Temperature dependent deformation of the B2 austenite phase of a NiTi shape memory alloy

Abstract

Temperature dependent deformation of the B2 austenite phase of a polycrystalline Ni49.9Ti50.1 (at.%) shape memory alloy was studied through combined macroscopic and microstructural characterization efforts. The objective was to determine mechanisms responsible for the macroscopic inelastic strains during isothermal loading of NiTi to 18% strain at temperatures above which the austenite phase exists in the nominally unloaded or stress-free condition (i.e., above the austenite finish temperature, Af). This study included in situ time-of-flight (TOF) neutron diffraction experiments used to follow the evolution of the lattice strains, texture, and phase fractions during deformation, ex situ macroscopic tensile experiments, and hot stage transmission electron microscopy (TEM). It was found that stress-induced martensite (SIM) formed at temperatures up to 310°C, which is well above the stress-free Af of 105°C. However, the stress induced martensite formed concurrently with general <001> slip processes and twinning by {114}B2 compound deformation twins, and did not occur as a separate distinguishable mechanism. Above the temperature that martensite cannot form with stress or the martensite desist temperature, Md, deformation was governed by the same slip and deformation twinning mechanisms, in addition to diffusion-assisted deformation processes. The overall results were combined to generate a deformation map that contained limits over which each of the identified deformation mechanisms was dominant in this Ni49.9Ti50.1 alloy.