Effects of HSHPT on the martensitic transformation behaviour of an NiTi alloy

Abstract

High speed high pressure torsion (HSHPT) is a novel severe plastic deformation technique that is used to produce bulk ultrafine-grained nickel-titanium shape memory alloys. In this study, the effect of grain refinement on phase transformation was investigated in a near equiatomic NiTi shape memory alloy subjected to processing by this technique. Phase transformations involving different degrees of deformation and stability of thermally-induced phase transformations were analyzed by differential scanning calorimetry (DSC). The measurements suggest that the martensitic transformation occurred even when the highest degree of deformation was applied. Optical microscopy (OM), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) investigations bring to light that the true strain applied controls the evolution of the microstructure. The results are presented and discussed in detail in this paper