Memory Metal - Demonstration

Austenite is the phase of NiTi stable at high temperature, while martensite is more stable at room temperature. Although austenite is the less dense phase, it is more rigid than martensite. At room temperature, the metal is in the martensite phase. When pressure is applied, the metal atoms easily slide by one another, and the shape changes. Heating the metal to about 50°C changes it to the rigid austenite phase. The dislocations between the crystalline grains of the metal (see topic "Simulation of Dislocations in Metals") will return to their original positions, and the metal will "remember" its original shape. Upon cooling, the metal returns to the martensite phase, but will retain its shape until pressure is applied.

To provide it with a new memory, the metal must be held in a new shape and heated to about 500°C (where there is enough energy to allow the defects to relax into new positions) and then slowly cooled. This is an annealing process, similar to the one demonstrated in the topic "Heat Treatment of a Metal Bobby Pin".