Understanding the contribution of the martensitic transformation to superelasticity

Nitinol, a nearly equiatomic alloy of nickel-titanium, can “remember” a previous deformation state and can recover deformation strains as high as 10% by deformation (superelasticity) or temperature changes (shape memory). These properties result from a reversible first-order phase transition between austenite (cubic, B2) and martensite (monoclinic, B19') phases.  As such, deformation mechanisms of Nitinol are more complex than the conventional modes of elastic and plastic deformation in traditional alloys. Consequently, the mechanical behaviour of highly textured Nitinol under multiaxial conditions is still relatively unpredictable, and the ability to improve quantitative prediction can only be achieved if the contribution of the martensitic phase transformation to the overall recoverable strain is better understood.

We will present results obtained from synchrotron x-ray diffraction measurements that distinguish between the conventionally elastic strain from the strain resulting from martensitic shape change and show dependences of these two different strain modalities affect the mechanical and fracture properties of NiTi in the superelastic regime.