[Mechanism]
 Shape-memory and super-elastic alloys-the difference in their characteristics depends on the trmperature range at which they's used.
The characteristics of Ni-Ti alloy very greatly depending whether it is in the austenitic or martensitic phase(10 or 15℃). Shape memory and super-elastic characteristics are achieved by utilizing this temperature difference. Shape-recovery trmperatures (Af:the temperature at which Ni-Ti alloy recovers its original shape)can be changed by slightly varying the Ni content.
Why does it recover its original shape? Shape-momory effect
Ni-Ti alloy assumes a crystalline structure, called the austenitic phase, when heated to a temperature exceeding the shape-recovery temperature, and returns to the martensitic phase when cooled. Material an external phase can be easily deformed by applying an external stress load. Deformation causes transformation of the crystalline structure to the martensitic phase. Heating of the alloy in martensitic phase retuens it to the austenitic phase, and material recovers its original shape.
Why does it exhibit super-elasticity? Super-elasticity effect
Ni-Ti alloy is tranformed into stress-inducing martensite under application of external stress it is in the austenitic phase, for which the temperature range is higher than the shape-recovery temperature; immediately after of stress, the alloy returns to the auasticity, rather than plastic deformation.
 
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