Shape Memory Alloys or SMAs, (sometimes also referred to as memory metals) can reverse low-temperature plastic or irreversible deformations by heating. In this process, the original state of deformation is returned by subsequent cooling (figures 1 and 2).
The shape memory effect is based on a thermoelastic martensitic transformation while the crystal structure is transformed depending on the temperature. In general, two crystal structures are distinguished: Austenite (high-temperature structure) and martensite (low-temperature structure). Both show an arranged lattice structure.
In case of the one-way effect, the material that was deformed at a low temperature recovers its original shape if it is heated to a higher temperature. The transformation starts at the austenite start temperature As that can be defined by a respective alloy composition. Additionally, shape memory alloys may also offer a two-way effect (they can remember two shapes, one at a high temperature and the other at a low temperature) that can be set by special thermomechanical material treatment processes. Finally, the transformation of the austenite to the martensitestructure may also be forced by shear stress. This effect is referred to as pseudoelasticity and acts rubbery and is, for example, an ideal property for spectacle frames.
Technically interesting for this are nickeltitanium alloys that form a martensitic lattice structure with relatively easy deformability in “cold” condition, austenitic structure with a highly-strength lattice structure in “hot” condition and that can be classified as temperature-dependent, allotropic modifications. Figure 3 illustrates the potential deformation of a NiTi alloy paper clip in an idealized raster image.