superelasticity

Superelasticity, also called pseudoelasticity, is the ability of certain materials to sustain large strains that are fully recoverable when the load is removed, due to a stress-induced phase transformation. It occurs in shape memory alloys above their austenite finish temperature, Af, so the stable phase at ambient conditions is austenite.

The mechanism relies on a diffusionless, stress-induced transformation between austenite and martensite. Under applied stress, the austenite phase transforms to martensite, accommodating substantial deformation. When the load is removed, the martensite reverts to austenite and the material recovers its original shape. This yields a characteristic stress–strain response with a plateau during loading and a small hysteresis upon unloading, and it avoids permanent plastic deformation.

The best-known superelastic alloy is nickel-titanium (NiTi, Nitinol); other shape memory alloys, including some copper- and iron-based systems, can also exhibit superelastic behavior under appropriate temperatures. Recovered strains in NiTi commonly reach about 6–8%, with maxima around 10% in favorable conditions. The effect is strongly temperature dependent and influenced by composition, heat treatment, and cycling.

Properties include near-complete recoverability of large strains, absence of residual deformation after unloading, and high damping. The behavior is non-linear and temperature-sensitive, requiring operation within a suitable temperature window relative to Af.

Applications exploit the combination of large, recoverable deformation and biocompatibility or corrosion resistance. Notable uses include medical devices such as stents and guidewires, orthodontic wires, endovascular implants, as well as actuators, couplings, and seismic dampers. See also shape memory alloys and martensitic transformations.