Role of microstructure on phase transformation behavior in Ni–Ti–Fe shape memory alloys during thermal cycling

Abstract

Two different microstructures of Ni–Ti–Fe shape memory alloys processed through different thermo-mechanical treatments with nearly similar grain size and in-grain misorientation but different crystallographic textures were subjected to series of thermal cycles without external loading. The microstructures and the phase transformation behavior of the samples were examined after every seventy cycles. The experiment involved treating the samples with liquid nitrogen (LN2) for complete martensitic (B19′) transformation and then heating it back to parent austenite (B2) condition. Thermal cycling introduced significant differences in microstructural parameters especially the grain boundary nature, stored elastic energy and the misorientation or defect densities. These microstructural alterations during thermal cycling were related to changes in transformation temperatures and enthalpy. Thermal cycling also brought out changes in crystallographic orientations of austenite grains. The present study aims to address the role of microstructure on the thermal fatigue behavior in Ni–Ti based shape memory alloys during cyclic transformations.