of the SMA spring and SMA actuator and the manufacturing process of SMA spring. Key words: spring, actuator, manufacturing process.

5.1

Introduction

Since the shape memory effect (SME) in the TiNi shape memory alloy (SMA) was discovered, many products have been developed that use the properties of SMAs (Table 5.1). SMAs are used in a variety of applications such as automobiles, electrical and home appliances, and housing. Most of these products use SMAs in the form of coil springs where they function as an actuator as well as a temperature sensor. The advantage of a coil spring shape is its large stroke compared with a wire as shown in Fig. 5.1, which compares the stroke of a straight wire and a coil of 30 mm free length. When the strain of 1.0% is given, the coil expands 60.3 mm while the stroke of the wire is only 0.3 mm. In this chapter, the design of SMA springs and actuator, and the manufacturing processes for SMA springs are discussed.

5.2

Design of shape memory alloy (SMA) springs

5.2.1 Difference from usual springs Conventional springs are designed to perform within the elastic region where Hooke’s law is obeyed. This allows easy design of the spring since the shear modulus and spring constant do not change within the elastic region. On the other hand, the relation between the deflection and load is not linear for the SMA spring since both shear modulus and spring constant change with strain. Moreover the characteristic changes greatly with small 63 © Woodhead Publishing Limited, 2011

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Shape memory and superelastic alloys Table 5.1 Application of the shape memory effect Application

Shape

Gearless transmission valve Automatic oil valve of Shinkansen Automatic desiccators Louver of air conditioner Coffee maker Rice cooker Camera Miniature robot Anti-scald valve Water purifier Thermostatic mixing valve Bathtub adapter for adding water Underfloor ventilating hole Easy-release screws by SMA washer Rock splitter

Coil Coil Coil Coil Coil Coil Wire Wire Coil Coil Coil Coil Coil Washer Rod

Wire diameter: 1.0 mm Outside coil diameter: 8.0 mm Wire turns: 30 turns Free length: 30 mm γ = 1.0%

30 30 Wire shape

Coil shape

30.3 90.3

5.1 Comparison of strokes of SMAs between wire shape and coil shape.

change in the alloy composition, shape memory treatment condition and operating condition such as temperature and strain. However, for the design of an SMA spring, the formulae of conventional springs are used by assuming the shear modulus is constant if the strain is small.

5.2.2 Selection of materials and shear modulus The materials used for the SMA spring can be classified according to the operation temperature (Table 5.2). The operation temperature of the SMA

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65

Table 5.2 Classification

Material

Use temperature (°C)

TiNi–Fe, Co, Cr

−20–40

TiNi TiNi–Cu

Thermal hysteresis (°C) 2–3

20–80 40–100

Application Actuators for low temperature (underfloor ventilating hole) Thermostatic mixing valve Actuators for high temperature (rice cooker / coffee maker)

2–3 10–15

10

TiNi–Fe

Load (N)

8

TiNi

6

4

2

TiNi–Cu

0 –20

0

20

40

60

80

100

Temperature (°C)

5.2 Load–temperature curves obtained by the fixed strain tests of the SMA springs.

spring is determined by its transformation temperature. The results of thermal cycling tests at the fixed strain for the SMA springs listed in Table 5.2 are compared in Fig. 5.2. It is clear that the recovery temperature increases by the addition of Cu but decreases by the addition of Fe. The shear modulus of the SMA spring exhibits a drastic change when the spring is cooled below the martensitic transformation temperature or heated above its reverse transformation temperature since the shear modulus of high temperature phase (parent phase) is substantially higher than that of the low temperature phase (martensite phase). For instance, the shear modulus of martensite phase is about 8000 MPa while that of the

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parent phase exceeds 20 000 MPa in Ti–Ni alloys. In TiNi–Cu alloys, the shear modulus of martensite phase is almost 0 MPa while that of the parent phase is about 16 000 MPa. It is generally acknowledged that the control of transformation temperature is difficult: only 0.1 at.% difference in composition results from a change of the transformation temperature of about 10 K. Thus the shear modulus of the SMA spring is also sensitive to the alloy composition.

5.2.3 Design of helical springs The characteristic parameters for design of springs are shown in Fig. 5.3. The notations used in the calculation are listed in Table 5.3 and fundamental formulae used for the design of springs are listed as follows:

φd L

5.3 Characteristic parameters of design of a spring. Table 5.3 The sign to use for the calculation Symbol

Meaning

Unit

d D1 D2 D

Wire diameter Inside coil diameter Outside coil diameter Mean coil diameter D = (D1 + D2)/2 Total coils Active coils Free length Load Deflection Shear stress Shear strain Shear modulus Spring index Spring constant

mm mm mm mm

N n L P d t g G C k

– – mm N mm MPa % MPa – N/mm

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D2

P

R

D

P

D1

α

Design of SMA coil springs for actuator applications

67

d = 8PD3n / Gd4

5.1

g = dd / pnD2

5.2

t = 8PD / pd3

5.3

G=t/g

5.4

C=D/d

5.5

The load and the stroke at the high temperature are often demanded as a design condition of the SMA spring. An example of designing the compression spring when the following design conditions are given is shown below. Design conditions The deflection and the load at the high temperature: PH = 5 N at dH = 10 mm. Stroke: St = 5 mm (deflection at low temperature: dL = dH + St = 15 mm) The shear modulus: GH = 20 000 MPa at the high temperature. The shear modulus: GL = 8000 MPa at the low temperature. Shear strain at high temperature: gH = 0.6% Spring index: C = 8 Design procedure The shear stress at the high temperature becomes tH = 120 MPa from equation 5.4. Wire diameter d is calculated using equations 5.3 and 5.5. d2 = 8PC / ptH = (8 × 5 × 8) / (p × 120) d = 0.92 Hereafter, the wire diameter d is assumed to be ϕ1.0 mm. Then, mean coil diameter D becomes ϕ8.0 mm from equation 5.5. The number of coils n is obtained from equation 5.1: n = GHd4dH / 8PHD3 = (20 000 × 1.04 × 10) / (8 × 5 × 8.03) = 9.8 Finally we obtain the dimensions of the spring: 10 active coils with wire diameter of 1.0 mm and mean coil diameter of 8 mm. Shear strain gL at the low temperature is calculated from equation 5.2 using the dimensions of the spring: gL = ddL / pnD2 = (1.0 × 15) / (p × 10 × 8.02) = 0.0075

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Then, load PL at the low temperature is obtained from equation 5.1. PL = dLGLd4/8D3n = (15 × 8000 × 1.04) / (8 × 8.03 × 10) = 2.9 As for deflection dL at the low temperature, less than 0.8% of the total deflection is preferable. The free length L of the spring is an important parameter for considering applications. The solid length Hs of the spring becomes, for instance, 13 mm from the equation d[(n + 2) + 1] if we assume one turn at both end. Then free length L becomes 31.75 mm from the equation L = 1.25dL + Hs. The specification of the SMA spring to fill the abovementioned design conditions is summarized in Table 5.4.

5.2.4 Point of SMA spring design Shear strain gL at low temperature It is preferable to decide gL(= gmax) in the beginning with tmax in consideration of the fatigue life. It should be note that the SMA spring must be used within gmax. The recommended gmax of the SMA spring is as follows: gmax = 0.8% or less for TiNi spring and gmax = 2.0% or less for TiNi–Cu spring. Spring index C C = 6–10 is recommended. The change of C causes the change dimensions of the spring.

5.3

Design of shape memory alloy (SMA) actuators

Conventional SMA springs reveal the one-way shape memory effect. Therefore, in order to operate repeatedly, the bias stress is required. Figure 5.4 shows an example of two-way SMA actuator combined with a bias spring where an SMA spring and a bias spring are set so as to oppose each other (Ohkata and Suzuki, 1998). When the SMA spring is heated above Table 5.4 The specification of the SMA spring Wire diameter Mean coil diameter Total coils Active coils Free length dH = 10 mm dL = 15 mm gL (gmax)

1.0 mm 8.0 mm 12 10 31.8 mm PH = 5 N PL = 2.9 N 0.75%

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Design of SMA coil springs for actuator applications SMA spring

69

Bias spring

Hot P

St Cool P

5.4 An example of a two-way SMA actuator combined with a bias spring and its actuation by temperature change.

the reverse transformation temperature, the axis moves to the right since the recovery force of the SMA spring overcomes the force exerted on the bias spring. The axis returns to original position when the SMA spring cooled below the martensitic transformation temperature. As a result, the SMA spring with a bias spring actuates by heating and cooling. The two-way motion generates a stroke of St. In general, the deflection and load of the SMA actuator can be estimated using the diagram as shown in Fig. 5.5. Figure 5.5 shows load–deflection curves of an SMA spring and bias spring. The slopes of load–deflection curves of the SMA spring and bias spring are spring constants and have opposite signs since they move in opposite directions. If there is an external force, the stroke is shortened to St′ for instance. Figure 5.5 also shows that a larger stroke can be obtained when the spring constant is small.

5.4

Manufacturing of shape memory alloy (SMA) springs

5.4.1 Coiling The SMA coil springs are fabricated using an automatic coil forming machine, which is the same machine that is used for a conventional coil spring. However, a larger forming pressure is required to form a required

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Shape memory and superelastic alloys 10 High temperature 8 Bias spring

Load (N)

6 A

A′ Low temperature

4 B′ 2

B

St′ St

0 0

5

10 15 Deflection (mm)

20

25

5.5 A load–deflection diagram for the SMA spring and bias spring in the two-way SMA actuator.

shape due to a larger spring-back effect of the SMA wires compared with conventional stainless wires or piano wires. Therefore a material and tool which can endure without burning are required. The coil diameter and the free length are changeable according to the adjustment of processing speed and improvement of slipping of the material.

5.4.2 Shape memory treatment (heat treatment) The SMA spring is heat treated after coiling in order to memorize the shape. The heat treatment condition is determined on the basis of productivity (cost) and durability. The spring is fastened on a jig so as to maintain the spring shape during heat treatment and heat treated at about 350–550 °C, followed by air cooling or water quenching. Heat treatment time is from several minutes to 60 minutes. The heat treatment condition is determined to obtain suitable properties such as transformation temperature, hysteresis and durability, which are also dependent on the alloy composition. It it is necessary to adjust the heat treatment condition for each lot of products when mass production, since the properties of each lot are always same even if the processing condition is same. Figure 5.6 shows the effect of heat treatment temperature on the transformation temperature and loads of an SMA spring (Ti–50.6 at.% Ni, g = 0.8%). It can be seen that the transformation temperatures (Af and As) decrease with increasing heat treatment temperature. The transformation

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Design of SMA coil springs for actuator applications

60.0

40.0

10

PH

8

Af

6

AS 4

Load (N)

Transformation temperature (°C)

80.0

71

PL 20.0

2

Ti–50.6 at.% Ni g = 0.8%

0

0.0 410

440

470

500

Heat treatment temperature (°C)

5.6 The effect of heat treatment temperature on the transformation temperature and loads of a SMA spring (Ti–50.6 at.% Ni, g = 0.8%).

temperature width (Af–As), i.e., the difference of Af and As, also decreases with increasing heat treatment temperature. The load at a low temperature PL decreases but the load at high temperature PH remains constant, resulting in the difference (PH–PL) increases with increasing heat treatment temperature. Figures 5.7 and 5.8 show the load–temperature curves for a Ti–50.6 at.% Ni spring heat treated at 470 °C for 60 minutes and 500 °C for 60 minutes, respectively. It is clearly seen that the hysteresis of transformation temperature increases with increasing heat treatment temperature: the spring heat treated at 470 °C exhibits an extremely small hysteresis of 2 °C. These results imply that the heat treatment at 470 °C is preferable to 500 °C since the small hysteresis leads to better durability and longer fatigue life. Figure 5.9 shows the effect of heat treatment temperature on the transformation temperatures and loads for the Ti–50.6 at.% Ni spring applied a strain of 1.0%. Similar dependences of heat treatment temperature were observed: the transformation temperatures, hysteresis and load at low temperature decrease with increasing heat treatment temperature. The load– temperature curve for the Ti–50.6 at.% Ni spring to which a strain of 1.0% was applied is shown in Fig. 5.10. When comparing Figs. 5.10 and 5.7, it should be noticed that the hysteresis of transformation temperature became larger by increasing the strain. The composition of the spring also affects the properties and heat treatment condition. Figures 5.11 and 5.12 show the effect of heat treatment temperature on the transformation temperatures and loads for a Ti–49.6 at.% Ni–Fe spring and a Ti–41.0 at.% Ni–Cu spring, respectively. For the

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Shape memory and superelastic alloys 10 HT 470 °C × 60 min

Ti–50.6 at.% Ni g = 0.8%

Load (N)

8

6

4

2

0 0

10

20

30

40

50

60

70

80

Temperature (°C)

5.7 The load–temperature curve for a Ti–50.6 at.% Ni spring (g = 0.8%) heat treated at 470 °C for 60 minutes.

10 HT 500 °C × 60 min

Ti–50.6 at.% Ni 8

Load (N)

72

g = 0.8%

6

4

2

0 –10

0

10

20

30

40

50

60

70

Temperature (°C)

5.8 The load–temperature curve for a Ti–50.6 at.% Ni spring (g = 0.8%) heat treated at 500 °C for 60 minutes.

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Design of SMA coil springs for actuator applications 10 PH 60.0

40.0

8 Af 6

PL AS

Load (N)

Transformation temperature (°C)

80.0

73

4

20.0

2

Ti–50.6 at.% Ni g = 1.0%

0

0.0 440

410

470

500

Heat treatment temperature (°C)

5.9 The effect of heat treatment temperature on the transformation temperature and loads of a SMA spring (Ti–50.6 at.% Ni, g = 1.0%). 10

Load (N)

8

6

4

2

Ti–50.6 at.% Ni

HT 470 °C × 60 min g = 1.0%

0 0

10

20

30

40

50

60

70

80

Temperature (°C)

5.10 The load–temperature curve for a Ti–50.6 at.% Ni spring (g = 1.0%) heat treated at 470 °C for 60 minutes.

Ti–49.6 at.% Ni–Fe spring, the dependences of transformation temperatures and loads on the heat treatment temperature are similar to those of the binary Ti–Ni spring. However, the best heat treatment condition is different: the spring heat treated at 550 °C exhibited excellent properties with a very small hysteresis for the Ti–49.6 at.% Ni spring as shown in Fig. 5.13.

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Shape memory and superelastic alloys

Transformation temperature (°C)

80.0

10 Ti–49.6 at.% Ni–Fe g = 0.8%

60.0

PH

8

6 40.0 PL 20.0

4

Af

Load (N)

74

2

AS 0

0.0 460

490

520

550

Heat treatment temperature (°C)

5.11 The effect of heat treatment temperature on the transformation temperature and loads of an SMA spring (Ti–49.6 at.% Ni–Fe, g = 0.8%). 10 Af 8 60.0

AS 6

PH 40.0

4 20.0

Ti–41.0 at.% Ni–Cu g = 0.8%

Load (N)

Transformation temperature (°C)

80.0

2

PL 0.0

0 410

440

470

500

Heat treatment temperature (°C)

5.12 The effect of heat treatment temperature on the transformation temperature and loads of an SMA spring (Ti–41.0 at.% Ni–Cu, g = 0.8%).

On the other hand, the Ti–41.0 at.% Ni–Cu spring revealed different temperature dependence: the transformation temperatures increased with increasing heat treatment temperature and the load differences at low and high temperatures are very large. However, it is noted that the Ti–41.0 at.% Ni–Cu spring exhibited higher transformation temperatures even though the hysteresis is large as shown in Fig. 5.14.

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Design of SMA coil springs for actuator applications 10 Ti–49.6 at.% Ni–Fe g = 0.8%

8

Load (N)

HT 550 °C × 60 min

6

4

2

0 –40

–30 –20

–10

0

10

20

30

40

Temperature (°C)

5.13 The load–temperature curve for a Ti–49.6 at.% Ni–Fe spring (g = 0.8%) heat treated at 550 °C for 60 minutes.

10 Ti–41.0 at.% Ni–Cu g = 0.8%

8

Load (N)

HT 440 °C × 60 min

6

4

2

0 20

30

40

50 80 60 70 Temperature (°C)

90

100

5.14 The load–temperature curve for a Ti–41.0 at.% Ni–Cu spring (g = 0.8%) heat treated at 440 °C for 60 minutes.

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In conclusion, it should be mentioned that not only the heat treatment condition but also strain have a strong effect on to the properties of the SMA spring. Furthermore, the alloy composition is another important factor controlling the properties of the SMA spring.

5.5

Reference

Ohkata I and Suzuki Y, (1998), ‘The design of shape memory alloy actuators and their applications’, in: Otsuka K and Wayman C M, Shape Memory Materials, Cambridge, Cambridge University Press, 240–266.

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6 Overview of the development of shape memory and superelastic alloy applications S. TAKAOKA, Furukawa Electric Co. Ltd, Japan

Abstract: This chapter introduces the historical overview of the application developments for shape memory and superelastic alloys in Japan. The history of applications of shape memory and superelastic alloys in Japan can be divided into four periods as follows. The first period began in 1981 and continued until 1985; main applications in this dawn period were toys and coil springs for home electronics. The second period began with the application of Ti–Ni shape memory alloys for bra wires in 1986. This sensational product attracted attention and made the alloys popular in Japan, and was followed by various applications for apparel, accessories and eyeglass frames. The third period started in 1992 with the application of superelastic Ti–Ni wires as antennas for mobile phones. The fourth period began in 2001 and continues to the present. Applications of superelastic alloys have been exploited in a number of medical devices since 2001. Shape memory wires and coil springs have been applied as actuators in automobiles and ecologic products. The developments of novel shape memory alloys, such as high temperature shape memory alloys and Ni-free shape memory alloys, and advanced fabrication technologies will further accelerate and expand applications of shape memory alloys. Key words: shape memory alloy (SMA), superelastic (SE) alloy, history, application.

6.1

Introduction

Application of shape memory alloys (SMAs) and superelastic (SE) alloys in Japan is characterized by a wide diversity of commercial products. The major practical applications in Japan are listed in Fig. 6.1. The application of shape memory has not been fixed in a certain field but has constantly changed to keep up with current trends and has stimulated the development of applications in new fields. This chapter introduces an historical overview of the application developments of shape memory and superelastic alloys in Japan.

77 © Woodhead Publishing Limited, 2011

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Year

All round SMA flower

Artificial flower

Orthodontic wire

Radifocus guide wire

Dental implant

Eyeglass frames(temple)

Bracelet

Brassiere

Fire door

Micro-arm robot

Portable fire alarm

Hot water ejector of toilet

Steam trap

Fishing line

Micro cylinder

Hat

Petticoat

Flow control valve

Camera

Tennis racket

Valgus hallux splint

Catheter (IABP)

Creel frame

Higher radiopacity guidewire Reamer shaft for orthopedics Gynecological examinatin equipment

Ferrous SMA Fishplate for crane rail

Fishing rod

Change of clothing house

Biliary Sendai stents

Thrombus filter

G Spiral catheter

Revo Wave guide wire

MD player

Comfortable apron Fishing balancer

Pin for watch band

SE Applications

SME Applications

Valve for continuously variable transmission

2005

Actuator for choking gas-passage

Drain of air conditioner

2000

Easy-release screws by SMA washer

High stiffness guidewire (not SEA)

Pipe coupling for extension wires

Shoulder pad of jacket

Underfloor ventilating hole

Flow contorol valve for water heater

Anti-scald valve

Bathtub adapter for adding water

Automatic oil valve of Shinkansen

1995

Headband of headphone

Antenna for cell-phone

Satellite

Pipe coupling for nuclear generator

Girdle

Shoes

Static rock splitter

Water purifier

Bathtub adapter for keeping hot

Paint gun for car bodies

Rice cooker

Outer vent control valve

Manual transmission

1990 Sterilizing tablewares storage box

Soft-boiled eggs cooker

Coffee maker

Louver of fog lamp

Eyeglass frames(rim)

Toy

1985

Electromagnetic cooker

6.1 The history of SMA applications in Japan.

Medical

Mobile communication

Apparel ∙ Leisure

Construction ∙ Housing

Hot-water supply system

Car ∙ Train

Louver of air conditioner

Microwave

Automatic desiccator

1980

Association of Shape Memory Alloys

Home electronics

Field

ASMA

Overview of the development of SM and SE alloy applications

6.2

79

History of the applications of titanium–nickel (Ti–Ni)-based shape memory alloys (SMAs) and superelastic (SE) alloys

Since the first commercial product was released in 1969, a variety of applications have been developed. The history of applications of SMAs and SE alloys in Japan can be divided into four periods as follows depending on the major application field.

6.2.1 Before the first stage (1964~) The shape memory effect (SME) in a Ti–Ni alloy was confirmed in 1964 in Japan, which is the same year in which the SME was found in America. However, it was commercialized by suppression of the SME by adding alloying elements because the object of the development was a structural alloy as anti-corrosive, heat-resistant and anti-abrasion materials. In the early 1960s, the SME of Ti–Ni alloys attracted attention. In this period, some application developments for heat engine and actuator were carried out in the research institutes of companies and universities but nothing was successfully commercialized. The basic properties and fabrication process of the SMAs were established in this period.

6.2.2 First stage (1981~) A Ti–Ni SE alloy was used as eyeglass frames in 1981. This was the first application of superelasticity. This application was highlighted by ‘metal with memory’ in mass communication and triggered a sensational boom in Japan. Inquiries increased enormously in a short period of time from a variety of industries including the automotive industry and home appliances industry. With the boom of ‘new materials’, the SMAs were picked up by the science magazines and media for the public. In particular, demonstrations of the SME and shape recovery by heating on TV attracted great attention from the public. Soon the SMA became well known and popular among the general public. Although a number of patents for the application of the SME were filed, there were few products that were commercialized successfully in this period. The applications in this period were limited to fancy goods rather than the main components in automotive and home appliance industries, mainly due to the high cost, low reliability and poor durability of the alloys.

6.2.3 Second stage (1986~) A SE TiNi alloy was adopted as bra wire by Wacoal Corp. in February of 1986. Owing to the extensive advertising campaign, the SMA attracted wide

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publicity again. This novel combination of ‘smart metal’ and ‘underwear’ created a sensation. This success caused many inquiries from the apparel industry on the possibility of applications of shape memory wires in accessories such as hats and shoes. The possibility was even considered that shape memory fibers could be interwoven with clothing materials. At a similar time, SE TiNi alloys started to be used as eyeglass frames and in orthodontics. However, after 10 years the application was discontinued, mainly due to the high cost compared with its benefits, and iron-based alloys started to be used instead. Many metal-making companies started to produce SMAs in this period. The number of companies that presented their products at exhibitions increased to 17. Many new technologies for making TiNi alloys were developed: TiNi alloys could be fabricated using powder metallurgy and selfpropagating synthesis from Ti and Ni wires. In addition to wire shape, TiNi alloys were fabricated as rolled plates.

