Superelasticity

The alloy is capable of deforming reversibly when subject to constraint.

This behaviour is also known as super-thermoelasticity or pseudo-elasticity. Shape memory alloys can bear a higher deformation than standards metal alloys such as steel. The principal is described below.

Stress is applied (red dotted line) on the alloy (at constant temperature) as shown on the state diagram (fig.1). Constraint-deformation diagram is then obtained (?,?) (fig.2).

SCHEMA

The curve on the figure 2 shows three areas :

  • I. From origin to ?Ms : austenite elasticity
  • II. From ?Ms to ?Mf : deformation due to martensitic transformation
  • III. Above ?Mf : martensite elasticity

We also see that during discharge the curve (black line on fig. 2) is analogous to the curve during charge but shifted from the hysteresis. This is due to the fact that constraint at the beginning ?As and at the end ?Af of reverse transformation differ from the beginning ?Ms and the end ?Ms of direct transformation.

Remark : From polycrystalline specimen, transition phases are less clearly defined (fig. 3). Moreover admissible reversible transformation is lower.