Shape Memory

Short answer: Shape-memory alloys can be bent out of shape when cool and then spring back when warmed. The best known is nitinol, an alloy of roughly equal parts nickel and titanium.

Its name comes from Nickel, Titanium and the Naval Ordnance Laboratory, where William J. Buehler's team discovered it in 1959. According to ASME, the team was looking for materials for tools to dismantle mines. Wikipedia recounts that, at a 1961 meeting, someone heated a folded strip of it with a pipe lighter and it snapped back to its original shape. The effect comes from a reversible change in crystal structure. Below its transformation temperature, nitinol is in a monoclinic martensite phase that deforms easily and holds its new shape. Heated above that temperature, it converts to a cubic austenite phase and returns to its original form. The remembered shape isn't automatic. ASME says the part must be shape-set by clamping it in a mold, heat-treating it at about 500 °C and cooling it quickly. Composition and heat treatment adjust the transformation temperature. Above that temperature, nitinol is also superelastic, recovering from strains of about 8%. The shape-memory effect was first seen in a gold–cadmium alloy in 1932 by the Swedish chemist Arne Ölander. Shape-memory polymers became commercially available in the late 1990s. Nitinol is now used in stents, orthodontic wires and eyeglass frames. The footage shows a bent wire and a coil over a red heating element, straightening and contracting as they warm.

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