6.2.4 Third stage (1992~) The third stage started in 1992 with the adoption of SE Ti–Ni wire as whip antennas for mobile phones. It was sold in large amounts with the rapid expansion of the market for mobile phones; it was the item that sold the most. SMAs were considered to be one of the general-purpose materials in this period. The requirements for antennas were not difficult to fulfill; straight wires could be used with a transition temperature less than room temperature Furthermore, it was not necessary to carry out the special shape memory heat treatment. This allowed convenient mass production and could reduce cost considerably. Unfortunately, the demand for whip antennas decreased drastically due to the development of internal antennas. Recently SE external antennas have been used again for one segment of broadcasting reception, but the number is far below that of the cases for whip antenna in the 1990s. In addition, SMA coils were utilized in water supply facilities in houses, such as in hot water cutoff valves, mixing valves and temperature controller regulators. In this period where the SME contributes an indispensable function of the items. The makers did not emphasize utilizing SMAs in their goods since it was not considered to be a special material during this period. It was expected that the SMA could be applied in other major industries such as the electronic, automotive and construction industries. However, the applications were not successful because of expense due to the high price of raw materials of Ti and Ni, poor workability and difficult shape memory heat treatment.

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6.2.5 Fourth stage (2001~) This stage has been characterized by the application to medical devices. The unique properties of TiNi SE alloys, such as large elastic deformation, good flexibility and excellent biocompatibility, attracted attention from the medical industry. In addition, the SMA makers also changed the target to the medical field, i.e., high profit business, because of the difficulty of costs as mentioned above. Although a Japanese company, Terumo Corporation, was awarded an international patent for guide wire for catheter, the utilization of TiNi SE alloys for angiography devices has been seriously delayed in Japan compared with western countries. Many medical devices utilizing TiNi SE alloys have been developed by a number of companies along with the improvement of the fabrication technology. The application to stents was made possible through the success in the fabrication of thin tubes of TiNi alloys. There is a great need for strict quality control for medical devices, thus the alloy makers have continued research and development for improving fabrication technologies; one of the important topics is reducing the inclusion size which causes breakage. As well as the applications for medical devices, applications for actuators became an attractive theme. The shape memory actuators, operated by electrical resistance heating, are expected to be utilized in portable electronic devices. The high martensitic transformation temperature is required to prevent malfunction of the SMA actuator. However, the higher martensitic transformation temperature causes reduced durability of TiNi actuators. Recently, active research has been carried out for utilizing SMA wire in the auto-focus system for lenses of mobile phone cameras.

6.3

Other shape memory alloys (SMAs)

6.3.1 Cu-based SMAs Besides TiNi-based alloys, the application development of CuZnAl alloys is the most advanced. Three companies, Dowa Metals (now Dowa Holdings), Mitsubishi Metals (now Mitsubishi Materials) and Sumitomo Special Metals (now Neomax), in Research Cooperative Union for Processing of SMAs, the predecessor of the present Japan Association of Shape Memory Alloys, developed Cu-based SMAs and some of them were commercialized by toys and cocktail stirrers in 1980s. Furukawa Electric Co. Ltd. developed a CuZnAl SMA for use in pipe coupling in air conditioners. However, manufacturing has been discontinued very early at all companies. In Cu-based shape memory alloys, the application development of a CuAlMn alloy is an

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innovation. It is expected to be applied to architectural fields and three-dimensional geometry products because the alloys exhibit good workability and the restraint device for shape memory treatment is not necessary.

6.3.2 Fe-based SMAs Nippon Steel Corporation developed Fe–Mn–Si-based alloys and succeeded in the application; now Awaji Materia Co. Ltd has succeeded to the business.

6.3.3 Other novel SMAs and SE alloys Ti–Ni-based high temperature SMAs The addition of alloying elements such as Zr, Hf, Pd, Au and Pt raises the martensitic transformation temperatures. Extensive research has been carried out for the practical applications of these high-temperature SMAs as electrically heated actuators. Ni-free SMAs Recently, Ni-free SMAs have been developed for medical devices and implants. In the Ti–Nb and Ti–Mo-based alloys, stable superelasticity has been confirmed at room temperature. Magnetic SMAs Magnetic SMAs such as Fe–Pd and Ni–Mn–Ga alloys have been investigated for potential use on actuating devices and sensors due to their faster response than classical SMAs which respond to temperature changes.

6.4

Examples of the main applications of titanium– nickel (Ti–Ni)-based alloys

This section briefly introduces the examples which have led to a change in the major application field. Details of the application examples in each field are presented in the following sections.

6.4.1 Eyeglass frames (rim) Eyeglass frames are the first practical application utilizing superelasticity in the world and the term ‘superelasticity’ was used for the first time. Plastic

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lenses easily detached from metal frames when exposed to cold atmospheres because plastic lenses shrink more than metal frames. The superelastic TiNi frame keeps the holding force constant even though plastic lenses shrink in a cold atmosphere. (1981: K. Hattori & Co. Ltd, now Seiko Holdings Corporation).

6.4.2 Toys Toys were the first commercial product utilizing the SME in Japan (1982: Takara Co. Ltd, now Tomy Company, Ltd).

6.4.3 Automatic desiccators An SMA spring was installed in an automatic actuating system in order to recycle the desiccant. This was the first practical equipment that applied the SME in Japan (1982: Toyo Living Co. Ltd).

6.4.4 Louver of air conditioner A sensor flap composed with a SMA spring and a bias spring detects the temperature change and switches the direction of wind; the flap guides the flow down when heating while it guides the flow up when cooling. This is the first practical item mass produced in Japan. During development research, the relation between R phase transformation and durability of the shape memory alloy was clarified (1984: Matsushita Electric Industrial Co. Ltd, now Panasonic Corporation).

6.4.5 Bras Superelastic wires were sewn up under the bust at the first time. Then the wires were applied in many areas of underwear such as the upper bust, corset and girdle. This item made SMAs famous among the public (1986: Wacoal Corp.).

6.4.6 Rice cooker A Ti–Ni–Cu alloy spring was applied in a rice cooker to release steam when steaming rice and to close the valve after cooking in order to retain the temperature of the vessel. This was the first practical application of Ti–Ni– Cu alloys where Cu was added to increase the shape recovery temperature (1988: Tiger Corp.).

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6.4.7 Oil controller in Shinkansen Oil-controlling devices composed of a shape memory spring and a bias spring were adopted for supplying the optimum amount of oil according to the oil temperature. This application is still being used in Shinkansen. The shape memory spring used in this device is the largest among practical applications (1998: Toyo Denki Seizo Co., Ltd, Central Japan Railway Company).

6.4.8 Static rock breaker A set of SMA cylinders was utilized for rock breaking because the large shape recovery force during heating could break rocks and concrete without dust and noise. It was developed by the research project of Japan Association of Shape Memory Alloys (1990: Nishimatsu Construction Co. Ltd).

6.4.9 Guide wire A high stiffness with no hysteresis could be achieved in the alloy with the same composition exhibiting superelasticity through thermomechanical treatment and it is applied to high stiffness guide wires (1999: ExelMedi, now AuBEX Corporation).

6.4.10 Camera The SMA wire was applied as an actuator in cameras for a safety lock switch, which is the first practical application for actuators operated by electrical current (2001: Minolta Co. Ltd, now Konica Minolta Holdings Inc.).

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7 Applications of shape memory alloys (SMAs) in electrical appliances T. HABU, Furukawa Techno Material Co. Ltd, Japan

Abstract: Shape memory alloy springs have been applied as sensors and actuators in various applications in combination with bias springs since they respond and generate force at the same time by temperature change. The shape memory spring actuator can also save weight and space and reduce the number of parts needed, such as a sequencer, a motor, a sensor and a power supply that are required for conventional actuators. Various ideas for the application of shape memory actuators have been suggested from home electronics and materials makers and some of them have been successfully commercialized. This chapter introduces commercial productions using shape memory actuators in household appliances and electrical equipment. Key words: shape memory, spring, heating, thermal sensor, actuator.

7.1

Introduction

There have been substantial efforts to develop applications of shape memory alloys (SMAs) since the early 1980s by utilizing their unique properties, superelasticity and shape memory effect. At first, the SMAs were commercialized as actuators and sensors in household appliances and electrical equipment. In addition, there were some practical applications of SMAs for heat engines and two-way types of application. The application of SMAs has been expanded to various fields in recent years, e.g., microelectrical devices, biomedical industry and transportation industry. Also there have been many trials to use SMA wire or spring into the artificial muscle of a robot as an actuator. In this chapter, products utilizing SMAs, mainly developed by Japanese companies, are introduced and basic operation mechanisms are also explained briefly.

7.2

Automatic desiccators

Figure 7.1 shows automatic desiccators equipped with an SMA actuator system and the schematic explanation of the operating mechanism. An SMA spring opens and closes the door to discharge moisture inside the box by heating and cooling, which also heat-recycles the desiccant. This was the first mass production application of SMA in Japan. 87 © Woodhead Publishing Limited, 2011

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Dehumidification

Bias spring

Main dry space

Moisture discharge

Bias spring

Main dry space

Dehumidifier compound

SMA spring

SMA spring

Dehumidifier compound

Dehumidification Heater off

Heater on

Moisture discharge

7.1 Automatic desiccator (courtesy of Toyo Living Co., Ltd).

7.3

Products utilizing shape memory alloys (SMAs)

7.3.1 Louver of air conditioner Figure 7.2 shows the ‘sensor flap’ of an air conditioner composed of an SMA spring and bias spring. The Ni–Ti tension spring at the front panel senses the temperature change of air flow from an air conditioner, and moves the flap to change the direction of the air flow automatically. When the air flow temperature is cold the flap guides the flow upward while warm air is directed downward. This driving system has advantages over conventional

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Air suction

Cool air Ni–Ti spring

Bias spring

Warm air

7.2 Louver of air conditioner (courtesy of Matsushita Electric Industry Co., Ltd).

Charcoal filter Mill Bias coil spring

Coffee beans

Ni–Ti coil spring

7.3 Coffee maker (courtesy of Matsushita Electric Industry Co., Ltd).

electromagnetic systems in cost, reliability and low noise. It was the first mass produced product of a shape memory alloy coil spring. It has sold more than one million pieces.

7.3.2 Coffee maker Figure 7.3 shows an electric coffee maker which utilizes a Ni–Ti–Cu spring as a valve controller. When the water in the pot is boiled, the shape memory spring expands and opens the valve. Then, very hot water is poured on coffee beans through a charcoal filter. This makes good tasting coffee. In this application, initially the spring actuated when the R-phase transformation of the Ni–Ti alloy was applied, but later it was replaced by a small

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Ni–Ti–Cu spring because of its large force difference between high and low temperatures. Additionally, the use of an Ni–Ti–Cu spring lowers the cost.

7.3.3 Hot water ejector of toilet The toilet stool that ejects a spurt of warm water for cleaning adopts the SMA spring as the actuator. Figure 7.4 shows an example of a toilet stool and the schematic explanation of operation. While cold water remains inside the ejector nozzle, the SMA spring does not react, so water comes

Nozzle cover

SMA spring

Bias coil With cold water

With hot water

7.4 Hot water ejector of toilet (courtesy of Toshiba Corporation).

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from the lower outlet. As the hot water arrives at the nozzle, the spring extends to switch the water path for ejection.

7.3.4 Rice cooker Figure 7.5 shows a rice cooker, which contains a Ni–Ti–Cu SMA spring in its valve system, and the operating principle of the valve. During boiling and steaming rice, the SMA spring expands when the temperature exceeds the transformation temperature of the spring and then the steam in the inner vessel comes out through the valve. The steam pressure is adjusted so as to cook the best tasting rice. The force of the shape memory spring decreases and the valve shuts after cooking, in order to hold an appropriate temperature in the inner vessel.

7.3.5 Water purifier An actuation system with a Ni–Ti SMA spring has also been applied in a domestic water purifier in order to protect the filter from accidental hot water exposure. The SMA spring has been designed to protect the filter from the flow of hot water. When the cold water is flowing, the SMA spring pulls the valve open to guide the cold water to the filter, while the SMA spring closes the valve when it is heated so that the hot water cannot flow through the filter as shown in Fig. 7.6.

7.3.6 Transformer overheating detector The SMA has been applied as a sensor for abnormal temperature increase of the power lines for trains. The SMA is clamped parallel to the power line. When the temperature of the power line rises above the transformation

NT spring Bias spring

Steam

7.5 Rice cooker (courtesy of Tiger Corporation).

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Cool water

Water filter

Spring

Valve plug

7.6 Water purifier (courtesy of TOTO and Nihon Trim Co., Ltd).

temperature of the SMA, it bends into a U-shape as shown in Fig. 7.7. The SMA detector allows easy detection of overheating from the ground.

7.3.7 Ventilating damper for sterilizing tableware storage box Figure 7.8 shows the ventilating damper for a sterilizing tableware storage box utilizing a Ni–Ti spring. This storage box is used in schools or large cafeterias. It holds the temperature at around 90 °C to sterilize tableware. A damper with an SMA coil spring on the top position keeps the box

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7.7 Transformer overheating detector (courtesy of Chugoku Electric).

Moisture discharging damper

SMA spring Bias spring

7.8 Ventilating damper for sterilizing tableware storage box (courtesy of AIHO company).

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airtight until 62 °C for quick heating. When the temperature has increased enough, the damper opens and vents the steam, resulting in a high sterilizing effect.

7.3.8 Gynecological examination equipment Gynecological examination machines are mainly used by doctors in obstetrics and gynecology departments for examinations that irrigate and dry the affected part. Figure 7.9 shows a gynecological examination machine equipped with a protecting valve from hot water actuated by a SMA coil: if the washing water temperature becomes higher than 50°C, the valve unit with SMA coil spring set inside cuts the flow.

7.4

Electric current actuator

As mentioned in this chapter, Ni–Ti SMA springs have been applied in various fields where they actuate by changing the environmental temperature. The actuation of the SMA is also possible by joule heating of SMA under a fixed load as shown in Fig. 7.10. The Ni–Ti SMA wire is heated simply by supplying electric current due to the high electrical resistance of

7.9 Gynecological examination equipment (courtesy of Takara Belmont).

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[Electricity on]

NT wire Cooling

Heating

Expansion

Contraction Weight

Weight

7.10 Electric current actuator.

Bias spring

NT wire

7.11 Miniature robot (courtesy of Toki Corporation).

the Ni–Ti alloy. When the current is cut off, the SMA wire elongates by the weight, which is due to the reorientation of martensitic variants. When the SMA wire is heated by the electrical current, the SMA wire contracts to the original length due to the reverse transformation. As a result, the SMA wire can actuate by controlling the electric current.

7.4.1 Miniature robot Figure 7.11 shows a miniature robot equipped with SMA wires that actuate its joints. The SMA wire contracts when it is heated electrically while it expands when the heat is radiated after cutting the current due to the bias spring. This actuating system provides immediate control of the robot’s joints (grip, wrist, elbow, shoulder) since it can be simply actuated by an electric on/off switch.

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7.4.2 Camera The SMA wire was utilized as an actuator for a safety lock switch in cameras (Fig. 7.12). The safety lock closes the rear cover during rewinding the film. This is the world’s first electrical current actuator operated by SMA wires.

7.4.3 Minidisk (MD) player There have been many efforts to reduce the weight and size of MD players. The SMA wire has been used as a drive source for the elevator style of

Schematic

Plate for opening and shutting rear cover

SMA wire Safety lock driving lever

Safety lock switch lever

Transfer cam

Coupling plate Button opening rear cover

7.12 Lock switch actuator for camera (courtesy of Konica Minolta Holdings Inc.).

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magnetic head (Fig. 7.13). Utilizing the SMA wire as an actuator makes it possible to minimize the required space and provide a simple system structure. The utilization of SMA wire for actuators successfully reduced the weight of MD players by 22% while offering long continuous playback time.

7.4.4 Soft-boiled egg cooker The Ni–Ti SMA spring was installed for keeping a suitable hot-water temperature in an egg cooker (Fig. 7.14). The Ni–Ti SMA spring in an

7.13 Electric current actuator for MD player (courtesy of Matsushita Electric Industry Co., Ltd).

Schematic Temp. <70 °C

Egg SMA spring Iron plate

Electromagnetic cooker Temp. >70 °C

7.14 Egg cooker (courtesy of Sony Corporation).

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electromagnetic cooker keeps water temperature at around 70 °C when it is suitable for cooking soft-boiled eggs.

7.4.5 Solar paddle actuator A solar paddle actuator1 composed of six SMA springs was used in a small satellite (Fig. 7.15). The actuator, which measures 100 mm in diameter and 127 mm in height and weighs approximately 660 g including the counterweight (340 g), can orient the solar paddle alone toward the Sun. The counterweight effectively compensated for the rotating motion of the small satellite in microgravity. Motion repetition experiments in vacuum were conducted to investigate the degradation properties of the SMA.

7.4.6 Toy SMAs have been also applied in toys due to their strange and mysterious performance (Fig. 7.16). These toys attracted the interest of many people in Japan, not only children but also adults, and help to make SMAs very popular. Also many kinds of educational toys that can help to teach the shape memory effect have been produced.

7.4.7 Artificial flower It is common to use an SMA spring in combination with a bias spring for two-way action. Also the SMA spring can create a two-way action by itself with training, cyclic treatment of heating and cooling, after shape memory heat treatment. However, there have been few practical applications because of poor reliability, slow reaction and small power. One of the typical examples utilizing the two-way action is the flowers shown in Fig. 7.17. The

7.15 Solar paddle actuator (courtesy of Actment Co., Ltd).1

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7.16 Toy (courtesy of Takara Tomy Company Inc.).

7.17 Artificial flower (courtesy of Create 7).

SMA wires exhibit good performance in smooth actuation, opening and closing the flowers: the SMA wires actuate to open the flower when heated by light and close slowly as they cool down after switching off.

7.5

Reference

1. Iwata T et al. (2008), Proceedings of the International conference on shape memory and superelastic technologies 2007, ASM International, 397–404.

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8 Applications of shape memory alloys (SMAs) in hot water supplies A. SUZUKI, Daido Steel Co. Ltd, Japan

Abstract: The application of shape memory alloys (SMAs) in a hot water supply system is described in this chapter. A faucet with a water temperature regulator is one of the most important applications of SMAs. The combination of a SMA spring that expands or contracts automatically by water temperature and a bias spring operated by a water temperature control knob manually can achieve a comfortable water temperature by the precise adjustment of the hot and cold water mixture. Other applications such as bathtub adaptors for adding water and for holding water temperature are also described. Key words: hot water supply, temperature regulator, valve, bathtub adaptor.

8.1

Shower faucet with water temperature regulator

A water temperature regulator for a shower faucet is one of the most well-know applications of shape memory alloys (SMAs). Figure 8.1 shows an example of a regulator containing an SMA spring and the schematic illustration is shown in Fig. 8.2. The regulator is composed of the following elements: (1) hot water and cold water inlets, (2) a shape memory spring and a bias spring which is made of stainless steel, (3) a flow control regulator valve for hot water and cold water and (4) a knob for temperature adjustment. When the outlet water temperature is higher than the set temperature, the SMA spring expands due to the reverse transformation so that the inlet valve for hot water closes and the inlet valve for cold water opens. As a result, the temperature of the mixed water decreases. On the other hand, when the outlet water temperature is lower than the set temperature, the SMA spring shrinks to open the valve for hot water and to close the valve for cold water, resulting in an increase in the outlet water temperature. The middle of the R-phase transformation temperature and the reverse transformation temperature of the SMA spring is about 40 °C and then the spring moves to control water flow in proportion to the difference between the set temperature and the outlet water temperature. 100 © Woodhead Publishing Limited, 2011

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Bias spring

101

SMA spring

8.1 An example of a regulator consisting of a SMA spring and a bias spring.

Bias spring

Cold water SMA spring

Mixed water

Knob for temperature Hot water adjustment

Flow control valve

8.2 Schematic of the regulator consisting of a SMA spring and a bias spring.

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d SUS, 1 d SUS, 0

d SMA, 0

C

→ F

KSUS

K50 K40

B Δ F0

A

Δ d0 Δ d1 → d

d0

d1

8.3 Load–deflection diagram explaining the balance points of the SMA spring and bias spring.

The movement of the SMA spring by changing the outlet water temperature can be explained using the load–deflection diagram as shown in Fig. 8.3. The SMA spring and bias spring are set out so that they work in opposition other than in the regulator. Therefore, the slopes of load–deflection curves have opposite signs. As shown in Fig. 8.3 the spring constant of the SMA spring is a function of temperature whereas that of the stainless spring remains constant, where K50 and K40 imply the spring constant of the SMA spring at 50 and 40 °C, respectively, and Ksus indicates the spring constant of the bias spring. When the set temperature is 40 °C, the SMA spring and the bias spring are balanced at point A where the deflections of the SMA and bias springs are dSMA,0 and dSUS,0, respectively. If the temperature becomes 50 °C, the balancing position shifts to point B. The bias force increases and then ΔF0 is applied to the flow control valve to move Δd0, resulting in the decrease of the mixed water temperature. When the set temperature is changed to 50 °C, the bias spring is compressed and also the bias force increases, resulting in the stroke moving to Δd1 which corresponds to the amount of adjustment of the knob. As a result, the balancing position moves to point C.

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8.4 Gas flow shielding device with a SMA disk.

8.2

Gas flow shielding device

There is a potential for gas leakage due to melting of the gas meter when fire occurs because the gas meter and joining parts are made of die-cast aluminum. This concern for gas leakage has led to the development of a gas flow shielding device operated by a thermal sensor that uses a SMA to cut the gas flow. As shown in Fig. 8.4, a thin circular SMA disk, which is bent at room temperature, is set in the gas flow path. If fire occurs, the atmosphere temperature rises above 70–80 °C and the SMA disk becomes flat. This closes the gas flow path, as shown in Fig. 8.5. There is a ring under the valve to fix the SMA disk in the gas flow path. A rod axis is installed at the upper side of the bent SMA disk as shown in Fig. 8.4 in order to smooth the flow of gas during ordinary use.

8.3

Bathtub adaptors

SMA springs are also applied in hot water safety valves for shower faucets and a bathtub adaptor for maintaining water temperature. The hot water safety valve for the shower faucet for preventing scalding is set at the connecting part to the boiler as shown in Fig. 8.6. Figure 8.7 illustrates the operating mechanism of the thermal protection valve by the combination

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Temperature rises

SMA plate

8.5 Operating principle of gas flow shielding device.

Shower Bathtub Boiler

8.6 Boiler with SMA hot water safety valve.

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SMA spring

Warm water

Hot water

8.7 Operating principle of hot water safety valves in bath.

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of a SMA spring and a bias spring. When hot water flows through a SMA spring, the SMA spring expands to close the valve because the recovery force overcomes the bias spring force. The SMA springs are also applied to bathtub adaptors for adding hot water safely. Two hot water safety valves are set in the outlet of the bath as shown in Figs. 8.8 and 8.9. The reverse transformation of the SMA spring is about 70 °C so that the SMA springs contract to close the valves when the hot water is higher than 70 °C. Both SMA valves cut hot water when the bathtub is not filled enough. On the other hand, only the upper spring

Bathtub Bathtub adaptor

8.8 Hot water safety valves in outlet of bath.

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Applications of SMAs in hot water supplies Supplying hot water

Hot water

Bias spring

SMA spring

Wall

Heating water and mixing

Hot water

Bias spring

SMA spring

Wall

8.9 Operating principle of hot water safety valves in bath.

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Adaptor with SMA spring

8.10 Cut-off valve for circulation of hot water in the tub.

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During warming Adaptor opens Burning

Water flowing

After warming Adaptor closes Stopping

No flow

8.11 Operating principle of cut-off valve for circulation of hot water.

operates if the bath is filled with hot water because the lower valve is cooled by water in the bath. This means hot water can be added safely in the bathtub. Another example is the cut-off valve for circulation of hot water in the tub. The valve is set in the outlet of the boiler and prevents the circulation of hot water in order to hold water temperature in the bath as shown in Figs. 8.10 and 8.11.

