Shape memory materials are materials that have the ability to transition from a deformed shape to an original (programmed or parent) shape when an appropriate external stimulus is provided. Typically, shape memory materials are metallic alloys or polymers, although other materials may also exhibit such properties. Many shape memory materials undergo the transition as a result of a change in temperature; however, other shape memory materials undergo a state transition in the presence of a magnetic field (ferromagnetic shape memory alloys), light, electricity, or chemical stimuli.
Shape memory alloys (SMAs) can undergo a phase change between two phases that have different mechanical properties. The two phases are commonly referred to as the martensitic and austenitic phases based on the Martensite and Austenite phases of Nitinol, which is the most widely used shape memory alloy. Nitinol is an alloy of nickel and titanium that has the ability to return to a predetermined shape when heated. The martensitic state is typically ductile and deformable. In contrast, the austenitic state is typically rigid and has a higher elastic modulus.
An important characteristic of SMAs is the transformation temperature at which phase transition of the SMA from martensitic phase to austenitic phase occurs. If the SMA is heated above the transformation temperature, it converts to the austenitic phase, where it can be programmed or trained into a parent shape. After the parent shape is programmed, the SMA is cooled back into the martensitic phase, where it can be deformed as a ductile material. If the SMA is subsequently heated above the transformation temperature, it will convert back to the austenitic phase and revert to the parent shape. This property of reverting back to the parent shape regardless of whether the alloy was deformed at the lower temperature, is the reason for the name “shape memory”. The SMA, when not in the parent shape, is referred to as being in its non-actuated shape or passive shape.