Recent Developments in High Temperature Shape Memory Alloys
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JENO BEYER, JAN.H. MULDER
University of Twente, Department of Mechanical Engineering, Laboratory of Materials Science, P.O.Box 217, 7500 AE Enschede, The Netherlands. ABSTRACT
The functional properties of Shape Memory Alloys (SMA's) are used succesfully at present in a variety of industrial and medical applications. The use of these materials in smart structures is now emerging in the field of aeronautic/space technology. Many applications require higher operating temperatures than available to date, or higher cooling rates and/or a higher number of cycles. For this purpose the properties and fabricability of commercial alloys as Ni-Ti-(X), Cu-Al-Ni or Cu-Zn-Al are being adjusted and improved. Other feasible alloys are being developed. The research and development is directed towards the control of the stress, strain, temperature and time dependence of shape memory properties for a stable in-service behaviour. In this paper the various approaches taken up in recent years by academic and industrial laboratories for developing high temperature SMA's are reviewed. INTRODUCTION
Shape memory materials have attracted considerable attention in recent years as functional materials with commercial applications in various engineering products. An extensive overview of the European activities has been given by Van Humbeeck recently [1]. Among the practical SMA's available today Ti-Ni alloys are being used most extensively because of the excellent mechanical properties, corrosion resistance and biocompatibility. The latter is important for biomedical applications, in particular for implants [2,3]. In recent years smart materials have been identified in the USA as an important emergent materials technology area for space and aerospace and a variety of other engineering fields [4,7]. SMA's belong to this group of materials because of their ability to perform both sensing and actuating funtions. [6,7]. The operating temperatures of most SMA's are limited by their thermo-elastic martensite transformation temperatures (TI) to 400K. However, there is an increasing interest and need for high temperature SMA's in electronic engineering, aircraft and space industry, automobile industry, chemical industry, (oil and gas) exploration and geothermal industry. The major concerns in developing these new types of SMA's are high TIT of the thermoelastic martensite transformation; because these alloys tend to operate at high temperatures, the stability of the alloy over a long period of time is equally important. In many applications the SMA will have to operate during many cycles either by repeated cooling and heating or loading and unloading. Therefore the stability of the shape memory behaviour has to be considered too [8]. Additionally the high temperature SMA's offer the possibility of shorther cooling times resulting in faster response, as compared to conventional SMA's. Cu-based alloys, among the best understood shape memory materials (with TI' limited to 453K), are cost effective, but show a lack of stability at high temperatures, in
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