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9 The use of shape memory alloys (SMAs) in construction and housing M. OZAWA, NEC TOKIN Corporation, Japan, A. SUZUKI, Daido Steel Co. Ltd, Japan and T. INABA, Nishimatu Construction Co. Ltd, Japan

Abstract: This chapter presents applications of shape memory alloys (SMAs) in various environmentally conscious technologies and products such as recycling, reducing noise and energy conservation. The underfloor ventilation hole with a thermal actuator composed of the SMA and bias springs can automatically open and shut depending on air temperature. SMAs have been utilized as noiseless rock breakers. The easy-release screw using a shape memory washer is effective in improving recycling efficiency in the consumer electronics industry. Key words: environment, energy conservation, recycle, actuator, shape memory alloy (SMA), breaker, easy-release screws.

9.1

Introduction

Recently technologies for reducing energy consumption and creating a clean environment have attracted attention. Shape memory alloys (SMAs) play an important role in the clean environment yield because of their unique properties. Ventilators in houses using a thermal actuator composed of an SMA coil and bias springs can automatically open and shut depending on air temperature. This ventilator hole is an energy conservation device since it operates without requiring an energy source, yet, by closing the ventilator in cold weather, improves the heat insulation of a house in a cold area. The noiseless rock breaker is another interesting example of SMAs. The SMA rock breaker generates the recovery force when it is heated by hot water or joule heating, and the recovery force acts as solid pressure in the rock and breaks the rock without causing a blast. Many applications of SMA rock breakers have been proposed, e.g; movement of the stones in golf courses, dismantling of buildings and removal of underwater structures in harbors. The easy-release screw using a shape memory washer could improve recycling efficiency in the consumer electronics industry. A conventional metal screw in combination with a washer made of an SMA makes it possible to disassemble components easily. The components are disassembled by simple heating because the shape memory washer expands 110 © Woodhead Publishing Limited, 2011

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and releases the screw. They can then be collected for recycling. This chapter introduces a number of uses of SMAs in construction and housing and explains their basic mechanisms.

9.2

Underground ventilator

Many houses need ventilators, which are often situated in the basement. Where there are ventilators in the basement, it is necessary to close the underground floor in the winter time to decrease heat dissipation, since the heat loss from the ventilator accounts for 18.1% of the total heat dissipation. To prevent the loss of energy, the Ni–Ti–Fe SMA coil with a bias spring is used for automatic opening and closing of the underground ventilator as shown in Figs 9.1 and 9.2. Another benefit of using the SMA actuator is that it is an environmentally friendly technology that does not use any electricity. Figure 9.3 shows the schematic explanation of the operation mechanism of an automatic underground ventilator by changing environmental temperature. When the temperature is 3 °C or below in the winter, the power of the SMA coil becomes weaker than that of the bias spring so that the ventilator is closed and no wind comes from underground. When the temperature is 15 °C or above in the summer time, the power of the SMA coil overcomes that of the bias spring so that ventilation is provided and moisture is removed.

9.1 Underground ventilator (courtesy SAHARA Co. Ltd).

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High

Low

9.2 Motion of underground ventilator by changing environmental temperature (courtesy NEC TOKIN Corporation).

9.3

Static rock breaker

In civil engineering, rock drills or blasters are used for crushing rocks, destruction of buildings and dredging harbors. However, it is considered that these methods cause environmental problems such as noise, vibration and dust. A static rock breaker utilizing SMAs has been suggested (Figs 9.4 and 9.5). The Ti–Ni SMA rock crushing device generates strength of 60 tons within 60 seconds with no sounds or vibrations.

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Hot season (>15°C)

Bias spring

SMA spring

Cold season (<3°C)

Bias spring

SMA spring

9.3 Schematic mechanism of underground ventilator (courtesy Furukawa Techno Material Co. Ltd).

SMA cylinders (f = 15 mm and L = 29 mm) with the reverse transformation start temperature (As) of 50 °C are compressed to 28 mm by a pressure of 30 tons in advance as shown in Figs 9.6. When the SMA cylinders are heated to over their As temperature, each SMA cylinder generates a power of 10 tons (Figs 9.7). The recovery force decreases with increasing recovery displacement. The schematic motion of a SMA cylinder is shown in Figs 9.8. The brief procedures are explained as follows (Figs 9.9). (1) Drill a hole

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9.4 A set of static rock breakers with SMA cylinders (courtesy NEC TOKIN Corporation).

9.5 Rock breaking using a static rock breaker (courtesy NEC TOKIN Corporation).

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300

Load (kN)

200

100

0

0

1 2 Displacement (mm)

3

9.6 Load–displacement curve of an SMA cylinder (courtesy NEC TOKIN Corporation).

Recovery force (kN)

150

100

50

0

0

0.5 1.0 Recovery displacement (mm)

1.5

9.7 Recovery force–displacement curve of an SMA cylinder (courtesy NEC TOKIN Corporation). Shrink Press

Shape recover Heat

9.8 Schematic motion of an SMA cylinder (courtesy NEC TOKIN Corporation).

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1. Boring

2. Set platen

3. Insert

4. Heating

5. Split

9.9 Schematic of static rock breaker (courtesy NEC TOKIN Corporation).

Inner platen 15 mm ∅

mm 45

Shape memory alloy Platen

9.10 Procedures of splitting a rock using the static rock breaker with SMA cylinders (courtesy NEC TOKIN Corporation).

45 mm in diameter in the rock or structure needs to be crushed. (2) Set six pieces of SMA cylinder which was compressed to 28 mm with inner platens between two platens as shown in Fig. 9.10. (3) Put the platens in the boring hole. (4) Heat the SMA cylinders above their reverse transformation start temperature by electrical current heating (joule healing). The SMA cylinders recover their original length 29 mm and then generate recovery force to break rocks. The SMA cylinders can be reused by compression to 28 mm again. It has been confirmed that the SMA rock breaker has a high crushing ability good enough for practical application in construction engineering. Operation of the SMA rock breaker is very simple and safe. Furthermore only electrical power is required for operation of the SMA rock breaker. Table 9.1 shows the comparisons of crushing performances between cement and SMA.

9.4

Easy-release screw

In Japan, the Home Appliance Recycling Law (an Act for recycling of specified kinds of home appliances) was enacted in April 2001. In January 2005, the Automotive Law for the Promotion of Effective Utilization of Resources was also enforced. These regulations encourage the effective use of resources and preservation of the environment. Televisions, air conditioners, refrigerators and washing machines are the four products that cannot be collected for mass disposal. One of the serious problems with recycling is that there

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Table 9.1 Performance comparison between cement and SMA

Crushing time Crushing power Power of expansion Direction of cracking Re-crushing Re-usability

Expanded cement

SMA

5–20 hours 0.4 t/cm2 3% (volume dilatability) Uncontrollable Needed to re-crush in case of failure Disposable

1 minute 6 t/cm2 5% (linear expansion %) Controllable Easy to re-crush even in case of failure Re-usable

Courtesy NEC TOKIN Corporation.

1.000 mm/div

1.000 mm/div

9.11 ‘C’-shaped SMA washer which is used in the easy-release screw unit (courtesy NEC TOKIN Corporation).

(a) Working

(b) Heated

(c) Released

SMA washer expands in diameter

SMA washer

Plate Screw

Base

9.12 Disassembling mechanism of easy-release screw unit with an SMA washer (courtesy Professor K. Yoshida, Tokai University).

is a large amount (about 600 000 tons) for disposal so the work of dismantling or demolition that involves releasing the screws one by one using an electric driver costs both time and money. Recently it has been suggested that the easy-release screw using a shape memory washer is effective in improving recycling efficiency in the consumer electronics industry. The easy-release screw unit is composed of a ‘C’-shaped SMA washer and a screw as shown in Figs 9.11. The ‘C’-shaped SMA washer expands by heating the assembled products above the reverse transformation temperature about 95 °C as shown in Figs 9.12. If the SMA washer expands to its

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Heating

Cooling

9.13 The process of the automatic disassembling system (courtesy Professor K. Yoshida, Tokai University).

9.14 Sharp cordless phone using the easy release screw unit (courtesy Sharp Corporation).

9.15 Easy-release screw and cross-section of charger in Sharp cordless phone (courtesy Sharp Corporation).

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original shape, the upper plate is detached from the base plate as shown in Figs 9.12. Using the SMA washer enables simple disassembling of the used products without the electric driver. Figure 9.13 shows the process of the automatic disassembling system. The discarded household electronic appliances can be dismantled just by passing through a heating zone by using the easy-release screws. The easy-release screws have been put to practical use for telephones and cordless phone machines from Sharp Corporation as shown in Figs 9.14 and 9.15. Other applications of the easy-release screws to home appliances are being planned.

9.5

Acknowledgements

The author would like to acknowledge the following for their contribution to the text: • •

Section 9.2: Furukawa Techno Material Co. Ltd Section 9.3: NEC TOKIN Corporation and Nishimatu Construction Co. Ltd • Section 9.4: Professor K. Yoshida, Tokai University.

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10 The use of shape memory alloys (SMAs) in automobiles and trains T. KATO, Piolax Inc., Japan

Abstract: This chapter introduces examples for applying shape memory alloys (SMAs) in the automotive and railway industries. SMAs have applied under a strict system of quality control in order to fulfil the specifications of the users. Applications in these industries have improved the reliability of the SMAs. Key words: automobile, railway, spring, actuator, valve.

10.1

Introduction

During the 1980s, various applications utilizing shape memory alloys (SMAs) were developed. In the automotive industry, the SMAs were used as thermal actuators in various applications such as radiator shutters, fan clutches, fuel management, climate control, engine control, brake ventilation, transmission control and rattling noise reduction. Among them, some have been applied in automobiles and rail vehicles. This chapter introduces the cases applying SMAs in the automotive and railway industries.

10.2

Shape memory alloys (SMAs) in automobiles

One of the practical applications is an actuator for transmission oil control. Below freezing temperature, transmission oil becomes viscous, which raises discharge pressure and uses excess energy. SMAs have been used to expand oil channels at lower temperatures, resulting in a reduction of oil pressure. At higher temperatures, transmission oil is less viscous, which increases sliding resistance and decreases gas mileage. To prevent it, some vehicles install an oil cooler whose channel is opened only at higher temperatures because continuous oil cooling lowers gas mileage. A switch valve which is mounted in a hydraulic circuit with a bias spring well matches a trend of miniaturization of parts. It is expected that SMAs will be continuously used for transmission control. Another example considered was a radiator fan. This application was based on an idea to warm air without rotating a radiator fan while coolant water was cold. SMA was examined as a device to sense cooling water 120 © Woodhead Publishing Limited, 2011

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temperature and operate a fan. However, SMA devices for mechanical actuation are no longer required since a new fan senses temperature and is turned on and off electrically.

10.3

Oil controller in Shinkansen

An SMA oil controller has been developed for the purpose of reducing loss in stirring lubricant in a gear of a Shinkansen, a Japanese bullet train. In the train, lubricant contained in a gearbox is dispersed by a pitch wheel and directly splashed on bearings. This lubricating method is not adequate for a high speed Shinkansen, because the high ram speed increases lubricant stirring loss. Particularly in the case of a rapid Shinkansen, such loss is significant, raising gear temperature. To reduce the loss, an automatic oil valve comprising the SMA spring and a constant-load spring has been installed. Transformation temperature of the SMA spring used is about 40 °C. As a bias spring, the constant-load spring is used since its load hardly varies, regardless of an extent of deflection (Fig. 10.1) (Ono, 2007). Concerning a bias spring, it is recommended to minimize the spring constant if possible to secure a certain stroke in a limited space. While a compression coil spring with a small spring constant may gain strong shape resilience at a high temperature, its force to push the SMA spring becomes weak at low temperature and sufficient stroke is not gained. If the device uses a constant-load spring, if a load that is sufficient to shorten the SMA spring at a low temperature is applied, significant shape resilience and stroke can be obtained at a high temperature (Ishii, 2007). Figure 10.2 shows an inner structure of the device. The lubricant is stirred by gear rotation. When lubricant temperature is low, its high viscosity hampers dispersal. As the gear stirs the lubricant, its temperature gradually

Force (torque)

Helical spring

Constant force spring

Deflection (stroke or number of turns)

10.1 Load-deflection curves of constant force spring and helical spring.

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Pinion Room B Oil pocket Bias spring

Bypass hole Gear Room A SMA spring

10.2 Automatic oil valve adjusting device for the Shinkhansen.

rises. When the temperature exceeds 40 °C, the SMA spring extends to reduce lubricant volume in room A and moves the piston to close the bypass hole. If lubricant in room A is reduced, stirring loss lessens, which avoids temperature rise of the gear. The device is currently used in the Shinkansen N700 series with a maximum speed of 270 km/h.

10.4

Steam trap

Steam has been used for heating passenger cars for rail transport in winter. Steam is circulated through a heat exchanger in passenger cars which is fed from a boiler in a diesel locomotive. As steam passes the heat exchanger, it is cooled and condensed. If such condensate is not drained, it hampers the flow of steam. To prevent it, bellows filled with compressed gas are used. Upon contacting steam, the bellows expand and shut off a valve, which increases condensate and lowers the temperature of the bellows, then it contracts and drains condensate. However, in midwinter, there have been accidents on the bellows; drained condensate has frozen and broken the bellows. By using an SMA spring in a steam-sensing element, the problem could be solved (Fig. 10.3) (Ohkata, 1993). The temperature of the valve decreases as condensate accumulates in a trap valve. When the temperature reaches the martensitic transformation finish (Mf) temperature of the SMA spring, a bias spring opens the valve so as to drain condensate in the trap. With the valve open, team flows and heats the SMA spring up to the reverse transformation finish (Af) temperature, then the valve closes again. Figure 10.4 shows how the valve operates in an actual train system at the ambient temperature of −10 °C. While steam is heating the SMA spring, the valve closes (for 3.6 minutes). When condensate accumulates, the temperature drops and the valve opens to drain condensate (for 4.2 seconds). In this way, the SMA spring has enabled efficient passenger car heating with few troubles.

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Steam

Drain

Bias spring

SMA spring

10.3 Structure of steam trap.

Valve open 4.2 s

3.6 min

120

Temperature (°C)

Valve close 100

80

60 Time

10.4 Temperature change of the steam trap valve in operation.

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10.5

Conclusions

Automotive parts have been improved along with technical innovation, which has led to changes in the way SMA devices are applied. In order to use SMA devices in vehicles, stable operation in harsh thermal environments from −40 °C to 100 °C is essential. Also, materials which are suitable for use in environments above 100 °C need to be developed. Automotive technology makes steady progress. In recent years, the number of hybrid cars and electric cars has increased in order to cope with environmental issues, which has led to the use of new parts and devices. Also, battery electricity becomes a major source of driving energy. It is expected that these new technologies will accelerate the development of new applications of SMA.

10.5

References

Ishii T (2007), SMST-2007 Proceedings, 443–444. Ohkata I (1993), Advanced Materials 93, V/B Shape Memory Materials and Hydrides, 18B, 1125. Ono H (2007), ‘Seminar of usage development of shape memory’, JSMA, 48–50.

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11 The use of shape memory alloys (SMAs) in aerospace engineering T. IKEDA, Nagoya University, Japan

Abstract: Low weight and high reliability are two of the most important considerations in the design of aerospace structures. Shape memory alloys (SMAs) have the functions of an actuator and a sensor, as well as having a degree of strength and an elastic modulus high enough for structural members. Accordingly, SMA elements can be used as structural members with these functions. This can reduce the number of parts and complexity of a system, and can lead to the system being light and highly reliabile. Therefore, SMAs are attractive materials in aerospace engineering and their potential in terms of replacing existing systems or developing new devices is an area that has been widely explored since they were discovered. In this chapter, the applications of SMAs in aerospace engineering are shown, which include both those already commercialized, and those used on a trial basis. Key words: shape memory effect, superelasticity, aerospace applications.

11.1

Introduction

The shape memory effect characteristic of nickel–titanium (NiTi) shape memory alloys (SMAs) was first discovered at the Naval Ordnance Laboratory in 1962. Nitinol is named after the NIckel-TItanium-Naval Ordnance Laboratory. The Raychem Corporation developed this technology and brought Tinel ® onto the market. The first commercial application of Tinel was in CryoFit ® coupling systems for the US Navy’s F-14 jet fighter (Aerofit, Inc., n.d.). NiTi SMAs have unique properties as a result of phase transformation. These properties include (i) large recoverable strains, (ii) large stress generation, (iii) large hysteresis loops in the stress–strain relationship, and (iv) variable electrical resistance as well as (v) a Young’s modulus measure that is sufficient for them to be used for structural members. SMA elements can be used as structural members (v) with the functions of actuator (i, ii), damper (iii), and/or sensor (iv) due to their unique properties. In this way, they can be used to reduce the number of parts and complexity of a system, which makes them an attractive commodity in the aerospace engineering industry, which demands low weight and a high level of reliability. For this reason, the potential applications of SMAs, for both the replacement of 125 © Woodhead Publishing Limited, 2011

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existing systems and the development of new devices, has been a major area of research in the field of aerospace engineering (particularly in terms of defense) ever since they were discovered (e.g. Anhalt et al., 2002; Hartl & Lagoudas, 2007; Kudva, 2004; Pitt et al., 2001; Simpson & Boller, 2008; Straub et al., 2004). In this chapter, the applications of SMAs in aerospace engineering are described. These include commercially available products; a tube coupling, a bolt separator, and a pin puller, and other devices that have been practically applied within real structures on a trial basis; a variable geometry jet engine nozzle, a hinge and deployment system for the solar array of a satellite, and an actuator in a space application.

11.2

Development and properties of CryoFit (Aerofit, Inc.)

11.2.1 Background to CryoFit ® (Harrison & Jervis, 1977) Many tubes are used in aircraft, such as hydraulic lines and fuel lines. When these tubes are broken, they often have to be repaired by people working in confined conditions. In this situation, it is very difficult to use welding and brazing because these processes require bulky equipment. For this reason, it was necessary to develop a coupling that would provide a reliable and leak-proof junction, and which could be installed without the use of such bulky or complex equipment.

11.2.2 Mechanism of CryoFit ® The CryoFit (Aerofit, Inc., n.d.) coupling (Fig. 11.1) is machined from an SMA bar. The interior diameter of this coupling is slightly smaller than the exterior diameter of the tubes which are to be joined at room temperature

11.1 CryoFit; image courtesy of Aerofit, Inc.

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in austenitic conditions (Fig. 11.2 (a)). Next, the coupling is immersed in liquid nitrogen (Fig. 11.2 (b)). At this cryogenic temperature it becomes martensitic. Then, it is mechanically expanded using a tapered steel mandrel so that the interior diameter of the coupling can be made larger than the exterior diameter of the tubes. It is stored in liquid nitrogen in order to preserve this expanded shape, and delivered to the user in this condition.

(a)

(b)

Immersing the coupling into liquid nitrogen

Tapered steel mandrel

Expanding the inner diameter

Liquid nitrogen

(c)

Warming up

11.2 Schematic diagram of mechanism of CryoFit. (a) Coupling just machined from an SMA bar. The coupling at room temperature is in the austenitic phase. (b) Coupling immersed in liquid nitrogen and mechanically expanded with a tapered steel mandrel. The coupling in the liquid nitrogen is in the martensitic phase. (c) Coupling positioned over a joint of the two tubes and warming up to shrink.

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Once on site, the coupling is positioned over the joint of the two tubes (Fig. 11.2 (c)). As it warms up, it shrinks down over the tubes with a high radial force as it tries to recover its original austenitic shape. The coupling has radial teeth formed circumferentially on the inner surface. The teeth enhance the tensile strength of the joint and the seal between the coupling and the tubes. Titanium–nickel–iron SMAs are used for CryoFit couplings, whose transformation temperature ranges from −150 to −100 °C.

11.2.3 Advantages of CryoFit® CryoFit couplings can be applied quickly, without the use of installation tools, and by people of relatively little skill. Furthermore, they can also be installed in a small space with limited access to the joint. They are reliable because they consist of a single moving part and are independent of tools or operators. They can be applied to a system with an operating pressure of up to 6000 psi (41 MPa). CryoFit couplings were designed in 1969 and qualified for the US Navy’s F-14 jet fighter shortly after. Since then, they have been used not only for military aircraft, but also for commercial aircraft during the production of original equipment, as well as during its modification and repair.

11.3

Development and properties of Frangibolt (TiNi Aerospace, Inc.)

11.3.1 Background to Frangibolt® (Johnson, 1992) The explosive bolts, which secure the components of a structure together and separate them quickly when activated, have been utilized for the deployment of payloads in space vehicles, for the release of emergency hatches, and other components. However, when the bolts are exploded, the resulting mechanical shock, together with the shrapnel from the bolts, can cause unintended damage to associated structures and equipments. There is no reliable non-destructive method for testing their integrity in a stand-by mode. Furthermore, they are relatively difficult to install. Therefore, it has been necessary to develop a separation device, which secures the components together and separates them non-explosively under controlled conditions.

11.3.2 Mechanism of Frangibolt® The Frangibolt (TiNi Aerospace, Inc., 2001) comprises an actuator, a bolt with a strain concentrated portion, a washer, and a lock nut (Fig. 11.3). The actuator is composed of a cylinder of NiTi SMA and a specially designed integrated heater. When a current is applied through the heater, the SMA

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129

Notched bolt Separation plane Broken bolt in tension

Heater and insulation

Actuator elongated

11.3 Schematic diagram of mechanism of Frangibolt. From TiNi Aerospace, Inc. (2001); image courtesy of TiNi Aerospace, Inc.

6.0

16.0 12.0

11.4 Compression equipment FBT-CT2. Dimensions are shown in inches (1 inch = 25.4 mm). From TiNi Aerospace, Inc. (2001); image courtesy of TiNi Aerospace, Inc.

cylinder elongates and fractures the bolt element, resulting in the separation of two or more components. The compression equipment for the Frangibolt actuator comprises a hydraulic hand pump, a two-stage cylinder, and a pressure gauge (Fig. 11.4). The Frangibolt actuator is installed in the fixture, the handle is then pumped

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until the prescribed pressure is reached, the pressure is released via the bleed screw, and the Frangibolt actuator is removed.

11.3.3 Advantages of Frangibolt® Frangibolts generate significantly less shock than explosive devices. They are easilty installed and can be used numerous times during acceptance and system level testing. The largest available Fragibolt actuator generates a force of more than 10 000 lbf (44 kN) which is used to fracture fasteners that are suitable for a load holding capacity of up to 5000 lbf (22 kN). Its weight is 100 g and its minimum life cycle is 60 cycles. Frangibolts were first used in outer space aboard the spacecraft Clementine in 1994. Since then, they have been qualified and used in numerous other space applications.

11.4

Development and properties of Pinpuller (TiNi Aerospace, Inc., 2001)

11.4.1 Background to Pinpuller (Bokaie et al., 1998) Pinpullers are release devices which remotely release a pin from engagement with a structure. They have been applied in the deployment of payloads from space vehicles, the release of emergency hatches, and so on. Pyrotechnic devices are inherently dangerous to manufacture and install, as there is always the possibility that they will ignite unexpectedly. They can be used only once. High output paraffin actuators can be used numerous times with reliable performance. However, they are large, complex, expensive, and slow to operate. If they are inadvertently released, they may introduce highly undesirable outgassing materials into the vacuum application. Therefore, it has been necessary to develop a release device, which is simple in design, compact in size, non-corrosive, non-outgassing, capable of being used for millions of cycles, and easily reset for another cycle of operation.

