Shape memory alloys (SMAs) are an important class of smart materials that have the ability to remember a shape. Current practical uses of SMAs are limited to below 100 degrees C which is the limit for the transformation temperatures of most commercially successful SMAs such as NiTi and Cu-based alloys. In recent years, the CoNiGa system has emerged as a new ferromagnetic shape memory alloy with some compositions exhibiting high martensitic transformation temperatures which makes CoNiGa a potential high temperature shape memory alloy (HTSMA). In this study, the microstructural evolution and martensitic transformation characteristics of CoNiGa (mainly Co46Ni27Ga27 and Co44Ni26Ga30 in at.percent) HTSMAs were investigated in as-cast and hot-rolled conditions as a function of different heat treatments. Heat treatment conditions were selected to introduce single, two, and three phase structures, where two precipitate phases (ductile Y and hard Y') do not martensitically transform. Calorimetry, X-ray analysis, scanning and transmission electron microscopy, thermo-mechanical process and cycling techniques are applied to understand the structural and chemical factors influencing the thermal stability and transformation characteristics. The main findings include improvement of ductility, most cyclically stable compositions with narrow transformation hysteresis (<40 degrees C) and transformation temperatures in the range of 100 degrees C to 250 degrees C, formation of new phases and their effects, and associated compositional changes in the matrix, on the transformation temperatures and on the microstructural evolution. In addition, Ms temperature depends linearly on the valence electron concentration (e/a) of the matrix, only if the Ga content is constant, and the samples with narrow transformation hysteresis demonstrate reversible martensitic transformation in constant-stress thermal cycling experiments.
