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A shape-memory alloy (SMA, smart metal, memory metal, memory alloy, muscle wire, smart alloy) is an alloy that "remembers" its original shape and that when deformed returns to its pre-deformed shape when heated. This material is a lightweight, solid-state alternative to conventional actuators such as hydraulic, pneumatic, and motor-based systems. Shape-memory alloys have applications in industries including automotive, aerospace, biomedical and robotics. ==Overview== The two main types of shape-memory alloys are copper-aluminium-nickel, and nickel-titanium (NiTi) alloys but SMAs can also be created by alloying zinc, copper, gold and iron. Although iron-based and copper-based SMAs, such as Fe-Mn-Si, Cu-Zn-Al and Cu-Al-Ni, are commercially available and cheaper than NiTi, NiTi based SMAs are preferable for most applications due to their stability, practicability〔Hodgson DE, Wu MH, Biermann RJ. (1990) ''Shape memory alloys''. ASM Handbook: ASM International. pp. 897–902〕 and superior thermo-mechanic performance. SMAs can exist in two different phases, with three different crystal structures (i.e. twinned martensite, detwinned martensite and austenite) and six possible transformations. NiTi alloys change from austenite to martensite upon cooling; ''Mf'' is the temperature at which the transition to martensite completes upon cooling. Accordingly, during heating ''As'' and ''Af'' are the temperatures at which the transformation from martensite to austenite starts and finishes. Repeated use of the shape-memory effect may lead to a shift of the characteristic transformation temperatures (this effect is known as functional fatigue, as it is closely related with a change of microstructural and functional properties of the material).〔(Shape Memory Materials ), K Otsuka, CM Wayman, Cambridge University Press, 1999 ISBN 0-521-66384-9〕 The maximum temperature at which SMAs can no longer be stress induced is called ''Md'', where the SMAs are permanently deformed.〔Duerig TW, Pelton AR. (1994) "Ti-Ni shape memory alloys". in ''Materials Properties Handbook: Titanium Alloys'', Gerhard Welsch, Rodney Boyer, E. W. Collings (eds.) American Society for Metals. pp. 1035–48. ISBN 0871704811.〕 The transition from the martensite phase to the austenite phase is only dependent on temperature and stress, not time, as most phase changes are, as there is no diffusion involved. Similarly, the austenite structure receives its name from steel alloys of a similar structure. It is the reversible diffusionless transition between these two phases that results in special properties. While martensite can be formed from austenite by rapidly cooling carbon-steel, this process is not reversible, so steel does not have shape-memory properties. File:Sma wire.jpg In this figure, ξ(T) represents the martensite fraction. The difference between the heating transition and the cooling transition gives rise to hysteresis where some of the mechanical energy is lost in the process. The shape of the curve depends on the material properties of the shape-memory alloy, such as the alloying. and work hardening. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Shape-memory alloy」の詳細全文を読む スポンサード リンク
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