11.4.2 Mechanism of Pinpuller The SMA actuated Pinpuller (TiNi Pinpuller) comprises an SMA wire, an output pin, a drive spring, a rest spring, balls, a ball keeper, a latch, a battery, and a switch (Fig. 11.5 (a)). When it is in the extended position, the output pin is loaded by the compressed drive spring. The pin remains firmly locked in this position due to the retractable detent system. This system is composed of an array of balls, which is held within the openings in the ball keeper. The latch, loaded by the rest spring, drives the array of balls outward.

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Drive spring

131

(b)

Battery

SMA wire

Rest spring Ball Ball keeper Latch

11.5 Schematic diagram of mechanism of TiNi Pinpuller. (a) Pinpuller in the extended position. (b) Pinpuller in the retracted position.

When the switch is closed, the joule heating causes the SMA wire to contract, and the wire drives the latch upwards (Fig. 11.5 (b)). The balls then roll inward and the pin is retracted due to the force of the drive spring. This Pinpuller is reset manually, either by pulling the pin out from the top, or by pushing it out from the bottom.

11.4.3 Advantages of Pinpuller TiNi Pinpullers are actuated in a millisecond. They are reset by simply reextending the output pin from the front or rear. They can be used hundreds of times during acceptance and system level testing. The available TiNi Pinpullers range from 5 lbf (22 N) of pull force and 0.250 in (6.35 mm) of stroke, to 100 lbf (444 N) of pull force and 0.625 in. (15.9 mm) of stroke. TiNi Pinpullers were first used in outer space aboard the Mars Global Surveyor spacecraft, which was launched in 1996. Since then, they have been qualified for use and implemented in numerous other space applications.

11.5

Development and properties of variable geometry chevrons (VGCs) (The Boeing Company)

11.5.1 Background to VGCs (Mabe et al., 2006; Nesbitt et al., 2004) Chevrons, serrated aerodynamic devices along the trailing edge of a jet engine nozzle, have been shown to greatly reduce jet noise during take-off,

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as well as reducing shock cell noise while cruising (Fig. 11.6). However, the immersion of these chevrons into the exhaust flow for the reduction of jet noise also causes drag or thrust loss, which can result in a significant penalty for flights with long cruise times. Therefore, it has been necessary to develop a noise reduction system which morphs the chevron between the shape optimized for noise reduction during take-off and the shape that reduces shock cell noise, without compromising engine performance during cruise. A dynamic tip motion of more than 2.3 cm was required, ranging from more than 1.5 cm into the fan stream at take-off (e.g., ∼75 °C the fan flow) to 0.8 cm into the free stream at the cooler cruise conditions (e.g., ∼ −40 °C free stream flow).

11.5.2 Mechanism of VGCs (Mabe et al., 2006) A VGC comprises SMA actuators and a stiff chevron-shaped carbon fiber composite substrate, which also behaves as a bias- spring (Fig. 11.7). Three SMA actuators are fastened to the substrate with two bolts (Fig. 11.8). Thin film heaters are attached on each actuator to control the temperature and hence the shape of the chevron. A flexible cover protects the actuators and wiring on the free stream side. The tip position of the chevron is estimated by measuring the strain at three locations on the surface of the substrate. Three thermocouples monitor the temperature of the actuators and substrate. In powered mode, a simple proportional-integral controller is applied to control each chevron’s tip position. In autonomous mode, the SMA actuators are

11.6 Chevrons. From Mabe et al. (2006); image courtesy of the Boeing Company.

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Actuator spring 60-Nitinol actuator with heater

Cover plate

Attach fasteners

+ Nitinol SMA

Assembly

Composite base

Composite substrate Free stream

Fan stream

11.7 Variable geometry chevron design concept. From Mabe et al. (2006); image courtesy of the Boeing Company.

Compliant cover seal

60-Nitinol with heaters

Strain gages (potted for protection)

11.8 Components of VGC. From Mabe et al. (2006); image courtesy of the Boeing Company.

activated without the application of power, instead using the variation in ambient temperature that occurs between take off and cruise altitude. The alloy 60-Nitinol (60% Ni, 40% Ti by weight) is used in actuators because the transition temperature can be set by a heat treatment process and does not require cold work. In addition, it has a notably high austenite modulus and a small creep rate. The size of the actuators is 25.4 cm in length

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by 3.8 cm in width. The thickness of the actuators decreases smoothly, from 0.44 cm at the centre, to 0.15 cm at either end. The martensite finish temperature and austenite finish temperature of the actuator are 15 °C and 60 °C, respectively.

11.5.3 Advantages of VGCs (Mabe et al., 2006) A VGC allows the chevron to morph between a shape that is optimized for noise reduction during take-off, and a shape that reduces shock cell noise during cruise, without compromising engine performance. The fact that the VGC can operate autonomously means it is possible for the chevron to morph independent of applied power. However, there is also a powered mode, which can be used to set and hold a chosen chevron configuration. VGCs were successfully flight-tested on a Boeing 777–300ER with GE-115B engines in 2005.

11.6

Development and properties of hinge and deployment system of lightweight flexible solar array (LFSA) on EO-1 (NASA and Lockheed Martin Astronautics)

11.6.1 Background of hinge and deployment system of LFSA on EO-1 (Carpenter and Lyons, 2002; Carpenter et al., 2001) When a spacecraft or a satellite is launched, its solar panels and communication antennae are folded and stowed. Once the spacecraft or satellite has arrived at its intended orbit or other extraterrestrial destination, they are redeployed. However, since they are very sensitive, it is essential that they are deployed with minimal vibration and shock. The conventional deployment system comprises pyrotechnic separation nuts and hinges with a spring-damper system. The disadvantage of the pyrotechnic devices is that they inherently produce shock loading. The hinges with spring-damper system are relatively complicated. Therefore, it has been necessary to develop a deployment device which has non-explosive actuators and hinges with a simple mechanism in order to reduce the weight and maintenance requirement of the device, and to improve its reliability.

11.6.2 Mechanism of hinge and deployment system of LFSA on EO-1 (Carpenter and Lyons, 2002) The LFSA was mounted on the NASA satellite Earth Observing 1 (EO-1) (Figs 11.9 and 11.10). Each SMA hinge of the LFSA comprises a pair of

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LFSA

11.9 EO-1 with LFSA mounted on. From GSFC (2000); image courtesy of ATK under contract to NASA Goddard Space Flight Center.

SMA hinges

Lightweight solar array EO-1 flight experiment

EO1 EXP 2-24-98-C-2-6

11.10 LFSA engineering model. From Beavers et al. (2002); reprinted with permission of the American Institute of Aeronautics and Astronautics, Inc.

thin SMA strips with an arcuate cross-section with thin flexible nichrome heaters bonded to the inner surface of each SMA strip, and metallic structural fittings (Figs 11.11 and 11.12). The SMA strips are heat treated in the deployed configuration. The fittings join their ends in a bi-lenticular configuration. In the martensitic state, the hinge is manually buckled and folded into the stowed configuration. When electric power is applied to the heaters, the SMA strips are transformed into the austenitic state, which causes them

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11.11 SMA deployment hinge. From Beavers et al. (2002); reprinted with permission of the American Institute of Aeronautics and Astronautics, Inc.

SMA strip

Fitting

Heater

11.12 Schematic assembly diagram.

to deploy the hinge. Once it is deployed, the power is turned off. Then the SMA strips are cooled and transformed into the martensitic phase. Although the martensitic phase is softer than the austenitic phase, the use of highly efficient section geometry in the deployed configuration allows the martensitic SMA hinge to support the lightweight solar array sections. The SMA used in this application has an austenite transition temperature of 70 °C.

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11.6.3 Advantages of hinge and deployment system of LFSA on EO-1 (Carpenter and Lyons, 2002; GSFC, 2000) The use of an SMA hinge facilitates a reduction in the number of requisite parts, a reduction in deployment shocks, and safer handling than conventional pyro-based systems. EO-1 was launched on 21 November 2000.

11.7

Development and properties of rotating arm for material adherence experiment (MAE) in Mars Pathfinder mission (NASA)

11.7.1 Background to rotating arm for MAE in Mars Pathfinder mission (Jenkins and Landis 1995; Jenkins et al., 1997) NASA missions to Mars rely on solar arrays for their primary power system. However, since the atmosphere on Mars contains large amounts of dust, it is vital to consider the effects of dust settling onto solar panels in the design of solar arrays. The Mars Pathfinder mission was to conduct a series of experiments to measure environmental effects on Mars. One of these experiments would measure the optical obscuration occurring due to the settling of dust from the atmosphere onto a solar cell. The Mars Pathfinder consists of a lander and a small, autonomous, six-wheeled rover vehicle, known as ‘Sojourner’. The MAE would be carried out on the front left corner of the Sojourner (Fig. 11.13). In MAE, the effect of the accumulation of dust on the solar array would be measured by comparing the short circuit current from the solar array with and without the cover glass. When the current of the solar cell without the cover glass was being measured, the cover glass would be moved from in front of the solar cell. Therefore, it was necessary to develop an actuator for MAE satisfying the following operational constraints: a power budget of 5 W for 10 s/day, a power distribution limited to 5V DC up to 1 A, a total footprint of 41.0 mm by 13.7 mm, a mass not to exceed 16 g, ability to complete at least seven cycles on the Martian surface, and so on.

11.7.2 Mechanism of rotating arm for MAE in Mars Pathfinder mission (Jenkins and Landis 1995; Jenkins et al., 1997) The actuator for rotating the cover glass comprises a rotating arm to which a cover glass is attached, an SMA wire, and a flat spring as a bias spring

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MAE

(a) MAE

11.13 Sojourner with MAE set up. (a) Location of MAE. From JPL (1997); image courtesy of NASA. (b) MAE setup. The right side is a solar cell with a cover glass mounted on a rotating arm. From Landis (1998); image courtesy of NASA.

(Fig. 11.14). The SMA wire is fixed to the axle at one end, and a stationary point at the other. The SMA wire is heated by passing a DC current through it. This causes the wire to contract and pulls on the axle and thereby to rotate the arm with the cover glass. When the current supply to the wire is turned off, the wire expands by the force of the flat spring and the cover glass returns to the rest position. The SMA wire has a diameter of 150 μm, an active length of 3 cm, and a transformation temperature of 90 °C. The arm is 3.5 cm long and rotates 32° to completely uncover the solar cell.

11.7.3 Advantages of rotating arm for MAE in Mars Pathfinder mission (Jenkins and Landis 1995; JPL, 1997) The actuator is simple in mechanism and light in weight compared with motors and solenoid actuators. The Mars Pathfinder was launched on 4 December 1996 and landed on the surface of Mars on 4 July 1997. This was the first multi-cycle electrically activated SMA actuator to be utilized in a space application.

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Solar cell Nitinol wire

Cover glass

Rotating arm in rest position

Restraint/guide channel

Rotating arm

Return spring

Stop (b)

Restraint

Rotating arm

Cover glass Solar cell

Nitinol anchor

Nitinol wire Nitinol anchor

Spring

Axle To electrical connections

1 inch

11.14 Mechanism of the rotating arm. From Jenkins and Landis (1995); image courtesy of NASA. (a) Top view (b) side view.

11.8

References

Aerofit, Inc. (n.d.), viewed 31 August 2009, 〈http://www.aerofit.com〉. Anhalt C, Breitbach E and Monner H P (2002), ‘Adaptronics in airliner design – a new structural approach’, Proceedings of SPIE, 4698, 364–375.

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Beavers F L, Munshi N A, Lake M S, Maji A, Qassim K, Carpenter B F and Rawal S P (2002), ‘Design and testing of an elastic memory composite deployment hinge for spacecraft’, Proceedings of 43rd Structures, Structural Dynamics, and Materials Conference, AIAA-2002–1452. Bokaie M D, Busch J D, Johnson A D and Petty B (1998), ‘Release device for retaining pin’, United States Patent 5,771,742, 30 June 1998, 11 September 1995 (filed). Carpenter B and Lyons J (2002), Lightweight Flexible Solar Array Validation Report, 29 January 2002, NASA/GSFC, viewed 31 August 2009, 〈http://eo1.gsfc. nasa.gov/new/validationReport/Technology/Documents/Reports/LFSA.pdf〉. Carpenter B F, Draper J L and Gehling R N (2001), ‘Shape memory alloy controllable hinge apparatus’, US Patent 6,175,989 B1, 23 January 2001, 26 May 1998 (filed). GSFC (2000), ‘Earth Observing 1 (EO-1)’, Earth Observatory, 15 November 2000, viewed 31 August 2009, 〈http://earthobservatory.nasa.gov/Features/EO1〉. Harrison J D and Jervis J E (1977), ‘Heat recoverable metallic coupling’, United States Patent 4,035,007, 12 July 1977, 29 October 1973 (filed). Hartl D J and Lagoudas D C (2007), ‘Aerospace applications of shape memory alloys’, Proceedings of the Institution of Mechanical Engineers, Part G: Journal of Aerospace Engineering, 221, 535–552. Jenkins P P and Landis G A (1995), ‘A rotating arm using shape-memory alloy’, Proceedings of the 29th Aerospace Symposium, NASA CP-3293, 167–171. Jenkins P P, Landis G A and Oberle L G (1997), ‘Materials adherence experiment: technology’, IECEC-97339, Proceedings of the 32nd Intersociety Energy Conversion Engineering Conference, 1, 728–731. Johnson A D (1992), ‘Non-explosive separation device’, United States Patent 5,119,555, 9 June 1992, 2 October 1990 (filed). JPL (1997), ‘Mars microrover power subsystem’, viewed 31 August 2009, 〈http:// marsprogram.jpl.nasa.gov/MPF/roverpwr/power.html〉. Kudva J N (2004), ‘Overview of the DARPA smart wing project’, Journal of Intelligent Material Systems and Structures, 15, 261–267. Landis G A (1998), ‘Measuring dust on Mars’, viewed 31 August 2009, 〈http://www. grc.nasa.gov/WWW/RT/RT1997/5000/5410landis.htm〉. Mabe J H, Calkins F T and Butler G W (2006), ‘Boeing’s variable geometry chevron, morphing aerostructure for jet noise reduction’, Proceedings of the 47th AIAA/ ASME/ASCE/AHS/ASC Structures, Structural Dynamics, and Materials Conference, AIAA 2006–2142. Nesbitt E H, Butler G W and Reed D H (2004), ‘Deployable segmented exhaust nozzle for a jet engine’, US Patent 6,718,752 B2, 13 April 2004, 29 May 2002 (filed). Pitt D M, Dunne J P, White E V and Garcia E (2001), ‘Wind tunnel demonstration of the SAMPSON smart inlet’, Proceedings of SPIE, 4332, 345–356. Simpson J C and Boller C (2008), ‘Design and performance of a shape memory alloy-reinforced composite aerodynamic profile’, Smart Materials and Structures, 17, 025028. Straub F K, Kennedy D K, Domzalski D B, Hassan A A, Ngo H, Anand V and Birchette T (2004), ‘Smart material-actuated rotor technology – SMART’, Journal of Intelligent Material Systems and Structures, 15, 249–260. TiNi Aerospace, Inc. (2001), viewed 31 August 2009, 〈http://www.tiniaerospace.com〉.

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12 Ferrous (Fe-based) shape memory alloys (SMAs): properties, processing and applications T. MARUYAMA, Awaji Materia Co. Ltd, Japan and H. KUBO, Kanto Polytechnic University, Japan

Abstract: Many ferrous (Fe)-based shape memory alloys (SMAs) have been discovered, such as Fe–Pt, Fe–Pd and Fe–Co–Ni–Ti. The Fe–Mn–Si system is one of the most distinctive SMAs in the sense that it is the only Fe-based SMA that has successfully been applied in the industrial field. The shape memory effect of the Fe–Mn–Si system was approved in 1982. The memory effect is associated with the strain field established by the formation of the hexagonal close packed (hcp) phase in the parent g phase. Various efforts have been made to develop these materials with an aim of producing high performance of the shape memory effect, high strength, high corrosion resistance, weldability and sufficient plasticity for industrial processing. It is currently possible to attain ∼4% shape recovery or 180 MPa stress in the Fe–Mn–Si–Cr SMA by heating the ferrous material up to 350 °C after 5∼8% deformation for martensite formation. The Fe–Mn–Si–Cr SMA has been applied in the industrial field as the material of a joint for pipes and rails. In this chapter, we will review the fundamental character of the Fe–Mn–Si–Cr SMA and report on the latest industrial applications of the steel pipe joints and the rail joint bar (fishplate) of heavy-duty crane rails. Key words: Fe-based shape memory alloy (SMA), stress-induced martensite, shape recovery strain, fishplate, pipe joint.

12.1

Introduction

In the 25 years since the discovery of Fe–Mn–Si shape memory alloys (SMAs), there have been hundreds of patents issued on many conceivable applications of Fe-based SMAs, yet the list of successful applications is still short. However, the scope for the application of these materials has gradually been opened to the industrial field. Indeed, a total of 25 tons of Fe– Mn–Si alloys has already been utilized in Japan. There are two different kinds of Fe-based SMAs. The first is a group containing alloys such as Fe–Pt (Wayman, 1971; Dunne and Wayman, 1973), Fe–Pd (Oshima, 1981) and Fe–Ni–Co–Ti (Koval et al., 1981; Maki et al., 1984) systems, which exhibit the typical characteristics of thermoelastic martensitic transformation, with a small thermal hysteresis (austenite finish temperature to martensite start temperature, Af –Ms < 10 °C). An ordered Fe–25% Pt alloy (L12) exhibits 141 © Woodhead Publishing Limited, 2011

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perfect shape memory effect, as well as the characteristic mechanical properties of pseudo-elasticity or super elasticity (Dunn and Wayman, 1973). The second is a group of alloys such as Fe–Ni–C (Kajiwara, 1985) and Fe– Mn–Si (Sato et al., 1982, 1986; Murakami et al., 1986) systems, which have a large thermal hysteresis (Af–Ms ∼150 K) in transformation, but still exhibit the shape memory effect. Among such Fe-based SMAs, the Fe–Mn–Si system is the only one that has been successfully adopted in industry. Since the discovery of the Fe– Mn–Si SMA in 1982 (Sato et al., 1982), there has been a great amount of effort and enthusiasm invested in the development of Fe–Mn–Si SMA with a view to replacing Ti–Ni alloys (Melton et al., 1998) in prime position in the industrial market. However, as a result of the fundamentally low performance of the Fe-based SMAs, they were not successful in taking even a part of the market from the Ti–Ni alloys. In the 2000s, however, a new market for Fe-based SMAs has gradually been explored. This market constitutes large products for which the Ti–Ni alloys are not suitable manufacturing materials. Large size joining pipes for tunnel construction (Maruyama and Kurita, 2004) and crane rail joint bars (fishplates) (Maruyama et al., 2008) are the latest examples which have helped open this new market up to the Fe-based SMAs. In fact, the Fe–Mn–Si SMAs have strongly inherited the characters of structural materials that are generally associated with stainless steel. Therefore, the market for structural materials with the subsidiary function of shape memory effect would be suitable for the Fe–Mn–Si SMAs. In this chapter, in addition to reviewing the history and development of Fe-based SMAs, we will also focus on the latest application of fishplates in the manufacture of crane rails, and describe the problems that emerged during the installation process, followed by the scientific and engineering solutions applied in the overall design of the fishplate as well as the installation process. The potential future developments in the manufacture and use of the Fe-based SMA are also discussed.

12.2

Iron–manganese–silicon (Fe–Mn–Si) shape memory alloys (SMAs)

Martensitic transformation occurs through the lattice deformation that alters the crystal structure of the parent phase to the martensite phase. Here, the mechanism of deformation is different from the slip of the crystal by dislocations, as shown in Fig. 12.1. The transformation happens in such a way that the nearest neighboring atoms will always retain their relative positions, unchanged within the lattice. Martensites can be formed by prohibiting the diffusion of atoms within the crystal, either upon quenching or on sub-cooling the material.

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Martensite transformation C

C

B

B A

A Heating Before deformation

After deformation

SMA (induced martensite transformation)

Slip C

B

C

A

A

B

Heating Before deformation

After deformation

Deformation by slip in general alloy

12.1 Generation of shape memory effect by stress-induced martensite in a shape memory alloy and slip deformation in non-shape memory alloy.

The thermal activation process is one way in which the martensitic transformation might occur. However, it can also result from the application of stress to the crystal in the parent phase. This stress-induced martensite plays a role similar to that of the dislocations in the deformation process in metallic materials. Therefore, it has long been considered that the stress-induced martensites could only exhibit the irreducible movement of the interface, and that the alloy could not go back to its original shape, either on heating or on applying the back stress. However, since the stress is equivalent to the temperature (thermodynamic potential) in the sense of being the driving force for the martensitic transformation, it would also be possible for the stress-induced martensites to generate the shape memory effect. In fact, the stress-induced martensite in the Fe–Mn–Si SMA yields the shape memory effect that is associated with the residual strain field yielded by martensites. It was in 1982 that Sato et al. confirmed the shape memory effect of an Fe–Mn–Si alloy. The shape memory effect of the Fe–Mn–Si alloy is governed by the formation of a stress field around the martensite of the hexagonal close packed (hcp) (e) phase, embedded in the parent fcc (g) phase. The development of the Fe–Mn–Si SMA for industrial applications began

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with the selection of the adequate composition of Mn and Si elements (Murakami et al., 1986). Mn and Ni are the elements that typically stabilize the g phase of Fe alloys. Nevertheless, only the Mn addition to the g phase of Fe alloys can decrease the stacking fault energy, which may increase the amount of the stress-induced e martensite produced. Increasing the Mn composition in the alloy, however, brings about a magnetic transition from the paramagnetic to the anti-ferromagnetic phase. This anti-ferromagnetic g phase is stable enough that no martensite transformation can be induced by deformation. To suppress the magnetic phase transition, Si is added as the third element of the Fe–Mn–based SMA, while the Si addition itself also demonstrates the capability of strengthening the alloy. One of the distinguishing characters of the Fe–Mn–Si SMA is that the stress-induced e martensite can be produced at room temperature and annealed out on heating at moderately high temperatures, such as ∼300 °C. The Ms (strictly speaking Md) temperature has to be set at around room temperature (Ms = −20–25 °C), while the Af temperature should be set at a moderately high temperature (Af = 130–185 °C) (Sato and Mori, 1991). The large temperature hysteresis makes it possible for the Fe–Mn–Si SMA to be processed at room temperature and still produce the shape memory effect. The stress-induced martensite may persist until the temperature is increased to Af, which means that it is possible to maintain the deformed shape at room temperature as long as is necessary. For those SMAs with small hysteresis, for example Af –Ms < 10 in a Ti–Ni alloy, the stress-induced martensites cannot persist at room temperature. When the applied stress is released, the reverse transformation causes them to destroy themselves. The shape memory process in a Ti–Ni alloy is not the same as in an Fe–Mn–Si alloy. The Ti–Ni SMA should be cooled to reach the martensite phase first and the deformation would then occur in the martensite phase. The content of each element in an Fe–Mn–Si SMA is determined so as to conform the Ms temperature at room temperature. The manifested base composition is Fe–32Mn–6Si (mass %) (Murakami et al., 1986). As a result of the issues arising from the industrial application of the Fe–Mn–Si SMA, such as corrosion resistance or anti-oxidation, the extra elements such as Ni or Cr are added to the base Fe–32Mn–6Si SMA. The typical innovated materials are as follows: • • • •

Fe–32Mn–6Si Fe–28Mn–6Si–5Cr Fe–20Mn–5Si–8Cr–5Ni Fe–16Mn–5Si–12Cr–5Ni

Of these materials, Fe–28Mn–6Si–5Cr SMA is the one which has most frequently been adopted for industrial applications.

