ReviewRecent developments in the research of shape memory alloys
Introduction
Shape memory alloys (SMA) attracted much attention in recent years, since they are smart (or intelligent) materials, as well as functional materials, which already exist. Since the unique shape memory effect (SME) and superelasticity (SE) realized in these alloys are caused by the martensitic (or displacive) transformation (MT) and its reverse transformation, we define the characteristic transformation temperatures as follows, as usual;
Then, SME and SE are characterized as follows, using the above terminology. SME is a phenomenon such that an apparent plastic strain given at a temperature below As recovers by heating to a temperature above Af, by virtue of the (crystallographically) reversible reverse transformation. SE, which is a pseudoelasticity occurring at a temperature above Af, is caused by a stress-induced martensitic transformation upon loading and by the subsequent reverse transformation upon unloading. The above crystallographic reversibility is characteristic of the thermoelastic martensitic transformation, which is characterized by a small temperature hysteresis (i.e. ΔT=Af−Ms) and mobile parent-martensite interfaces. Since most of SMAs are ordered alloys or intermetallics, they are related to the present journal in that respect. In the present overview, we picked up the subjects in nearly recent 10 years, and we assumed that the readers have some introductory knowledge on MT, SME and SE. For those who are not familiar with those fundamentals, please refer to Refs.1, 2, 3, 4for MT, and Refs.5, 6, 7, 8for SME and SE.
In the following, the subjects are divided into three Sections (2–4). In Section 2, recent developments in Ti–Ni based alloys are discussed, since Ti–Ni based alloys are most important SMAs and they are most actively investigated. Besides, they are distinct from other SMAs in various respects, such as in crystal structure of martensite, elastic properties, mechanical properties etc., as will be clear later. In Section 3, more general properties common to all SMAs are discussed, and the applications of SMAs are discussed in the final chapter.
Section snippets
Phase diagram of Ti–Ni alloy system
Fig. 1 is a recent phase diagram of Ti–Ni alloy system by Massalski et al.[9]Our interests are restricted in the central region bounded by Ti2Ni and TiNi3 phases, since the single phase “TiNi” (B2 type ordered phase) transforms into a monoclinic phase martensitically. The phase diagram in the above region have been controversial for many years. The main problems are the presence or absence of a eutectoid decomposition of TiNi→Ti2Ni+TiNi3 at 630°C[10], and the assessment on the nature of Ti3Ni4
Defect densities in intermetallics—are there any constitutional vacancies in intermetallics?
As mentioned earlier, most of SMAs are in fact intermetallic compounds (shape memory intermetallics) because they are ordered alloys and many of them remain ordered near to the melting temperature (e.g. Ti–Ni, Au–Cd, etc.). Therefore, point defects we discuss here include both vacancy and anti-site defect (ASD).
In comparison to the relatively recent knowledge about the role of point defects on mechanical properties of structural intermetallics for high temperature applications[69], the
Applications of shape memory alloys
Since SMAs have unique properties which ordinary metals do not have, they have high potentiality for many applications. In fact, more than 10,000 patents have been proposed in the past, and it is difficult to classify all the applications. Thus, in the following we introduce only successful applications utilizing shape memory effect and superelasticity, respectively.
Acknowledgements
The authors are grateful to Professor Tetsuro Suzuki for useful discussions on pretransformation phenomena and ductility of Ti–Ni alloys. They also appreciate the useful discussions with Mr. T. Tateishi at Terumo Corp. on medical applications. The present work was supported by Grant-in-Aid for Scientific Research on Priority, and partly supported by Project Research A from University of Tsukuba. Area on Phase Transformations (1997-9) from the Ministry of Education, Science and Culture of Japan.
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