Superelasticity, Pseudoelasticity, and the Shape Memory Effect

Superelasticity refers to the ability of certain materials to undergo unusually large elastic deformations over a certain temperature range. In the most well-known superelastic material, nitinol (NiTi 51% Ni), up to 11 percent recoverable strain is observed (as much as 25 percent in specific directions in single crystals ). As a comparison, only 

At low levels of stress, the material s elastic response is linear. Upon further loading, there is very little change in stress, but a large increase in strain, s. At higher loading, the material s elasticity is again linear. 

Although the elastic response is linear at low and high stresses, the modulus of elasticity (the slope of the linear portions of the stress-strain curve) in the two regimes is different. 

There is a cyclic stress-strain hysteresis with loading and unloading, with zero residual strain at the end of the cycle, which accounts for the superelasticity. 

This peculiar superelastic behavior is owing to a stress-induced transformation from a high-strength crystalline phase to a very structurally similar, yet deformable, crystalline phase. On removing the stress, the deformed material transforms back to the initial 

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