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It has frequently been suggested that the Fe–Mn–Si alloy could be conveniently melted and processed by utilizing the production facilities of steels or stainless steels because it is a similar high Mn stainless steel. However, such facilities for the mass production of steels could be utilized only once a massive amount of the SMA had been secured. In the initial stages of small-scale production, it is inevitable that advanced furnaces and facilities will be used. The materials cost would inevitably be high in comparison with that of conventional stainless steel.

12.3

Shape memory effect of the iron–manganese– silicon (Fe–Mn–Si) alloy

The shape memory effect of the Fe–Mn–Si alloy is explained by the reverse transformation of the hexagonal (e) martensite to the face centered cubic (fcc) (g) austenite phase (Sato et al., 1982). The alloy, which is deformed at room temperature, contains a stress-induced martensite of thin plates embedded in the bulk parent phase. The coherent interface of the martensite is composed of an array of Shokley partial dislocations. These are supposed to establish the oriented stress field by themselves. In comparison to the thermoelastic Ti–Ni SMA, the mechanism of assigning the shape memory effect is fundamentally different. A unique crystallographic path for the reverse transformation can be established by the configuration of ordered atoms in the Ti–Ni SMA. However, in the disordered Fe–Mn–Si SMA, the reverse transformation should be guided by the anisotropic residual strain field yielded by the martensite formation. Therefore, the Fe–Mn– Si SMA does not necessarily take the same path in the forward (g→e) transformation as it does in its reverse (g ←e) transformation. Owing to the difference in the mechanism of the reverse transformation mentioned above, the Fe–Mn–Si SMA is widely considered to be inferior to the thermoelastic SMAs in terms of its properties. As a matter of fact, TiNi and Cu–Al–Ni alloys can recover their original shape by ∼8% in strain (van Humbeek and Stalmans, 1998) while the shape recovery achieved by the Fe–Mn–Si–Cr SMA is at most 4% (Otsuka et al., 1989). The Fe–Mn–Si SMA exhibits one-way shape memory effect only, and it does not exhibit the super elasticity or two-way shape memory effect as usually demonstrated by Ti–Ni alloys. This is because of the large temperature hysteresis that exists between Af and Ms (ΔT∼150 °C) in the transformation of the Fe–Mn–Si SMA. However, this seemingly inferior character of the Fe–Mn–Si SMA can work in the favor of the joint in the sense that, once recovered, Fe–Mn–Si SMA would never change its shape, even if the temperature were to change again. Generally, thermoelastic SMAs are recognized as functional materials. In fact, the Ti–Ni SMAs are mostly applied as functional materials which

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repeat the fundamental action of movement by expansion and contraction. However, the Fe–Mn–Si SMA can offer its shrinkage ability only once as a functional material. It is inevitable that it will then be used to serve as a structural material at the place of installation thereafter. Accordingly, the industrial applications have gradually been diversified to encompass a series of large size joints or components, which can not be served by Ti–Ni SMA. Pipe joints (R∼400 mm f) for the curved pipes of tunnel construction (Maruyama and Kurita, 2004) and the crane rail SMA fishplates (Maruyama et al., 2008) are the typical examples of these large size industrial parts using SMAs. That is, the Fe–Mn–Si SMA is a structural material with the subsidiary capability of a functional material, which can perform only one cycle of shape memory effect.

12.4

Mechanical properties of iron–manganese– silicon (Fe–Mn–Si) shape memory alloys (SMAs)

The fundamental characteristics of the Fe–28Mn–6Si–5Cr SMA are shown in Table 12.1. The important factors to be considered in SMAs are shape recovery strain and shape recovery stress. Various efforts have been made towards improving the shape recovery strain or stress of the SMA. One of the most fundamental and significant treatments that can be applied to the SMA to obtain a greater proportion of shape recovery is the so-called

Table 12.1 Fundamental properties of Fe–Mn–Si SMA Item

Unit

Values

Proof stress Ultimate tensile stress (UTS) Ductility Hardness (Hv) Density (25 °C) Melting point Thermal expansion (0–500 °C) Thermal conductivity Specific heat Specific resistance Young’s modulus Shear modulus Poisson ratio Ms Af Recovery strain Recovery stress Magnetic property

MPa MPa %

200–300 680–1,000 16–30 190–220 7.2–7.5(7.454) 1320–1350 16.5 × 10−6 0.02 0.13 100–130 × 10−6 170.0 65.0 0.359 −20–25 130–185 2.5–4.5 150–200 Paramagnetism

g/cm3 °C °C−1 cal/cm⋅deg⋅sec cal/g⋅deg Ω⋅cm GPa GPa °C °C % MPa

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training treatment (Otsuka et al., 1989), which is also known as the cyclic thermomechanical training. Once the first cycle of 5–8% deformation has been set up, it is followed by heat treatment at 600 °C in order to anneal out the stress-induced martensite and leave the oriented nucleation sites, which are favorable to the stress-induced martensites. In the training treatment, the SMA may repeatedly be subjected to the same thermomechanical training. However, the training treatment has mostly been applied only once, because the shape recovery strain is rapidly saturated through the repetition of the training treatment. The shape recovery strain of the Fe–28Mn–6Si–5Cr SMA is about 2% without any training treatment (see the next section) and can achieve at most 4% with the training treatment. On the other hand, for the shape recovery stress, the maximum possible value is estimated at 130 MPa (again, see the next section) for the materials without training treatment, which increases to a value of 180 MPa after training. The amount of pre-deformation in the shape memory process required to bring about such large values of shape recovery is considered to be 5–8%. The Fe–Mn–Si SMA ought to serve as a structural material at the place of installation after having completed the simple one-way shrinkage of its shape recovery ability. For this reason, its mechanical properties are also important in the industrial applications stage. Figure 12.2 shows the stress– strain curve obtained using the 6 mmf rod sample. In this case, a strain measurement as great as ∼40% has been recorded, and the fracture has occurred shortly beyond the neck of the rod. The proof stress of 250 MPa is rather low in comparison to common steels, but exhibits significant workhardening, which results in a tensile strength of 700–850 MPa. For this reason, this material is found to exhibit fairly good characteristics for a structural material. The only disadvantage is that the thermal conductivity

Stress (MPa)

800

σT = 800 MPa

600 400 σY = 270 MPa

200 0

0

10

20

30

40

Strain (%)

12.2 Stress–strain curve of 6 mmf of Fe–28Mn–6Si–5Cr SMA.

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of SMA is about six times smaller than that of a normal stainless steel. All other characteristics, such as tensile strength and thermal expansion coefficient, are almost the same as other steels. The plastic deformation of an SMA may be induced by either slip or by the formation of martensites, as shown in Fig. 12.1. On estimating the temperature dependence of 0.2% proof stress, one can distinguish the critical stress for inducing martensitic transformation sm from the yield stress sY of the SMA. Figure 12.3 indicates a temperature dependence of 0.2% proof stress in the case of the Fe–32Mn–6Si SMA (Otsuka et al., 1989). Solid solution treatment was applied at 1422 K for 1.8 ks. The SMA was deformed by 5%. The straight line with a positive slope indicates the critical stress for inducing martensitic transformation sm, and the line with a negative slope represents the critical yield stress sY of the SMA. Therefore, the shaded area in the figure is the region where martensites can be formed without yielding to any slip of the lattice planes in the SMA. One can clearly see that, as a result of the training treatment, the critical transformation stress sm decreased by 30–80 MPa and, on the contrary, the yield stress sY of the SMA increased by ∼100 MPa. That is, the training treatment suppresses the slip in the SMA, but accelerates the formation of martensite. The critical stress for martensitic transformation decreases more steeply with a small amount of deformation (4%) than that of the SMA deformed by a large amount of deformation (8%). However, the yield stress for the plastic deformation increases more in a case of heavy deformation. As a result, the critical stress for inducing martensitic transformation and the yield stress for dislocation movements could compete to affect the shape memory effect.

500

0.2% proof stress (Mpa)

450 400 B

350 300

Critical yield stress for slip σY

250 200 150

Critical transformation stress σm 1st def

A

2nd def (1st = 4%) 25

75 125 Temperature (°C)

175

12.3 Critical transformation stress sm and critical yield stress sY of Fe–32Mn–6Si SMA (with the permission of MRS).

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From a microscopic point of view, the thermally induced or spontaneous transformation of e martensite variants is known to be generally selfaccommodated (Gaunt and Christian, 1959; Sato et al., 1984; Yang and Wayman, 1992). The e band usually consists of three laminated variants with different shear directions in order to minimize the total shape strain of the transformed region. In contrast, the stress-induced e transformation is generally accomplished by the selective motion of a single type of Schockley partial dislocation, being most favorable to the direction of applied stress (Maki and Tuzaki, 1992; Yang and Wayman, 1992). There is no special surface treatment necessary for the Fe–28Mn–6Si–5Cr SMA working in a dry air atmosphere. The corrosion resistance of the Fe– Mn–Si SMA is almost equivalent to that of the 5Cr steel. Tungsten inert gas (TIG) welding is available with fillers of SMA wires. Thus, it is concluded that the Fe–Mn–Si–Cr SMA exhibits good corrosion resistance, good weldability and good mechanical properties. Such potential characteristics of the Fe–Mn–Si SMA have facilitated the development of various applications, such as the pipe joint of large curved steel pipes in civil engineering (Maruyama and Kurita, 2004) and heavy duty joint components for rails (Maruyama et al., 2008).

12.5

Proper process for shape memory effect

The fundamental scheme of the process for an SMA to generate shape memory effect is shown in Fig. 12.4. The alloy materials are melted in the induction furnace and the ingot is processed by hot rolling and forging, followed by solid solution treatment. Heat treatment is carried out at 950– 1150 °C for the release of the residual stress that has been introduced in the previous process and in order to homogenize the SMA. The homogenized materials are shaped into the desired form by processing, dressing and cutting. This process would ideally be performed at high temperatures over 950 °C. In the event that the shaping is completed at room temperature, the SMA should be heated to 950 °C with the desired shape fixed. The constrained shape at 950 °C is preserved as the original shape of the SMA. Following the shaping process which is carried out at high temperature, the SMA is slightly deformed to induce martensites in the bulk materials. The induced martensites are annihilated during the process of shape recovery. The extent of the shape recovery caused by the annihilation of the martensites is determined from the shape recovery rate (= (shape recovery strain/applied strain) × 100%) and the maximum recovery strain. It would be 5–8% in the case of the Fe–Mn–Si SMA (Otsuka et al., 1989). The deformed SMA recovers its original shape when the material is heated up to ∼350 °C. The reverse transformation starts to take place at ∼90 °C and continues until the material has reached ∼35 °C. Figure 12.5 depicts the

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Shape memory and superelastic alloys [Preliminary process] melting, hot process, etc.

Solid solution treatment

Shaping the original shape

by the process of hot forging, rolling, dressing, cutting, etc.

Shape memory treatment

by heating over 950 °C under the constraint on the original shape by extension, compression or twisting at room temperature

Deformation

training is an additional process for SMA to exhibit better shape memory

Training Heating

once shape recovery at 350 °C and pre-deformation for shape recovery

Deformation

Heating for shape recovery

shape recovery to the original shape by heating to 350 °C

12.4 Fundamental scheme of the process for a SMA to produce the shape memory effect.

4 Inner diameter shrinkage Outside diameter shrinkage

3

2

26 f 19 f

Shrinkage rate (%)

150

1

36.7

0

0

100

200

300

400

500

600

Temperature (°C)

12.5 Shape recovery rate of radius shrinkage (after one cycle of training).

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change in the shape recovery rate of the joining pipe with increasing temperature. The joining pipe has been preliminarily expanded at room temperature with mandrels. The change in its inner diameter is measured to obtain the shrinkage rate upon heating the joining pipe. In the industrial application of Fe–Mn–Si SMA, it is essential to focus either on the shape recovery stress or on the shape recovery strain. Of course, the stress and strain are compatible with each other, but in terms of practical application, one of them would serve as a guideline in the design of the product. For example, the final stress level to be attained at the joint part of the crane rail should be high enough to resist the force threatening to open the gap at the joint part, which is exerted by the heavy duty crane through the crane wheel. Then, the fishplates would be designed so as to satisfy the requisites of due stress resistance against the heavy crane movements on the rail. During the heating of fishplates installed on both sides of the crane rail, the additional stress that could be induced by the thermal expansion of SMA has a significant effect on the fishplate itself, with the inhomogeneous recovery stress resulting in the curved fishplates. Figure 12.6 illustrates such a situation. Two similar curves of shape recovery stress (one solid and one

Recovery stress (MPa)

400

c

300

200

After training d

b

c1

f e

b1

d1

100

Before training 0 –200

a –100

0

100

200

300

Temperature (°C) Martensite

12.6 Shape recovery stress on the heating and cooling cycle of Fe–28Mn–6Si–5Cr SMA (with the permission of Trans. Mat. Res. Soc. Jpn).

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dashed) are drawn in the stress–temperature diagram. For convenience, the solid curve of the SMA that has been subjected to one cycle of training treatment is analyzed. The shape recovery stress is measured by fixing both ends of the bar sample in the tensile test machine. Once the Fe–28Mn–6Si– 5Cr SMA has been subjected to the training treatment, it is again deformed in advance by 5% so that the sample may shrink on heating. On heating the whole sample, the stress-induced martensites start to annihilate and then a contracting stress is generated in the sample. It applies extension stress to the tensile machine, as depicted by the curve a–b in Fig. 12.6. In this period of increasing temperature, the thermal expansion of the specimen is supposed to extend the fixed ends of the specimen. At approximately 330 °C, the stress is almost saturated at ∼200 MPa. On cooling, the material behaves in reverse. Thermal contraction makes the specimen shrink and then pulls both the ends of the sample. The tensile stress increases due to the contraction force generated in the specimen as shown in Fig. 12.6. After taking the maximum value at point c, the stress decreases with decreasing temperature and approaches the point d as it reaches room temperature. This stress level d is exactly the available stress of the Fe–28Mn–6Si–5Cr SMA in the shape memory effect. As can be seen from the points d and d1 in Fig. 12.6, the available stress could also be increased by training treatment. At the point c in the stress–temperature curve, the stress-induced martensites start to be formed in the parent phase. The critical stress indicated by point c in Fig. 12.6 is the martensite start temperature, Md, by tensile deformation. In the case of the Fe–28Mn–6Si SMA, the point c should lie on the Md line A–B, as shown in Fig. 12.3. The above-mentioned shape recovery stress d is exhibited as the compression stress at the joint of the crane rail. The shape memory effect in the Fe–Mn–Si SMA is initiated by the pre-deformation of the SMA in advance. In the stress–strain curve, it is represented by the deformation line O→s→a→b, as shown in Fig. 12.7, where the line Os indicates the elastic part and the line sa indicates the stress–strain relationship in the martensite transformation. The strain accumulated in the SMA after unloading to the point b is designated as Ob. The reverse martensitic transformation is supposed to start at the point of critical compressive stress of the induced martensitic transformation, which is designated as Y on the stress–strain curve. The SMA can recover its original shape at the end point E of no stress in the stress–strain diagram. The SMA at the point b is to be heated to ∼35 °C. We create an image of the track of crane rails set with the initial open space (see Fig. 12.8) by SMA fishplates. When the fishplates start to shrink on heating, the opposing tracks are pulled in to close the open space. But, at first, they exhaust the available stress bd in vain until they have made contact with each other. In this situation, the SMA still possesses an available stress Of = Oh (elastic) + hf (martensite). The final amount of stress

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a

f'

g' k'

130– 200 MPa

s e' d'

h' k

O

g

d b

2.5–4% e

h

Strain

Y

f

E

Reverse martensitic transformation

12.7 Stress–strain curve OsabYEO of the SMA and the shape recovery stress df ′, gg′, kk′, which are available for the compressive stress at the joined part of the crane rail. The curve f ′g′k′d is usually approximated by the straight line.

Of would be attained when all the deformation-induced martensites have been released upon heating to 350 °C. The area defOd represents the stress– strain region that can contribute to the generation of the compressive stress at the joint part of the crane rail. For a better explanation of the phenomenon, it is described in the first quadrant of stress–strain diagram as depicted in Fig. 12.7 by d′e′f′dO, where the curve f′g′k′d is usally approximated by a straight line. Then, if the initial gap is opened according to d, g, k, the compressive stress to be attained at the joint part of the crane rail would be df′, gg′, kk′ respectively, and if the gap is too wide, it is not possible to take out the compressive stress from the SMA.

12.6

Applications of iron–manganese–silicon (Fe–Mn–Si) shape memory alloys (SMAs)

12.6.1 Fishplates for crane rail For the heavy duty crane, rails are connected by four bolts with normal steel fishplates sandwiching the rail, placed on both sides of the web. At the time

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of installation, the rails are pulled into contact with each other and connected with no gap at the joint part. However, such rails gradually become separated at the joint part and can be damaged by broken flakes in the gap and dents during the heavy duty operation of the crane. To eliminate such problems, a sufficient and controlled compressive stress is required at the joint part to resist the stress responsible for creating the gap. For this reason, the fishplates are made by an Fe–Mn–Si SMA. The rails are connected by bolts with the SMA fishplates extended in the longitudinal direction in advance. On heating the SMA fishplates, they shrink according to the shape memory effect and the tight connection at the joint part of the crane rail is obtained with the satisfactory compressive stress (Fig. 12.8). Since the autumn of 2004, SMA fishplates have been applied in series to crane rails in steel factories. To this date, they have worked consistently without any trouble. The weight of the fishplate shown in Fig. 12.9 is 10 kg. To achieve shape recovery the fishplates are baked directly in the blaze of a gas burner. The total time required for the installation of the fishplate is

SMA fishplate

(1) Connecting with bolts (gap is between rails)

(2) After heating (gap disappeared)

12.8 Connection of rails with SMA fishplates.

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12.9 SMA fishplates for crane rails.

about 10 minuites. The initial open space set between the two butting rails has been closed completely upon heating the fishplates up to ∼35 °C. The compressive stress yielded at the joint part of the rails prevents the rails pulling apart. As the result, the smooth movement of the crane on the rail has prevented the generation of dynamic loads at the joint part, which serves to eliminate the eventual damage of the electric system due to overloads. Thus, the SMA fishplates for the crane rail have regularly been adopted by factories. No problem has been reported so far, after 5 years of use.

12.6.2 Pipe joints for steel pipes Recently, a new construction method for tunneling work, the so-called curved boring method, was developed. This method is noted as an important technology in the construction of tunnels deep underground, in particular, in unstable ground. Curved steel pipes are dug outward into the soil surrounding the tunnel circumference from the small platform of the tunnel, which constitutes a curved pipe roof of whalebone structure. The space of the main tunnel is created under this curved pipe roof as shown in Fig. 12.10. Because the work space provided by the platform is relatively small, the curved long pipes should be cut to the appropriate length in order to be carried in the platform and jointed with the SMA joining pipes. A series of such processes have to be performed efficiently and safely in the small space under ground. Therefore, from an early stage of developing the technology in civil engineering, the use of Fe–Mn–Si SMA pipe joining has been expected to work in place of welding or bolts. There have been hundreds

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Shape memory and superelastic alloys Working space to connect curved pipes

Curved pipes (100∼300 mm f × ∼1 mL)

Propulsive machine for curved boring

A series of curved pipes with a length of∼1 m are joined together in a small working space provided underground beforehand

Pipe joints 4∼10 m

After a structure like ribs has been constructed the tunnel is driven

required to joint in short times without damaging the environment in the small working space

12.10 Schematic illustration of curved boring method to construct the underground space in a tunnel.

of problems for pipe joining to overcome in the curved boring method. One of them is the requirement to resist the large pull-out force of the pipes. The resistance force of the pipe joining must be as high as that of welding in terms of producing a defending force against the pull-out of the pipes. A method of reinforcement by C-type steel rings has been developed, in which steel rings are used to fill in the grooves set at the end part of the pipe, as well as on the inner side of the joining pipes. As a result of such the fundamental investigations of the SMA pipejoining technique, the curved boring method was adopted on a trial basis in 2003. The 250A curved pipes (outside diameter 267.3 mm Ф, bend radius 6000 mm) were joined by the Fe–Mn–Si–Cr SMA joining pipes in the platform, as shown in Fig. 12.11. The SMA joints are produced by centrifugal casting with a composition of Fe–28Mn–6Si–5Cr. The joint is heated by high-frequency induction heating. This method can be performed by fewer workers in fewer working hours compared with welding.

12.7

Future trends

In the industrial application of SMA, we can easily identify merits such as the fact that SMA joining does not waste time in the joining of pipes or rails, and that it does not require experienced welders. However, the Fe– Mn–Si SMA is used as a functional material only in the installation process, and it is inevitable that it will serve as a structural material after this point. Therefore, SMA joining has not been applied until the total cost has been

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12.11 Pipe coupling with SMA joint.

made low enough in comparison with the other existing joining methods. We have always been confronted with the difficulty of overcoming the low cost problem in terms of the product parts. Specifically, in the case of the Fe–Mn–Si SMA, an increase in the total amount of the product used does not solve the problem. An important point to be observed is that the SMA contains a large amount of Mn elements, which constitute over 20% in the alloy. One ought to use a small induction furnace to melt any raw materials containing a large amount of Mn. A mass production electric furnace cannot be adopted for the melting of any SMA containing a large amount of Mn elements and being of high heat capacitance. Besides, in the case of an SMA containing over 20% Mn, the impurities contained in the raw Mn materials give rise to fatal faults in the mechanical properties of the SMA. In order to produce a healthy and functional shape memory effect, the SMA requires at least 15% elongation in the deformation process. Therefore, we have to pick up the high price and high purity Mn materials. Alternatively, if we could decrease the Mn content of the SMA, we could utilize the mass production furnace for a large amount of the SMA product, and the total cost would be quite reasonably decreased. In the production of the pipe joints for tunnel construction and the crane rail fishplates, the price of the SMA materials is the key factor. The second important point in determining the low cost of the SMA is to explore the new markets which consume a large amount of SMA. Recently, the Fe–Mn–Si SMA has received considerable attention in terms of its industrial application to damping materials (Sawaguchi et al., 2006). The shape memory effect has already been explained in terms of how it is

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generated by the formation of the stress-induced martensites and their reverse transformation. The SMA does not induce the fatal degradation of materials by means of plastic deformation. That is, we may apply the SMA parts to the part on which the repeated stress is applied. For example, the materials could be applied to high-rise buildings as an absorption damper in order to eliminate the severe damage caused by earthquakes. Now, the development of low cost materials and the exploration of their new industrial applications are the two main directions in which we may proceed in this SMA field. Recognizing the fact that the Fe–Mn–Si SMA exhibits only one cycle of the shape memory effect and thereafter serves as a structural material at the place of installation, we are positive about establishing a new application field characteristic of the Fe–Mn–Si SMA.

