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Phase transformations in shape memory alloys

Shape memory alloys (SMA)are one example of new materials, which provide also new properties. Shape memory is the ability of special alloys to "remember" their original shape after having been exposed to a change in temperature or mechanical stress. This anomalous behavior is based on diffusion less phase transformation know as martensitic transformation. Martensitic transformation best known in steels heat treatment exists in many other alloys, even in some ceramic and polymeric materials. Shape memory effects (pseudo-elasticity, one-way effect or pseudo-plasticity and two-way effect) are related to solid, mostly crystalline alloys. There are several alloy types that show such effects: Ni-Ti-, Cu-Zn-, Fe-Ni- and Fe-Mn-. Reversible martensitic transformation which is the basic prerequisite for shape memory behaviour is characterised by fractal austenite-martensite microstructural change. For better shape memory and other exploitation properties SMAs have to be thermally and thermomechanically treated in a special way. There is a wide of SMA applications in technical and medical fields. Primarily Ni-Ti-alloys, but recently some other not so expensive alloy types have found technical and medical application as well.

martensitic transformation; shape memory effect; SMA

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