Which correctly describes the mechanism of superelasticity in the Ni-Ti alloy (about 50 at%Ni) used for orthodontic wires and stents? The service temperature is above the reverse transformation finish temperature Af.

Answer: C. Under load the parent phase transforms to martensite (stress-induced martensitic transformation), and on unloading it reverts and the strain recovers

Above Af, the parent phase with the B2 structure is stable. When stress is applied, the parent phase transforms to martensite at a certain stress (stress-induced martensitic transformation), and a large strain develops at a nearly constant stress. On unloading the martensite becomes unstable and returns to the parent phase, so strains of up to about 6–8% recover. This property of deforming greatly and returning at a nearly constant force is used in orthodontic wires that move teeth with a constant force and in stents that are folded thin and inserted into blood vessels. Plastic deformation by dislocation slip is a permanent strain that does not recover on unloading. Strengthening with precipitates only raises the yield stress; the Young's modulus hardly changes, and it does not explain superelasticity.