12.8

References

Dunne D P and Wayman C M, (1973) ‘Effect of austenite ordering on martensite transformation in Fe–Pt alloys near composition Fe-3Pt’ Metall. Trans., 4, 137–145, 147–152. Gaunt P and Christian J W, (1959), ‘The cubic-hexagonal transformation in single crystals of cobalt and cobalt–nickel alloys’ Acta Metall., 7, 529–533. Kajiwara S, (1985), ‘Nearly perfect shape memory effect in Fe–Ni–C alloys’ Trans. JIM, 26, 595–596. Koval Y N, Kokorin V V and Khandros L G, (1981), ‘Shape memory effect in Fe–Ni–Co-Ti alloys’ Phys. Met. Metall., 48, no.6, 162–164. Maki T and Tsuzaki K, (1992), ‘Transformation behaviour of e martensite in Fe–Mn–Si shape memory alloys’ Proc. ICOMAT-92, 1151–1162. Maki T, Kobayashi K, Minato M and Tamura I, (1984), ‘Thermoelastic martensite in an ausaged Fe–Ni–Ti–Co alloy’ Scr. Metall., 18, 1105–1109. Maruyama T and Kurita T, (2004), ‘Ferrous shape memory alloys and their applications’, Kinzoku, 74, no.2, 48–51. Maruyama T, Kurita T, Kozaki S, Andou K, Farjami S and Kubo H (2008), ‘Innovation in producing crane rail fishplate using Fe–Mn–Si–Cr based shape memory alloy’ Matrs Sci and Tech, 24, 908–912. Melton K N (1998), ‘General applications of SMA’s and smart materials’ , in: Otsuka K and Wayman C M, Shape Memory Materials, Cambridge, Cambridge University Press, 220–239. Murakami M, Otsuka H, Suzuki H and Matsuda S (1986), ‘Complete shape memory effect in polycrystalline Fe–Mn–Si alloys’ Proc. of ICOMAT-86 (JIM), 985–990. Oshima R (1981), ‘Successive martensitic transformations in Fe–Pd alloys’ Scr. Metall., 15, 829–833. Otsuka H, Murakami M and Matsuda S, (1989), ‘Improvement in the shape memory effect of Fe–Mn–Si alloys by the thermomechanical treatment’ Proc. MRS Int. Mtg. Adv Mater, 9, 451–456. Sato A and Mori T (1991), ‘Development of a shape memory alloy Fe–Mn–Si’ Matrs Sci Eng, A146, 197–204.

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Sato A, Chishima E, Soma K and Mori T (1982), ‘Shape memory effect in gamma reversible epsilon transformation in Fe–30Mn–1Si alloy single-crystals’ Acta Metall., 30, 1177–1183. Sato A, Chishima E, Yamaji Y and Mori T (1984), ‘Orientation and composition dependence of shape memory effect in Fe–Mn–Si alloys’ Acta Metall., 32, 539–547. Sato A, Yamaji Y and Mori T (1986), ‘Physical-properties controlling shape memory effect in Fe-Mn-Si alloys’ Acta Metall., 34, 287–294. Sawaguchi T, Sahu P, Kikuchi T, Ogawa K, Kajiwara S, Kushibe A, Higashino M and Ogawa T (2006), ‘Vibration mitigation by the reversible fcc/hcp martensitic transformation during cyclic tension–compression loading of an Fe–Mn–Si-based shape memory alloy’ Scr. Mater., 54, 1885–1890. van Humbeek J and Stalmans R, (1998), ‘Characteristics of shape memory alloys’ in: Otsuka K and Wayman C M, Shape Memory Materials, Cambridge, Cambridge University Press, 149–183. Wayman C M (1971), ‘On memory effects related to martensitic transformations and observations in b-brass and Fe3Pt’ Scr. Metall., 5, 489–492. Yang J H and Wayman C M, (1992), ‘Self-accommodation and shape memory mechanism of epsilon-martensite’ Mater. Characterization, 23–35 and 37–47.

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13 Applications of superelastic alloys in the telecommunications industry T. HABU, Furukawa Techno Material Co. Ltd, Japan

Abstract: This chapter introduces various examples of super elastic use in the telecommunications, sports, leisure and other industries. These products utilize the characteristics of superelastic alloys, i.e., high flexibility and large recoverable deformation, in daily use. Key words: superelasticity, flexible, high recovery, high strength, anti-abrasion.

13.1

Introduction

Superelastic Ti–Ni alloys have attracted considerable attention in various fields of industries due to their large recoverable deformation, which provides exceptionally high flexibility. Also a constant recovery force during unloading gives significant advantages over other metals. Numerous ideas for superelastic use have been suggested and many of them have been successfully commercialized in various fields such as telecommunications, sports and leisure. This chapter introduces some popular examples which utilize superelasticity.

13.2

Products utilizing superelastic alloys in the telecommunications industry

13.2.1 Antenna for cell-phone An antenna wire for cellular phones is one of the most sold products utilizing superelasticity. A superelastic Ti–Ni wire has been used for the core wire of antennas of cellular phone (Fig. 13.1). Previously piano wire or stainless steel wire was used; however, this wire was easily deformed permanently when it was bent in pockets or simply by being stretched out and folded into the phone many times. Thus, the superelastic alloy with a large recoverable deformation has been the best solution for a new antenna free from permanent deformation. 163 © Woodhead Publishing Limited, 2011

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13.1 Antenna for cell-phone (courtesy of NEC Corporation).

13.2.2 Antennas for other applications Domestic demand for the superelastic antenna for cell-phones in Japan has dropped due to the increasing demand for smaller cellular phones, which has led to the antenna being mounted in the actual phone. However, many superelastic antennas are sold abroad, though fewer than before. Besides antennas for cell-phones, there are similar applications of the superelastic antennas for radios and radio controlled cars (Fig. 13.2). It is anticipated that superelastic Ti–Ni antennas will be used for domestic land-based digital television broadcast reception.

13.2.3 Headbands of headphones Superelastic wires have been used in the headbands of headphones (Fig. 13.3). The headphones can be folded into a compact shape owing to the high flexibility of the superelastic wires. Furthermore, a constant low pressure on the ears gives a comfortable feeling when in use.

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13.2 Antenna for radio controlled car.

13.3 Headband of headphone (courtesy of Sony Corporation).

13.2.4 Fishing There are many applications of superelastic wires being used in fishing tools. This is because the superelastic wires satisfactorily meet the required characteristics for the fishing tool such as high flexibility, high strength and good sensitivity.

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Fishing line A thin Ti–Ni wire has been used as a metal fishing line for sweetfish (ayu) fishing (Fig.13.4). Sweetfish is a small fresh-water fish and fishing for it is a very popular activity in Japan. The Ti–Ni superelastic wire for this application is thinner but stronger than nylon wire, and is also more elastic than conventional metal wire; it has been known as ‘the metal line with nylon feeling’. Furthermore the Ti–Ni wire is strong against shocks and is stainfree. The wires are made in several sizes, 0.045 mm to 0.085 mm diameter, and colored blue or gold by adjusting the thickness of the oxide film of the wire surface. Fishing balancer There are some advantages in using a superelastic Ti–Ni wire for fishing balancer, Spinnerbait (Fig. 13.5): it can bend easily and does not get caught by obstacles under water such as seaweed or rock. It is also compact for convenient carrying. The Ti–Ni line is much thinner than other balancers so it has less water resistance and extra sensitivity. Furthermore, the Ti–Ni wire exhibits greater corrosion resistance in sea water compared with a stainless steel wire. Fishing rods and other items for fishing A superelastic Ti–Ni tube has been used on the tip of fishing rods (Fig. 13.6). Use of a superelastic tube provides high sensitivity, flexibility and toughness, and the high performance of the fishing rod has satisfied many people. There have been various other applications utilizing superelasticity for fishing items, i.e., guide wire for fishing pipe rod, creel frame and lure, etc.

13.4 Fishing line (courtesy of Morris Co. Ltd).

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1. Free form Wire is a shape memory alloy (SMA) superelastic

2. When sinking into water Same form as conventional balancer

3. When it settles down into water When the weight and hook are shaken by water wave, SMA balancer absorbs the vibration and keeps balance between hook and rod

4. Pulls up the line when fish is caught When a fish is hooked, a balancer becomes a straight line and hooks the fish well

13.5 Fishing balancer (Spinnerbait) (courtesy of Yoshimi Inc.).

13.6 Fishing rod (courtesy of Daiwa Seiko Inc.).

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13.7 Paint gun for car bodies (courtesy of Anest Iwata).

13.2.5 Paint gun for car bodies A superelastic Ti–Ni wire has been used as the pin electrode in a paint gun (Fig. 13.7). The high resistance to permanent deformation endures high pressure of the paint gun and improves the service life of the pin electrodes.

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14 Applications of superelastic alloys in the clothing, sports and leisure industries T. HABU, Furukawa Techno Material Co. Ltd, Japan

Abstract: This chapter introduces the products which are used in contact with or near the human body among superelastic applications, in other words, the products exhibit superelasticity at body temperature. These products exhibit stable superelastic properties at body temperature regardless of environmental temperature. They give a pliant and comfortable feeling of wearing, making it hard to believe that they are made of metal. Typical examples are bras and glasses. Key words: superelasticity, flexible, comfortable, human body temperature, wearing.

14.1

Introduction

The superelastic alloys have been successfully applied in many types of wearing items, such as eyeglass frames, bras, etc., mainly due to their exceptionally high flexibility and soft feeling. The high flexibility of superelastic wires in such wearing items makes people comfortable when they wear them. This chapter introduces popular wearing items utilizing the superelastic properties of shape memory alloys.

14.2

Products utilizing superelastic alloys in the clothing, sports and leisure industries

14.2.1 Eyeglass frames The first practical use of superelasticity in non-medical fields was as eyeglass frames, such as rim and temple. The use of superelastic alloys allows eyeglass frames to recover their original shapes after a large deformation or hard bending, as shown in Fig. 14.1. Superelasticity of the frames also gives a constant low stress and make the frames very comfortable to wear. The superelastic eyeglass frames were first applied in ladies products due to their soft and comfortable feeling, but later came to be used in the products for men. They are also used for children’s products since they do not break when stepped or hit accidentally. In addition the contraction stress upon cooling fastens the lenses effectively and causes the lenses not to be pulled out from the frames. 169 © Woodhead Publishing Limited, 2011

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14.2.2 Bras Superelastic wires were utilized in bras for a beautiful body line, soft feel and flexible fitting. This was the first mass-produced application utilizing superelasticity and made ‘superelastic’ a common and familiar word.

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Originally, wires exhibiting the shape memory effect were used in bras because they returned to their original shape by heating even if they had deformed during laundry. However, developers found that it was very comfortable to use the superelastic effect on body temperature, and finally superelastic wires were used in bras. Many pieces have been used for the under-wire of bras in Japan (Fig. 14.2). Their beautiful silhouette and soft and fine fitting were superior to those of the conventional iron wire. They were also used for the upper-wire or in girdles as well, as shown in Fig. 14.2.

14.2.3 Shoes Superelastic Ti–Ni wires were used in the heels of shoes (Fig. 14.3), where the wires retain the shape of shoes and then make comfortable to wear.

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14.2.4 Hat Superelastic wires were also used in the rims of hats as shown in Fig. 14.4. In addition to similar functions of superelastic wires as stated above, they allow the hat to be folded up into a compact size for carrying.

14.2.5 Shoulder pads of jacket Figure 14.5 shows shoulder pads containing a superelastic wire. The superelastic wire supports the pads to keep the shape and makes a comfortable fit.

Superelastic alloy wire

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14.2.6 Petticoat Ti–Ni superelastic wires of 1.6 and 1.8 mm diameter were used for the bones of petticoats for dresses as shown Fig. 14.6. Compared with the conventional piano wire bones, the superelastic wires allow the dress hem to move naturally and the petticoat can be folded to a small size when carrying it around.

14.2.7 Comfortable apron Recently, Ti–Ni superelastic wires have been used in a comfortable apron for assisting heavy-lifting workers in agriculture and health care workers (Fig. 14.7). There are superelastic wires in both sides of the apron, and these wires support bending and recovery of a waist by the superelastic recovery power. They are effective in preventing lumbago from work with a heavy stoop. For example, when you hold a person to provide care, the apron can support you to raise the patient or to keep your bending posture for a long time.

14.2.8 Accessories Another common application of superelastic wires is as accessories such as bracelets, necklaces, earrings, and straps with pearls or beads as shown in

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Fig. 14.8. The necklace is very comfortable to wear and it keeps a more round shape than normal metal-cored necklaces without drooping. In the case of bracelets, it is not necessary to put a clasp since the superelastic bracelet fits flexibly and gently around the wrist.

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15 Medical applications of superelastic nickel–titanium (Ni–Ti) alloys I. OHKATA, Piolax Medical Devices Inc., Japan

Abstract: This chapter introduces the medical treatment devices that use Ni–Ti shape memory alloys. The superelastic effect of Ni–Ti medical devices has contributed to new clinical techniques (such as orthodontic surgery and minimally invasive treatment) as well as to the improvement of clinical performance. Key words: medical devices, superelastic (SE) wire, orthodontic wire, guide wire, catheter.

15.1

Introduction

Twenty years have passed since Ni–Ti shape memory alloys (SMAs) were first used in practical applications. It has now been widely acknowledged that Ni–Ti SMAs can be applied as industrial materials, as well as having very useful properties of metallic materials. These properties led to their use in the active development of medical devices. There are six possible reasons why SMAs are used in the medical field. • • • • • •

They possess a low modulus of elasticity. They do not deform easily. They can support a constant load, regardless of flexibility. They are capable of recuperation, owing to the SMA effect. They are capable of a high level of corrosion resistance, and biological adaptability. They are a high value-added product.

Compared with stainless steel, super elastic (SE) materials have a modulus of 40 N/mm2, which is about a quarter of the modulus of a stainless steel that has 1950 N/mm2. A low modulus of elasticity, low deformability and compatibility with the human body are great properties that no other metallic elements possess.

15.2

Hallux valgus

Hallux valgus is a structural deformity of the bones and joint between the foot and the big toe. It may cause the big toe to turn inward, in the direction 176 © Woodhead Publishing Limited, 2011

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of the second toe, and the tissues surrounding the joint may become swollen and tender. The term ‘hallux’ refers to the great toe and ‘valgus’ refers to an abnormal angulation of the great toe, which is commonly associated with deformities of the bones. Hallux valgus causes the sufferer pain when wearing shoes and difficulty in walking. Today, this disorder is a social issue and tends to be greater among women, who more commonly wear closetoed shoes, such as high heels. Hallux valgus may be caused by a variety of conditions intrinsic to the structure of the foot, such as flat feet (where the arch of the foot collapses, with the entire sole of the foot coming into complete or nearly complete contact with the ground) and excessively extended toes (where all five toes are extended on either side). In the case of patients with flat feet, Hallux valgus may be treated by using an arch support to raise the arch of the foot. However, no braces and shoe gears for patients with extended toes have yet been developed. Although orthopedic treatments for correcting the projection of a joint, such as the wearing of a splint at night time, have been utilized, it has not been possible to make such treatment suitable for application while walking. A shape memory splint consists of a three-dimensional textile, which is made of Teflon and Ni–Ti SE wire (diameter of 0.5–1.0 mm, room temperature). This splint is formed into a shape that fits the patient’s foot by wrapping it in a plaster cast and subjecting it to heat treatment at 450 °C, as shown in Fig. 15. 1. The shape memory splint tightens the arch of the foot

The SE net memorizes

SE net

15.1 A shape memory splint and its application.

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and both sides of the foot with a constant pressure, which suppresses excessive extension of toes and the collapse of the arch of the foot. In the case of this splint, the tightening pressure increases inside shoes due to the superelastic properties of the Ni–Ti alloy because the temperature inside the shoes is 10–20 °C higher than room temperature. Furthermore, this splint can tolerate repeated deformation due to the excellent superelastic properties of the Ni–Ti SMA.

15.3

Orthodontic wire

In dentistry, SE alloys are successfully used as orthodontic wires in the correction of irregular teeth. Brackets, the metallic attachments shown in Fig. 15.2, are cemented to the surface of the teeth by an orthodontist. The bracket serves as a means of fastening the arch wire to the band. When using stainless steel or Co–Cr, the strain range of the wire is 0.5% at most. In order to obtain a good spring-back, it is necessary for the wire to be bent into a U-shape or in a loop, although this causes stress to the patient. On the other hand, SE wires are flexible to the point where they can return back to their original shape after having been deformed. With this characteristic, it is not necessary to re-apply the wire every time there is movement of the teeth, so the number of reapplications per patient decreases, as shown in Fig. 15.3. This results in a shorter treatment time. Orthodontic wires using the orthorhombic martensitic transformation of Ni–Ti–Cu have been developed. Less tension is required for the installation of Ni–Ti–Cu than for Ni–Ti, so it is less stressful for the patient. Since the loop of the stress hysteresis is narrow, the recovery range of the Ni–Ti–Cu orthodontic wires is sufficient to produce and accommodate the movement of the teeth. The small stress hysteresis of Ni–Ti–Cu allows for a similar recovery range to that which can be obtained in Ni–Ti.

15.2 U-shaped conventional metallic orthodontic wire.

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15.3 Superelastic Ni–Ti orthodontic wire.

Spring coil with PTFE coating

Safety wire

Core wire

15.4 A spring coil guide wire using stainless steel (PTFE: polytetrafluoroethylene).

15.4

Guide wire

15.4.1 Application of SE alloy to guide wire Surgery was once thought of as a procedure in which abdominal or thoracic incisions were made. However, these days, minimally invasive treatment, such as endovascular treatment and endoscopic treatment, is becoming increasingly popular. The wide use of endovascular treatment began in Sweden in 1951, when Sven-Ivar Seldinger developed a catheterization method. The method involves the insertion of a tube called a ‘catheter’ into a blood vessel, combined with the injection into a blood vessel of a contrast medium, which makes the vessel visible through the X-ray machine. An angiographic guide wire is a medical device that is used along with a catheter, and acts as a guide through the blood vessel in reaching a targeted lesion. Prior to 1990, the core wire of an angiographic guide wire was made from austenite stainless steels in mainstream production. A guide wire was originally structured from a stainless steel core wire covered by a stainless steel coil (Fig. 15.4). Later, a hydrophilic guide wire, the surface of which had been coated with a hydrophilic polymer to reduce

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friction, with a catheter began to appear. The core wires of the guide wires were originally made only from an Ni–Ti alloy, but these days the wires are made from an Ni–Ti alloy with SE characteristics, so they can occupy the main position in the market. In the future, the greatest focus will be on micro-guide wires (which measure less than 0.5 mm (0.018 inches)) that are used for abdominal blood vessels and peripheral arteries. The general angiographic guide wires and micro-guide wires are shown in Figs 15.5 and 15.6. The structure of the guide wire consists of an Ni–Ti core wire with a tapered distal portion, covered with polyurethane and a hydrophilic polymer on the top to reduce any friction between the inner lumen of the catheter and the vascular wall. The micro-guide wire, on the other hand, has a smaller outer diameter (below 0.5 mm, 0.018 inches) with gold, platinum or tungsten wrapped around the coil, which is buried in the tip of the core wire. Gold, platinum and tungsten are metallic elements that are visible through an X-ray, so it is easier for the surgeon to locate the position of the tip of the micro-guide wire through the radioscopic image.

15.4.2 Characteristics demanded of a core wire The function of the guide wire is to guide the catheter to the targeted lesion. It is necessary to insert the guide wire prior to advancing the catheter, because it will have to go through bifurcated and bent blood vessels. Depending on where in the body the guide wire is being inserted, the placeHydrophilic coating

Core wire

Polyurethane

15.5 A hydrophilic coated guide wire.

Radiopaque marker coil

Polyurethane

Hydrophilic coating

Core wire

15.6 A hydrophilic micro-guide wire.

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ment of emphasis in terms of the performance of the guide wire will be different. We test the guide wire along with the catheter using a blood vessel model and selecting the materials of the core wire, as shown in Fig. 15.7. In selecting the core wire of the micro-guide wire that is used for an abdominal blood vessel, we take the following four factors into consideration: (1) straightness, (2) flexural rigidity, (3) resistance to deformation and (4) toughness. The first factor is the straightness of the guide wire. The straightness affects the torque transmission and operation of the guide wire. When inserting the wire into diverged blood vessels, the straightness becomes very important. The lower the straightness of the guide wire, the more difficult it is for the distal point to be rotated by hand. Even though it is rotated, it quickly flips, so it becomes difficult for the guide wire to be inserted into the targeted blood vessel. Therefore it is necessary to produce a core wire of about 2 meters in length, with no visible bends. The second factor is the flexural rigidity. Flexural rigidity is measured by the performance of the wire in a three-point bending test. Flexural rigidity influences the pushing performance and operational feeling of the peripheral blood vessel. If the flexural rigidity is high, the pushing power of the hand is easily transmitted to the distal tip of the guide wire. The flexural rigidity is proportional to the Young’s modulus. The Young’s modulus of an austenite stainless steel is twice that of an Ni–Ti superelastic alloy. Therefore, the material of the core wire has a large influence on the operation of the guide wire. Increasing the flexural rigidity of an Ni–Ti SE alloy, just like stainless steel, is likely to become an important subject in future materials development. The third factor is the resistance of the wire to deformation. Resistance to deformation can be confirmed by the level of residual distortion after the application of a certain fixed distortion. The problem with austenite steel is that it is easily deformed. This causes a decrease in performance

15.7 A blood vessel model for selecting the materials of the core wire.

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because the austenite steel wire is easily deformed when it is inside the blood vessel. On the other hand, the Ni–Ti SE alloy is excellent in its resistance to deformation, so its performance does not fall even, after inserting and extracting the guide wire. As for the widespread use of the Ni–Ti superelastic alloy as the core wire material of the guide wire, the characteristic of being resistant to deformation plays a huge role. Once the micro-guide wire has been inserted into a blood vessel, especially into peripheral blood vessels, the advantages of the core wire material of the micro-guide wire being a superelastic alloy will become very significant. The fourth factor is the toughness of the wire, which is the characteristic most greatly concerned with safety. There should never be a situation in which a core wire can become fractured inside the body. Although the Ni–Ti SE is an extendable material, it is also an intermetallic compound, so the crack made at the abrasive part of the tip of the core line becomes a starting point for a fracture, which then spreads quickly. The risk of the inserted micro-guide wire breaking inside a blood vessel that is crooked and has a small diameter cannot be disregarded. It is especially important to be cautious of any cracks and coarseness of the surface of the core line. It is necessary to consider the following points in addition to the four characteristics stated above. The distal tip of the core wire is tapered with a grinder or other chemical polishers. There is a possibility that the Ni–Ti alloy will be weakened if hydrogen enters the core wire. If the grinding is done mechanically, it is important to be careful not to develop any scratches, because these may lead to fracture when in use. Furthermore, the tip of the core wire must have a coil which is connected inside. The connection uses a laser, etc. in welding and soldering. The Ni–Ti SE alloy has a weaker soldering ability than that of the austenite steel. The plating process can improve its soldering ability but cannot eliminate the possibility of hydrogen entering into the core wire. The diameter of the tip of the wire is less than 0.1 mm. This means that the ratio of the surface area is larger than that of the volume, so it can be said that hydrogen may easily enter. It is understood that hydrogen that enters into the material influences the likelihood of a fracture being formed through bending fatigue. Furthermore, the high workability of the Ni–Ti SE alloy allows for the change in its characteristics which occurs through heat treatment, resulting in a change in the characteristics of the core wire through secondary heat treatment (as shown in Fig. 15.8) and thus giving a merit to the micro-guide wire in terms of acquiring a new application.

