Shape Memory Alloy

A shape-memory alloy is a metal that can be bent or deformed while cold and then made to spring back to an earlier, remembered shape simply by heating it, a property that comes from a reversible change between two crystal structures: a lower-temperature martensite phase, which deforms easily by rearranging its crystal twins rather than breaking atomic bonds, and a higher-temperature austenite phase, which the metal returns to on heating, pulling the deformed shape back with it. Shape-memory behavior in various alloys was observed as early as the 1930s and 1940s, but the alloy that made the effect practically useful, a nickel-titanium compound later marketed as Nitinol, was developed at the United States Naval Ordnance Laboratory in 1962 and 1963; according to the standard account, its shape-memory property was discovered by accident when a researcher held a lighter under a bent sample during a meeting and watched it straighten back out as it warmed. Nitinol and related shape-memory alloys are now used in applications including dental braces, medical stents, aircraft components and seismic bracing for buildings.