Austenite Phase. Temperature is above transition temperature. The transition temperature varies depending upon the exact composition of the nitinol alloy; commercial alloys usually have transitional temperatures between 70C to 130C (158 to 266F). The yield strength with which the material tries to return to its original shape is considerable; 35,000 to 70,000 psi. Crystalline structure is cubic.

Martensitic Phase. Low temperature phase. The crystal structure is needlelike and collected in small domains. Within the small domains the needle-like crystals are aligned. The alloy may be bent or formed easily. Deformation pressure 10,000 to 20,000 psi. Bending transforms the crystalline structure of the alloy producing an internal stress.

Annealing Phase. High temperature phase. The alloy will reorient its (cubic) crystalline structure to "remember" its present shape. The annealing phase for the nitinol wire we are working with is 540C.

When nitinol is at room temperature it is in the martensitic phase. When the alloy is bent, the needle-like crystalline structure within the domains deforms, creating internal stress. When the alloy is heated above its transitional temperature (Austenite phase), the crystalline structure changes from needle-like to cubic. The cubic structure of the alloy doesn't fit into the same space as the needle-like domain structures formed when the alloy was bent. The alloy relieves the stress by returning to its "remembered" crystalline cubic shape.

If the alloy hasn't any been deformed or stressed, the crystalline structure changes still occur, but it doesn't result in any net movement.