15.4.3 Summary The Ni–Ti superelastic alloy is currently thought to be the most suitable material for a guide wire. However, it does present a problem in that it has

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Value of residual strain (mm)

1.40 1.20 1.00 0.80 0.60 0.40 0.20 0.00 450

500

550

600

650

Heat treatment temperature (°C)

15.8 The heat treatment temperature dependence of a residual strain.

a low degree of rigidity, although it has a high resistance to deformation. The development and improvement of a material that offers rigidity which is not inferior to stainless steel, and which possesses SE characteristics akin to Ni–Ti would be highly desirable to clinical doctors.

15.5

Biliary stents

A stent made from an SMA has been used in the human body for many purposes. The name ‘Stent’ comes from Dr Charles Stent, who practised in the nineteenth century.1 In 1969, Dr Dotter suggested that stents could be used in the treatment of humans.2 In 1991, Dr Parodi treated an aortic aneurysm using a stent graft.3 Today, a lot of variously designed stents have been launched worldwide. Stent therapy is a minimally invasive treatment, which offers a reduced burden on the patient, compared to that suffered by surgery, so it improves the quality of life (QOL) of the patient. Recently interest in stents has increased among doctors and patients due to their usefulness.

15.5.1 Category of the stent Stent material is divided into two groups: metal and polymer. The term metallic material encompasses nitinol, stainless steels and other metallic alloys. The term polymer material includes nylon, polyurethane and other polymers. The category of metallic stent is also divided into two types of expansion methods: a self-expandable type and a balloon expandable type. Nitinol stent is a self-expandable type due to its SE characteristics.

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The stent is inserted into a human’s narrowed lumen. Once inserted, the lumen is expanded by the stent, leading to improved blood flow. In other words, a self-expandable type of a stent is inserted into the delivery system. The delivery system consists of a small diameter tube of approximately 2.3 mm. The diameter of the nominal lumen of a bile duct is approximately 6 to 10 mm and a stent can expand over four times, for example from 2.3 to 10 mm (Fig. 15.9).

15.5.2 Stent design and components A stent consists of a braided wire or a laser cut from a tube. A braided stent is flexible and it fits better in the lumen. A laser cut stent has an excellent expansion force with high effectiveness in strong stenosis. Stent design has also produced a lot of ideas for new implantation positions or purposes, e.g., having less shortening of the design before and after expansion, a fitted shape for curved organs, and other ideas. It also includes the covering design. A covered stent prevents the ingrowth of the tissue going through a stent mesh. Cover material is also selected according to implantation position. For aneurysm purposes, covering materials such as ePTFE or polyester are selected. For bile ducts, silicone or ePTFE materials are selected in order to be able to tolerate the bile juice. The delivery system is also an important item. The characteristics of a delivery system require a small diameter, easy access to the implantation positioning and easy deployment. The delivery system consists of an outer

15.9 Expanding stent from the delivery system.

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sheath and an inner sheath. A stent is inserted at the distal point of the outer sheath (Fig. 15.10). For a biliary stent, the length of the delivery system is different depending on the implantation method. There are two methods, which include the use of an endoscope, and a percutaneous approach. The percutaneous approach involves the direct puncture of the bile duct using ultrasound. Thus, a delivery system of short length is acceptable in this instance. Its length is approximately 450 to 700 mm. However, the endoscopic approach requires a length of 1700 mm or more. In the case of an implantation method which makes use of an endoscope, the delivery system has to pass through the esophagus, stomach and duodenum in order to reach the bile duct. The delivery system is also inserted through the endoscope. To be inserted into the bile duct, the delivery system becomes a curve of an acute angle. When the stent is being deployed, an axial force hangs at the delivery system. However, the deploying force cannot travel directly to the distal point of the delivery sheath because the delivery system has a curved acute angle at the end of the endoscope. When the delivery system is in use, the outer sheath applies an elongation force and the inner sheath applies a compression force. If the delivery system is elongating or shortening, it is not possible for the stent to be deployed at the targeted position, so the design of the delivery system is just as important as stent design.

15.5.3 Characteristic of the biliary stent The biliary stent requires the following characteristics: good flexibility, inner lumen preservation, mild radial force and visibility. In this instance, the

15.10 Stent mounted into the delivery system.

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‘good flexibility’ refers to the ability to fit at the bile duct of the inner lumen and the stent. If a stent does not have good flexibility, a gap will form between the bile duct and the stent so that biliary sludge will fill the gap, causing an occlusion of the bile duct. The characteristic ability of the biliary stent to preserve the inner lumen preservation is closely related to flexibility. If the stent has good flexibility but is not able to preserve the inner lumen in the curved position, the stent will not be able to drain the bile juice4 (Fig. 15.11). The purpose of the stent is to expand the narrowed lumen. For this reason, the radial force is an important characteristic of the stent. However, any extra radial force gives extra stimulation to the tissues, which can result in re-stenosis. The radial force is controlled by the strut design, width and thickness of the stent. Visibility is also an important characteristic in terms of deciding the position of the stent. To position the stent, the physician uses an X-ray machine and a contrast medium. After deciding the position of the stent implantation, the delivery sheath is inserted into the implantation position. The physician carefully deploys the stent from the delivery

2

0

1

Stent A

2 0 1

Stent B 2

2

0

1

0

Stent C

Stent D

15.11 Comparison of inner lumen at a curved state.

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sheath using the X-ray. If the stent has poor visibility, the physician cannot confirm the expanded stent position. This presents a risk of positioning the stent in the wrong place.

15.6

Regional chemotherapy catheter

15.6.1 Medical tubule used for the port system type of transcatheter arterial infusion (TAI) The port transcatheter arterial infusion (TAI) is one of the treatments that have been developed and utilized exclusively in Japan. The catheter, which is a narrow tube used for endovascular treatment, is inserted from the foot of the femoral artery or from the artery which is located below the clavicle. The catheter then enters into the hepatic artery and is used to deliver anticancer drugs, which are administered directly into the tumor location in the liver. The tip of the catheter is connected to an implanted port and is buried underneath the surface of the skin. A needle is used to puncture the skin and is then inserted through the skin to the port. Anti-cancer drugs are administered through the needle to the lesion of the liver. There are two advantages for the TAI of using the implanted port. First of all, the TAI can administer a high dosage of anti-cancer drugs that are targeted to the tumor, a method which results in the patient suffering fewer side effects compared with the method of administering drugs to the whole body through a vein. Secondly, it is possible for patients to move freely after the drugs have been administered because the port and catheter are completely buried under the skin. When the TAI was introduced in the 1980s, there were two types of complication that occurred to some extent: (1) the closure of the hepatic artery and (2) the migration of the tip of the catheter. A slight vibration of the tip of a catheter caused by body movement results in a fracture of the intima of a blood vessel, which may in turn lead to vessel occlusion. Furthermore, if the tip of a catheter moves, the drugs it is carrying drift into areas beyond the target area, which leads not only to inadequate results in terms of the intended medication, but also to the patient developing ulcers or other side effects. The method that is generally used today is that the tip of the catheter is inserted and fixed into the gastro-duodenal artery or a peripheral hepatic artery, while the drugs are administered through a hole in the side of the catheter, which is located in the hepatic artery. Recently, there has been another catheter launched with a helical-shaped tip that has an SMA coil buried inside it. Compared with an implanted catheter with a straight tip, such as was previously in use, the tip of the catheter is much more easily fixed into the blood vessel.

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15.6.2 Necessary charactericitcs of catheters Excellent biocompatibility and the capability to survive long-term implantations are two important elements demanded of an implanted catheter for the TAI. The materials of a catheter are coated with silicone and polyurethane to prevent blood clots and fibrin stains. High tractability is also required of a catheter in order that it can move smoothly through the guide wire.

15.6.3 Structures and characteristics of the implanted catheter As mentioned above, there is currently a catheter for TAI on the market that has the SMA coil implanted into it. Figures 15.12 and 15.13 show a photo of the catheter and its schematic illustration, respectively. A flat wire made from Ni–Ti is used as an SMA. At first, the flat wire is wound up into a helical shape to form a wire and to make it into a three-dimensional coil shape when heat is added to it (Fig. 15.14). The three-dimensional SMA coil of the catheter is then installed inside the polyurethane-made catheter. Since the SMA is a hollow coil, the guide wire can be inserted through it (Fig. 15.15). When the guide wire is pulled out from the catheter, it returns to its original coil shape with the tip of the catheter fixed inside the blood vessel. The end tip of the catheter is structured so as to have no contact with the intima of the blood vessel, a design that prevents any damage being done to the

15.12 The tip of the implanted catheter for hepatic arterial infusion chemotherapy.

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Markers SMA coil Side hold

20 cm (O.D. 1.30 mm)

60 cm (O.D. 1.70 mm)

80 cm

15.13 Illustration of the implanted catheter for hepatic arterial infusion chemotherapy.

15.14 An SMA coil.

blood vessel. At both ends of the SMA, there is a platinum marker, which is bonded. Since the marker is made from platinum, the X-ray does not penetrate it, which makes it easier to see and locate the position of the SMA coil and the catheter in the X-ray image. Figure 15.16 shows the comparison of fixed power by the presence of the SMA. The catheters have four times or more resistance and greater fixation due to the presence of an SMA coil compared with catheters which do not contain an SMA coil. The catheter is made of polyurethane which has strong biocompatibility. The contrast medium is mixed with polyurethane and the whole catheter can be seen from the X-ray image. The surface of the catheter is coated with a hydrophilic polymer. When a hydrophilic polymer comes into contact with

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Catheter Guide wire

SMA coil

15.15 A picture and illustration of the SMA coil inserted guide wire.

Sliding resistance load 30.0

Resistance load (g)

With SMA coil No SMA coil 20.0 N=5 10.0

0.0 Catheter

15.16 Measurement of sliding resistance load of the implanted catheter for hepatic arterial infusion chemotherapy.

blood, it swells and becomes lubricated so that even though the blood vessels are crooked and spiral-shaped it is able to reach the targeted lesion. Figure 15.17 shows a catheter being inserted into a bent and crooked artery. Even though the blood vessel comprises a turn of nearly 180 degrees, the fact that the polyurethane surface has been coated with a hydrophilic polymer provides lubricity, and this together with the fixation of the SMA

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Markers

15.17 Placement of the implanted catheter with arterial redistribution.

coil makes it possible for the catheter to operate successfully. The catheter is stably fixed inside the blood vessel after the implantation. There are some cases where the catheter is implanted for more than a year.

15.7

Endoscopic guide wire

15.7.1 Purpose and function An endoscopic guide wire is inserted with the catheter (or with other therapeutic devices) through the endoscopic channel, which starts at the mouth and continues to the duodenum. The guide wire serves in the same way as a rail for a train, guiding the device to the targeted lesion, as shown in Fig. 15.18.

15.7.2 How it is different from angiographic guide wire Through the endoscopic channel Endoscopic guide wires are inserted through the endoscopic channel. Their total length is longer than that of an angiographic guide wire. A guide wire with a length of 400–500 cm is commonly used. The outer diameter of a guide wire is 0.89 or 0.64 mm (0.035 or 0.025 inches), depending on the diameter of the inner lumen of the catheter or other device.

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Bile duct Endoscope

Duodenum

Guide wire Stenosis

Pancreatic duct

15.18 A schematic illustration of endoscopic channel.

Less fluid In comparison with the inside of a blood vessel, the gastro intestinal duct and the endoscopic channel have less fluid. In these conditions, the longterm use of a hydrophilic guide wire results in a reduction in lubricity.

15.7.3 Classification of guide wires Endoscopic guide wires are classified into two types according to their intended use and surface properties which are, namely, the hydrophonic type and the fully hydrophilic-coated type. Hydrophobic type: for delivery of catheter and other devices This guide wire is made out of two portions. (Fig. 15.19) Its main shaft is covered with a PTFE heat shrink tube, and the portion that reaches up to 20 cm from the distal tip is covered with polyurethane, which is coated with a hydrophilic polymer on the surface. The PTFE heat shrink tube has twocolored striped patterns and has hydrophobic properties. The striped pattern is effective in terms of monitoring the movement of a guide wire under an

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Nitinol wire

193

Hydrophobic polymer

Hydrophilic coating Radiopaque coil

15.19 A hydrophobic type guide wire.

Nitinol wire

Radiopaque coil

Polyurethane

Hydrophilic coating

15.20 A fully hydrophilic-coated type guide wire.

endoscope. Compared with the second type of guide wire to be explained next, the hydrophilic portion of this guide wire is very short. As hydrophobic polymer coats most of the surface of the guide wire, the guide wire is not damaged much in terms of maneuverability when it becomes dry. Since PTFE is a very inactive material, the damage caused by contrast media and other drugs with high concentrations is extremely small. Fully hydrophilic-coated type: for seeking the target lesion The core wire is entirely covered with polyurethane, while its surface is coated by hydrophilic polymer (Fig. 15.20). It is basically used for approaching the targeted lesion in cases where this is an area of complex anatomy. After reaching the lesion, it is exchanged for a hydrophobic guide wire to guide other devices. This type of guide wire has high lubricity that makes it easy for it to approach the lesion. However, as time passes, the hydrophilic coating becomes dry, which results in it suffering a reduction in lubricity. The structures of the core wires listed above are guide wires that are made of Ni–Ti alloys. There is a marker on the tip that is visible under an X-ray. The marker of the core wire is made of a visible metal coil or a polyurethane-contained visible metal. It is useful for physicians to confirm the position and movement of a guide wire under the guidance of an X-ray.

15.7.4 Recent requirements of endoscopic guide wires Since its advent, endoscopy has been used mainly for diagnosis. However, the therapeutic use of endoscopy has become more popular and is starting to increase. Recently, therapeutic devices such as a catheter, stone basket, balloon catheter and stent have been developed and improved.

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The requisite characteristics for guide wires to be able to advance smoothly and break through a stricture are as follows: • •

greater lubricity greater rigidity.

The balance between the two characteristics is very important. It is a kind of trade-off problem because a rise in rigidity results in a reduction in lubricity, whilst a rise in lubricity results in a reduction in rigidity.

15.7.5 New concept of endoscopic guide wire for development The application of a rugged surface is a possible solution for high rigidity with high lubricity. The hydrophobic type is chosen for the basic structure of the new guide wire in order to meet the demands of the age of therapeutic endoscopy. An Ni–Ti core wire has a PTFE-coated shaft with a distal tip coated with hydrophilic polymer. In general there are four approaches to improving its lubricity. These include: the selection of the material, the creation of a smooth surface, the chemical modification of a surface and the reduction of contact with the surface. PTFE is one of the most lubricious materials among the polymers used for medical devices. It has one of the smoothest surfaces and has very inactive properties. We changed striped patterns from wide pitches to narrow pitches. The wide-striped patterns (Fig. 15.21) were changed into narrow patterns of PTFE with each ridge colored (Fig. 15.22). As can be seen in Fig. 15.23, the narrowed pitches reduced insertion resistance because there was less area in contact compared with that of the wide pitches. The era of therapy using endoscopy has just begun. New devices will be introduced each month in the near future. In consequence, there will also be greater demands on endoscopic guide wires, such as the

Device or blood vessels

Wire contacted part

Surface of a guide wire

15.21 Striped patterns of guide wire surface with wide pitch.

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Device or blood vessels

Wire contacted part

Surface of a guide wire

15.22 Striped patterns of guide wire surface with narrow pitch.

Insertion resistance 1.2 1.0 0.8 0.6 0.4 0.2 0 (N)

Narrow pitch

Wide pitch

15.23 Comparison of the insertion resistance of wide pitch and narrow pitch guide wires.

need to have smaller diameters, higher rigidity and greater lubricity. Continuous efforts to develop the endoscopic guide wire will be needed for the therapeutic age of endoscopy.

15.8

Device for onychocryptosis correction

An ingrown toenail is a nail disease in which the edge of a nail cuts into the adjacent skin fold, and is one of the most common foot maladies. Some of the symptoms include: pain along the margins of the nail, difficulty in walking, etc. Signs of infection include redness and swelling of the area around the nail. While in-growth can occur in both the nails of the hand and of the feet, it occurs most commonly in toenails. Some of the possible causes include: wearing narrow-toed shoes, imcorrect trimming of the nails, trauma or excessive external pressure to the nail plate or toe. Although

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15.24 Ingrown nail correction device.

there are various non-surgical therapeutic modalities such as taping, gutter treatment and superelastic wire, as well as surgical ones that include nail removal with phenol cauterization of the nail matrix, they sometimes lead to a relapse or severe nail damage. The ingrown nail correction device is a clip-type that is made of Cu–Al– Mn SMA, which is a suitable material because it possesses the good coldworkability characteristic necessary for forming the clip-on device. The non-invasive method using the device is very simple; clamping the tip of an ingrown nail with this device generates the continuous recovery force of superelasticity and then achieves the straightening of the curved nail, as shown in Fig. 15.24. Patients can probably apply or detach it easily by themselves at home.

15.9

References

1) Heineman Modern Dictionary for Dental Students (2nd edn), William Heinemann Medical Books Ltd, 1973. 2) Dotter CT: Transluminally-placed coilspring endarterial tube grafts: long-term patency in canine popliteal artery. Invest Radiol 4: 329–332, 1969. 3) Parodi JC, Palmaz JC, Barone HD: Transfemoral intraluminal graft implantation for abdominal aortic aneurysms. Ann Vasc Surg 5: 491–499, 1991. 4) Nashihara H: Development of self-expandable stent, Annual design Revi Japanese Soc Sci Design 11(11): 52–57, 2006.

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Appendix: History of the Association of Shape Memory Alloys K. SHIMIZU, Osaka University, Japan

The Association of Shape Memory Alloys, the editorial committee responsible for this book, was established in 1993 in order to promote further research and development in the areas of fundamentals and applications of shape memory alloys (SMAs). In this appendix, the circumstances of the establishment, as well as the recent activities, of the association will be introduced in brief. As is well known, in 1963, Dr Buehler’s group at the US Naval Ordinance Laboratory recorded the appearance of a unique phenomenon of the shape memory effect (SME) in a familiar Ti–Ni alloy, although a similar phenomenon had already been observed in unfamiliar Au–Cd and In–Tl alloys in 1951 and 1954, respectively. Because of its remarkable uniqueness, the SME was immediately investigated to discover potential applications in the manufacture of the machine parts of industrial products and even in living essentials, mainly in the United States and the Netherlands. However, the results of this investigation could not be developed for practical use. After a while, in about 1970, the Raychem Corporation in the United States developed a Cryofit coupling and an electrical pin-and-socket contact made of the Ti–Ni SMA and offered them for sale in large quantities; the former coupling was typically applied in the production of the aircraft hydraulic tubing of the US F-14 fighter plane. On the other hand, at about the same time, the fundamental mechanism of the SME was investigated and clarified in relation to a thermoelastic martensitic transformation in ordered alloys. During the investigation, researchers discovered another unique phenomenon, super elasticity (SE), which was closely related to the SME. Since then, the SME and SE phenomena have been found not only in Ti–Ni alloys but also in many noble metal base and other alloys. In 1975, the first international symposium on SMAs was held at Toronto, Canada, and many academic and technological researchers participated from all over the world. By 1980, the US patented right for the processing and technological applications of Ti–Ni SMAs had lapsed, and extensive research had developed globally concerning the fundamentals and applications of Ti–Ni base, noble metal base and other SMAs. In the circumstances mentioned above, two organizations were established in Japan to promote the development research into the fundamentals 197 © Woodhead Publishing Limited, 2011

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and applications of SMAs; one (a semi-governmental organization) was a ‘Committee for Promotion of Applications of SMAs’ formed in Osaka Science and Technology Center in 1982, and the other (a private organization) was a ‘Research Cooperative Union for Processing of SMAs’ formed by six manufacturing companies of SMAs in 1983. The former organization, ‘Committee for Promotion of Applications of SMAs’, consisted of members from typical universities, governmental research institutes and manufacturing companies, and had conducted extensive investigations into the existing circumstances of the development research for the processing and applications of SMAs and of the test characterization methods for those SMAs. The organization had further examined the subjects and systems required to promote this research and development. As a result, the first work of the organization was the standardization of the terminology relating to SMAs used in academic and technological fields and of the methods of measuring martensitic transformation temperatures. After a little while, this organization was succeeded by a ‘Committee for SMAs’, which was a subdivision of the Committee for Investigations on Standardization of Test Characterization Methods on New Materials used as Electric Power Source instead of Petroleum, which was specially established in the Osaka Science and Technology Center with governmental support as one of the national projects intended to activate research into the various kinds of functional materials that were newly developed about that time. The investigation work carried out in the ‘Committee for SMAs’ effectively took over from that of the previous ‘Committee for Promotion of Applications of SMAs’. Thus, six items relating to the standardization of terminologies and test characterization methods of SMAs were established via careful discussion in the Japan Industrial Standards (JIS) Committee, which were JIS H7001, JIS 7101, JIS H7103, JIS H7104, JIS H7105 and JIS H7106; the first two were based on the investigations of the previous committee. After the establishment of the six JISs, the ‘Committee for SMAs’ was closed in 1992. The latter organization, the ‘Research Cooperative Union for Processing of SMAs’, was formed by six manufacturing companies and it was fortunately able to obtain a governmental grant-in-aid to carry out improvement work on processing technologies for SMAs under the national support system for the development of industrial technology research. Three of the six manufacturing companies were those which produced Ti–Ni SMAs. In alphabetical order, these were: Daido Special Steels, Furukawa Electric Industry and Tohoku Kinzoku (now NEC TOKIN). The other three companies were manufacturers of Cu–base SMAs: Dowa Mining, Mitsubishi Metals and Sumitomo Special Metals. All the companies had investigated individually and/or cooperatively with governmental

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support for various topics relating to SMAs, such as the minimization of inclusions, control of martensitic transformation temperatures, improvement of SMA and SE characteristics due to the addition of a third and/or forth element and due to thermomechanical treatments, and so on. Thus, those companies could be successful in producing various products (lines, pipes, plates, thin foils and coil springs) from SMAs with better SME and/ or SE characteristics, and the products were supplied for application in the manufacture of various industrial machine parts and medical interposition devices in the human body. The switch of a dry box, the sensor flap of an air conditioner, a medical stent and many others, as have been introduced in this book. This private organization had been acting in close cooperation with the former semi-governmental organization, contributing to the establishment of the above-mentioned six JISs for the SMAs. It was closed in 1993, having obtained the expected results to some extent although, with very few exceptions, noble metal-based SMAs could not be supplied for practical uses because of their being to some degree unsuitable for such applications. The above two organizations were closed as mentioned, but some members of those organizations had promptly advocated the establishment of another new organization. Ti–Ni SMAs had attracted an increasing amount of attention, not only among professional workers but also among ordinary people worldwide, and research and development into the fundamentals and applications of SMAs were required to promote them more extensively and strongly than before. Thus, the Association of Shape Memory Alloys (ASMA) was established in October 1993, as mentioned at the beginning. At the start, the ASMA was constituted of several individual members and of six supporting members from industrial companies. These were, in alphabetical order: Daido Special Steels, Furukawa Electric Industry, Kato Spring (now Piolax), Mitsubishi Material, NEC TOKIN and Sogo Spring. The ASMA has expanded little by little, and it now consists of 39 individual members and of nine supporting members from industrial companies. The individual members join the association voluntarily from universities and other research institutes, and the supporting members typically come from nine companies in the manufacturing industries, coil spring makers and those working with applications of Ti–Ni and ferrous SMAs. The ASMA is operated under a board of trustees and a general meeting, the former being constituted of one president, one secretary-general, eight trustees, one inspector and one counselor. The objective of the ASMA is to promote progress and development in the science and technology of SMAs and to contribute to the development of related industries. In order to achieve this objective, the following projects have been enforced:

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• • •

investigation and research on SMAs and their trusts; holding of lectures and research meetings on SMAs; publication of journals and other documents on the activities of the ASMA; • connection and cooperation with other related academic and technological organizations in domestic and foreign countries; • other necessary projects in order to achieve the objective of the ASMA. These projects, as well as the annual budget for them, are planned and implemented via discussion among the board of trustees and the general meeting. After its establishment in 1993, the ASMA has actively carried out various projects, such as the holding of short courses and symposiums on SMAs every year, the publication of a newsletter and of summary booklets of patent reports and new models for practical uses, financial support to several domestic and international meetings on SMAs, the holding of the Japan–China Bilateral Symposium on SMAs (1997), the organization of the International Conference on Shape Memory and Superelastic Technology (2007) and many others. The ASMA has also produced a standardization work on the Ti–Ni SMA wire itself, JIS H7107, in addition to some corrections and supplements to the other six JIS previously established. As has been mentioned above, the ASMA is now continuing its activity steadily, although some projects have been inevitably reduced due to the economic depression, and has greatly contributed to the progress and development of the science and technology of SMAs and to the development of SMAs industries in Japan and also throughout the world.

