Certain metallic alloys (and also some polymeric and ceramic materials) exhibit unusual behaviour when subjected to mechanical load and/or temperature change. This is due to shape changes being generated by Martensitic Phase Transformations, rather than by conventional elastic (bond stretching) or plastic dislocation glide (Introduction to dislocations TLP) deformation. Something very similar happens during Deformation Twinning. However, unlike the case of twinning, these phase transformations are reversible, at least under certain conditions, and hence the associated shape change can also be reversed. This can lead to interesting and useful effects, such as a capacity to cycle a component between two different macroscopic shapes by cycling the temperature. These alloys are commonly known as Shape Memory Alloys (SMAs).
The most common of these alloys is an equi-atomic alloy of Ni and Ti, known as ‘Nitinol’, although both Ni-rich and Ti-rich alloys are also used. Other examples include Cu-Zn or ternary alloys like Ni-Cu-Ti or Ni-Hf-Ti. They have been used for a variety of applications, including pipe couplings, earthquake dampeners, eyeglass frames, orthodontic wires, mobile phone antennas, micro-actuators, and a variety of biomedical devices.