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Index

accessories, 173–4 actuator applications shape memory alloy coil springs design, 63–9 SMA actuators design, 68–9 SMA spring design, 63–8 SMA spring manufacturing process, 68–76 aerospace engineering, 125–39 CryoFit, 126–8 Frangibolt, 128–30 LFSA on EO-1 hinge and deployment system, 134–7 Pinpuller, 130–1 rotating arm MAE in Mars Pathfinder mission (NASA), 137–9 variable geometry chevrons, 131–4 alloying elements, 44–5 effect on titanium-niobium (Ti-Nb) based alloys, 35–8 anti-ferromagnetic γ phase, 144 artificial flower, 98–9 Association of Shape Memory Alloys (ASMA), 197–200 automatic desiccators, 83, 87, 88 automobiles, 120–1 bathtub adaptors, 103–9 boiler with SMA hot water safety valve, 104 cut-off valve for hot water circulation, 108 cut-off valve operating principle, 109 hot water safety valves in outlet of bath, 106 hot water safety valves operating principle, 105, 107 biliary stents, 183–7 bras, 83, 170–1 ‘C’-shaped SMA washer, 117 calcia, 56

camera, 84, 96 Clausius–Clapeyron equation, 10, 24 clothing, sports and leisure industries superelastic alloys applications, 169–75 accessories, 173–4 bras, 170–1 comfortable apron, 173, 174 eyeglass frames, 169–70 petticoat, 173 shoes, 171 shoulder pads of jacket, 172–3 coffee maker, 89–90 cold wire drawing, 59–60 comfortable apron, 173, 174 Committee for Promotion of Application of SMAs, 198 Committee for SMAs, 198 composition control furnace, 43–4 distribution of transformation temperature in the alloy, 45 equipment for adding alloying elements, 45 example, 44 copper-based alloys, 48–9, 81–2 fabrication of pipe by press moulding, 49 appearance of press moulding pipe of Cu–26.0Zn–4.1Al alloy, 50 prospect for practical application, 48–9 temperature dependence of recovery stress in Cu–6Al–1.6Si–16Zn wire, 50 thermal cycle properties, 49 CryoFit, 125, 126–8 advantages, 128 coupling image, 126 mechanism, 126–8 schematic diagram of mechanism, 127 crystallographic theory, 19 curved boring method, 155–6 schematic illustration, 156 cut-off valve, 108 operating principle, 109 cyclic thermomechanical training, 147

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Index

Daido Special Steels, 198, 199 deformation behaviour, 22–6 differential scanning calorimetry, 6–7, 44, 58 diffusional transformation, 6 diffusionless transformation, 6 Dowa Mining, 198 easy-release screw, 116–19 ‘C’-shaped SMA washer, 117 charger in sharp cordless phone, 118 disassembling mechanism, 117 process of the automatic disassembling system, 118 sharp cordless phone, 118 electrical appliances, 87–99 automatic desiccators, 87, 88 electrical current actuator, 94–9 artificial flower, 98–9 camera, 96 lock switch actuator for camera, 96 miniature robot, 95 minidisk player, 96–7 soft-boiled egg cooker, 97–8 solar paddle actuator, 98 toy, 98, 99 products utilising SMAs, 88–94, 91–2, 93 coffee maker, 89–90 detector for overheat of transformer, 91–2, 93 gynaecological examination equipment, 94 hot water ejector of toilet, 90–1 louver of air conditioner, 88–9 rice cooker, 91 ventilating damper for sterilising tableware storage box, 92, 93, 94 water purifier, 91, 92 electrical current actuator, 94–9 SMA joule heating under a fixed load, 95 endoscopic guide wire, 191–5 classification, 192–3 fully hydrophilic-coated type, 193 hydrophobic type, 192–3 difference from angiographic guide wire, 191–2 new concept for development, 194–5 insertion resistance of wide and narrow pitch guide wire, 195 striped patterns of surface with narrow pitch, 195 striped patterns of surface with wide pitch, 194 purpose and function, 191

recent requirements, 193–4 eyeglass frames, 82–3, 169–70 fatigue life, 27–9 heat treatment on fatigue life, 29 Fe–25% Pt alloy, 141–2 Fe–Mn–Si alloy see iron–manganese–silicon shape memory alloys Fe–Ni–C alloy, 142 ferrous based shape memory alloys, 82, 141–58 future trends, 156–8 iron–manganese–silicon shape memory alloys, 142–5 applications, 153–6 generation of shape memory effect by stress-induced martensite, 143 mechanical properties, 146–9 proper process for shape memory effect, 149–53 fundamental scheme, 150 shape recovery rate of radius shrinkage, 150 shape recovery stress on heating and cooling cycle, 151 stress–strain curve OsabYEO and shape recovery stress, 153 shape memory effect of iron–manganese– silicon, 145–6 fishing superelastic alloys applications, 165–7 fishing balancer, 166, 167 fishing line, 166 fishing rod, 167 fishing rods and other items for fishing, 166 flexural rigidity, 181 Frangibolt, 128–30 advantages, 130 compression equipment FBT-CT2, 129 mechanism, 128–30 schematic diagram of mechanism, 129 Furukawa Electric Co. Ltd, 81, 198, 199 gas flow shielding device, 103, 104 operating principle, 104 with SMA disk, 103 Gibbs free energy, 12 ‘good flexibility,’ 186 graphite, 56 guide wire, 84, 179–83 gynaecological examination equipment, 94

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Index hallux valgus, 176–7 helical springs design, 66–8 characteristic parameters of a spring, 66 conditions, 67 procedure, 67–8 sign to used for the calculation, 66 Hooke’s law, 63 hot drawing, 46–8 dies in hot-drawing machine, 47 machine, 47 hot water ejector of toilet, 90–1 hot water safety valves, 104 operating principle, 105, 107 in outlet of bath, 106 hot water supplies shape memory alloys applications, 100–9 bathtub adaptors, 103–9 gas flow shielding device, 103 shower faucet with water temperature regulator, 100–2 industrial processing forming and shape memory treatment, 60–2 continuous furnace for shape memory heat treatment, 60 shape memory treatment temperature effect on TiNi shear modulus, 61 melting process, 54–8 composition, 54–5, 56 ingot composition control, 58 melting techniques, 55–8 relation between TiNi composition and transformation temperature, 55 TiNi100–xXx alloy martensite start temperature, 56 vacuum arc remelting, 57 vacuum induction melting, 57 titanium–nickel shape memory alloys, 53–62 fabrication process, 54 impurity concentration, 58 tensile strength, 59 working process, 58–60 cold wire drawing, 59–60 hot processing, 58–9 Ti–55 wt%Ni wire stress-strain diagram, 60 iron–manganese–silicon shape memory alloys, 141, 142–5 applications, 153–6, 157 connection of rails with SMA fishplates, 154

203

fishplates for crane rail, 153–5 pipe coupling with SMA joint, 157 pipe joints for steel pipes, 155–6 SMA fishplates for crane rails, 155 fundamental properties, 146 future trends, 156–8 mechanical properties, 146–9 critical transformation stress and critical yield stress, 148 stress–strain curve, 147 shape memory effect, 145–6 Japan Industrial Standards Committee, 198 Kato Spring see Piolax lightweight flexible solar array on EO-1 hinge and deployment system, 134–7 advantages, 137 engineering model, 135 EO-1 with LFSA mounted on, 135 mechanism, 134–6 schematic assembly diagram, 136 SMA deployment hinge, 136 louver of air conditioner, 83, 88–9 MAE see material adherence experiment magnetic shape memory alloys, 82 Mars Global Surveyor spacecraft, 131 Mars Pathfinder mission, 137–9 martensitic transformation, 5–9, 142–3 crystallography of titanium-nickel (Ti-Ni) based alloys, 17–19 crystal structures of the parent and martensite phases, 18 crystal structures of the R-phase formed by elongation, 18 DSC curves showing multi-step transformation, 9 DSC measurement, 7 illustration of atomic arrangements, 8 thermodynamics, 12–13 unit cell shape change on martensitic transformation, 6 material adherence experiment, 137–9 medical applications, 176–96 biliary stents, 183–7 biliary stent characteristic, 185–7 category of the stent, 183–4 expanding stent from delivery system, 184 inner lumen at curved state, 186

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Index

stent design and components, 184–5 stent mounted into delivery system, 185 device for onoychocryptosis correction, 195–6 ingrown nail correction device, 196 endoscopic guide wire, 191–5 classification, 192–3 difference from angiographic guide wire, 191–2 endoscopic channel schematic, 192 fully hydrophilic-coated type, 193 hydrophobic type, 193 new concept for development, 194–5 purpose and function, 191 recent requirements, 193–4 guide wire, 179–83 blood vessel model for selecting materials of core wire, 181 characteristics demanded of core wire, 180–2 heat treatment temperature dependence of residual strain, 183 hydrophilic coated guide wire, 180 hydrophilic micro-guide wire, 180 SE alloy application, 179–80 spring coil guide wire using stainless steel, 179 hallux valgus, 176–7 shape memory splint and its application, 177 hepatic arterial infusion chemotherapy illustration of implanted catheter, 189 sliding resistance load of implanted catheter, 190 tip of implanted catheter, 188 orthodontic wire, 178–9 superelastic Ni–Ti orthodontic wire, 179 U-shaped conventional metallic orthodontic wire, 178 regional chemotherapy catheter, 187–91 elements and materials requested of catheters, 188 implanted catheter structures and characteristics, 188–91 medical tubule for transcatheter arterial infusion, 187 placement of implanted catheter with arterial redistribution, 191 SMA coil, 189 SMA coil inserted guide wire, 190 miniature robot, 95 minidisk player, 96–7

Mitsubishi Material, 199 Mitsubishi Metals, 198 NEC TOKIN, 198, 199 Ni-free shape memory alloys, 82 Ni hypersensitivity, 16 nickel, 54, 55, 58, 61 nickel–titanium shape memory alloys, 125 Nitinol, 125 60-Nitinol, 133 oil controller in Shinkansen, 84, 121–2 automatic oil valve adjusting device, 122 load deflection curves, 121 onoychocryptosis, device for correction, 195–6 orthodontic wire, 178–9 paint gun for car bodies, 168 petticoat, 173 phase diagram, 16–17 equilibrium phase diagram, 17 phase transformation see martensitic transformation Pinpuller, 130–1 advantages, 131 mechanism, 130–1 schematic of mechanism, 131 Piolax, 199 precision casting, 46 casting of pipe of Ni–Ti alloy using graphite mould, 46 schematic of precision casting machine, 46 press moulding, 49 radiator fan, 120 railways, 121–4 Raychem Corporation, 125, 197 recrystallisation, 61 regional chemotherapy catheter, 187–91 Research Cooperative Union for Processing of SMAs, 198 resistance to deformation, 181–2 rice cooker, 83, 91 rotating arm MAE in Mars Pathfinder mission, 137–9 advantages, 138–9 mechanism, 137–8 mechanism of rotating arm, 139 Sojourner with MAE set up, 138 self-accommodation, 19 shape memory alloy actuator, 68–9, 111

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Index shape memory alloy springs, 63–8, 69–76, 103–9 design, 63–8 characteristic parameters of a spring, 66 classification, 65 conditions, 67 difference from usual springs, 63–4 helical springs, 66–8 load-temperature curves, 65 materials and shear modulus selection, 64–6 point of SMA spring design, 68 procedure, 67–8 shear strain γL at low temperature, 68 sign to used for the calculation, 66 specification, 68 spring index C, 68 heat treatment temperature effect on transformation temperature and loads Ti–50.6 at.% Ni, γ = 0.8%, 71 Ti–50.6 at.% Ni, γ = 1.0%, 73 Ti–41.0 at.% Ni–Cu, γ = 0.8%, 74 Ti–49.6 at.% Ni–Fe, γ = 0.8%, 74 load–temperature curve Ti–50.6 at.% Ni spring (γ = 0.8%) heat treated at 470 °C, 72 Ti–50.6 at.% Ni spring (γ = 0.8%) heat treated at 500 °C, 72 Ti–50.6 at.% Ni spring (γ = 1.0%) heat treated at 470 °C, 73 Ti–41.0 at.% Ni–Cu spring (γ = 0.8%) heat treated at 440 °C, 75 Ti–49.6 at.% Ni–Fe spring (γ = 0.8%) heat treated at 550 °C, 75 manufacturing process, 69–76 coiling, 69–70 shape memory treatment (heat treatment), 70–6 two-way SMA actuator with bias spring, 69 load–deflection diagram, 70 shape memory alloys, 53, 77 aerospace engineering, 125–39 CryoFit, 126–8 Frangibolt, 128–30 LFSA on EO-1 hinge and deployment system, 134–7 Pinpuller, 130–1 rotating arm MAE in Mars Pathfinder mission (NASA), 137–9 variable geometry chevrons, 131–4 applications development, 77–82

205

Cu-based SMAs, 81–2 Fe-based SMAs, 82 magnetic SMAs, 82 Ni-free SMAs, 82 Ti-Ni-based high temperature SMAs, 82 titanium–nickel based alloys, 79–81 applications history in Japan, 78 automobiles and railways, 120–4 automobiles, 120–1 oil controller in Shinkansen, 121–2 steam trap, 122–3 basic characteristics of Ti-Ni-based and Ti-Nb-based alloys, 15–41 coil springs design for actuator applications, 63–76 design of SMA actuators, 68–9 design of SMA springs, 63–8 manufacturing of SMA springs, 69–76 construction and housing, 110–19 easy-release screw, 116–19 static rock breaker, 112–16 underground ventilator, 111–12, 113 electrical appliances applications, 87–99 automatic desiccators, 87–8 electrical current actuator, 94–9 products utilising SMAs, 88–94 fundamentals, 5–12 martensitic transformation, 5–9 SME, 9–10 superelasticity, 10–12 history of Association of Shape Memory Alloys, 197–200 hot water supplies applications, 100–9 bathtub adaptors, 103–9 gas flow shielding device, 103, 104 shower faucet with water temperature regulator, 100–2 main applications of Ti-Ni-based alloys, 82–4 mechanisms and properties of shape memory effect and superelasticity, 3–14 atomic arrangements during martensitic transformation, 8 change in unit cell shape on martensitic transformation, 6 DSC curves showing multi-step martensitic transformation, 9 DSC measurement of martensitic transformation temperatures, 7 mechanism of superelasticity, 11 mechanism of the SME, 9 stress–strain curves, 4

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Index

temperature dependence of transformation stress, 11 tensile stress–strain curves of Ti–50.6Ni, 5 unit cell shape change on martensitic transformation, 6 properties, 4–5 thermodynamics of martensitic transformation, 12–13 titanium nickel and copper-based, development and commercialisation, 43–52 objective and status of achievement for R & D: Cu based alloy, 52 objective and status of achievement for R & D: Ni-Ti based alloy, 51 research on Cu–Ni-based SME, 48–9 research on Ti–Ni-based SME, 43–8 shape memory effect, 9–10, 63 mechanism, 9 titanium–niobium (Ti–Nb) based alloys, 33–5 shape recovery stress, 152 shear modulus, 63, 64–6 martensite phase, 65–6 parent phase, 65–6 Shinkansen, 121–2 shoes, 171 shoulder pads of jacket, 172–3 shower faucet with water temperature regulator, 100–2 load–deflection diagram, 102 regulator with SMA spring and a bias spring, 101 regulator with SMA spring and a bias spring schematic, 101 SMA see shape memory alloys SME see shape memory effect soft-boiled egg cooker, 97–8 Sogo Spring, 199 ‘Sojourner,’ 137, 138 solar paddle actuator, 98 static rock breaker, 84, 112–16 load–displacement curve of SMA cylinder, 115 performance comparison between cement and SMA, 117 procedures of splitting a rock using, 116 recovery force–displacement curve of SMA cylinder, 115 rock breaking, 114 schematic, 116 SMA cylinder schematic motion, 115 with SMA cylinders, 114

steam, 122–3 steam trap, 122–3 structure, 123 temperature change, 123 stents see biliary stents straightness, 181 stress cycling effect, 26–7 cycling deformation effect on stress–strain curves with various heat treatment, 28 Sumitomo Special Metals, 198 superelastic alloys, 77 applications development, 77–82 applications in clothing, sports and leisure industries, 169–75 accessories, 173–4 bras, 170–1 comfortable apron, 173, 174 eyeglass frames, 169–70 hat, 172 petticoat, 173 shoes, 171 shoulder pads of jacket, 172–3 telecommunications, sports, leisure and other applications, 163–8 antennas for other applications, 164, 165 cell-phone antenna, 163–4 fishing, 165–7 headbands of headphones, 164, 165 paint gun for car bodies, 168 radio controlled car antenna, 165 superelastic nickel–titanium alloys medical applications, 176–96 biliary stents, 183–7 device for onoychocryptosis correction, 195–6 endoscopic guide wire, 191–5 guide wire, 179–83 hallux valgus, 176–7 orthodontic wire, 178–9 regional chemotherapy catheter, 187–91 superelasticity, 10–12, 83, 197 mechanism, 11 temperature dependence of transformation stress, 11 titanium–niobium (Ti–Nb) based alloys, 33–5 switch valve, 120 TAI see transcatheter arterial infusion telecommunications industry superelastic alloys applications, 163–8

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Index antennas for other applications, 164 cell-phone antenna, 163–4 headbands of headphones, 164–5 Terumo Corporation, 81 thermal activation process, 143 thermodynamic potential, 143 thermodynamics, 12–13 thermoelastic martensitic transformation, 15 thermoelastic shape memory alloys, 145 Tinel, 125 Ti–Ni-based high temperature shape memory alloys, 82 TiNi Pinpuller, 130, 131 Ti–Ni superelastic wire, 166 titanium–nickel (Ti–Ni) based alloys, 16–29, 43–8 alloying elements, 44–5 applications history, 79–81 first stage (1981), 79 before the first stage (1964 ), 79 fourth stage (2001), 81 second stage (1986), 79–80 third stage (1992), 80 crystallography of martensitic transformation, 17–19 crystal structures of the parent and martensite phases, 18 crystal structures of the R-phase formed by elongation, 18 deformation behaviour, 22–6 critical stresses for inducing martensitic transformation, 25 schematic phase diagram in a temperature–stress co-ordinate, 27 schematic stress–strain curves at various temperatures, 23 schematic typical stress–strain curves at specific temperatures, 26 fabrication of thin sheet using continuous casting and rolling method, 48 thin sheet from continuous casting and rolling method, 48 fabrication process, 54 fatigue life, 27–9 heat treatment on fatigue life, 29 hot drawing, 46–8 dies in hot-drawing machine, 47 machine, 47 industrial processing, 53–62 forming and shape memory treatment, 60–2 melting process, 54–8 working process, 58–60

207

main applications, 82–4 automatic desiccators, 83 bras, 83 camera, 84 eyeglass frames (rim), 82–3 guide wire, 84 louver of air conditioner, 83 oil controller in Shinkansen, 84 rice cooker, 83 static rock breaker, 84 toys, 83 phase diagram, 16–17 equilibrium phase diagram, 17 precision casting, 46 casting of pipe of Ni-Ti alloy using graphite mould, 46 schematic of precision casting machine, 46 in situ composition control furnace, 43–4 distribution of transformation temperature in the alloy, 45 equipment for adding alloying elements, 45 example, 44 stress cycling effect, 26–7 cycling deformation effect on stress– strain curves with various heat treatment, 28 transformation strain, 19–22 orientation dependence of calculated strain induced by R-phase transformation, 21, 22 transformation temperature, 22 Ni content dependence of M8 temperature, 23 titanium–niobium (Ti–Nb) based alloys, 29–40 crystallography and transformation temperature, 29–33 content dependence of the transformation strain, 32 lattice correspondence among β, α″, α, 30 Nb content dependence of Ms for Ti-Nb binary alloys, 32 Nb dependence of b′/a′ and c′/a′ for the α″-orthorhombic martensite phase, 31 orientation dependence during the martensite transformation from the β to α″ in the Ti–22Nb alloy, 31 effect of alloying element, 35–8 stress-strain curves from strain

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Index

increment cyclic loading and unloading tensile tests, 37 Zr and Ta addition on the martensitic transformation temperature Ms, 36 Zr and Ta addition on the transformation strain, 37 effect of heat treatment condition, 38–40 stress–strain curves obtained by cyclic loading–unloading tensile tests, 40 stress–strain curves of loading and unloading at various temperature, 39 shape memory and superelastic properties, 33–5 inverse pole figure for rolling direction of specimen solution treated at 1173K for 1.8ks, 35 stress–strain curves at loading and unloading at various temperatures, 33 stress–strain curves for strain increment cyclic loading and unloading tensile tests, 34 Tohoku Kinzoku see NEC TOKIN toughness of wire, 182 toys, 83, 98, 99 training treatment, 147 transcatheter arterial infusion, 187

transformation strain, 19–22 transmission oil control, 120 underground ventilator, 111–12, 113 motion by changing environmental temperature, 112 schematic mechanism, 113 SMA coil with bias spring, 111 vacuum arc remelting, 56–8 schematic view, 57 vacuum induction melting, 56–8 schematic view, 57 VAR see vacuum arc remelting variable geometry chevrons, 131–4 advantages, 134 chevrons, 132 components, 133 design concept, 133 mechanism, 132–4 ventilating damper for sterilising tableware storage box, 92, 93, 94 VGC see variable geometry chevrons VIM see vacuum induction melting Wacoal Corp., 79 water purifier, 91, 92 water temperature regulator, 100–2

© Woodhead Publishing Limited, 2011