Microstructure and martensitic transformation characteristics of CoNiGa high temperature shape memory alloys

Microstructural evolution and martensitic transformation characteristics of Co46Ni27Ga27 and Co44Ni26Ga30 high temperature shape memory alloys were investigated in as-cast and hot-rolled conditions as a function of heat treatment. Heat treatments were selected to introduce single-, two-, and three-phase structures, where the precipitate phases do not martensitically transform. The effects of these precipitates, and associated compositional changes, on transformation temperatures, thermal hysteresis, and microstructural evolution during thermal cycling, were revealed. It was found that martensite start temperature linearly depends on the valence electron concentration (e/a) of the matrix, if the Ga content is constant. For a given e/a, the higher the Ga content is, the higher the transformation temperatures become. The presence of γ′ precipitates and the volume fraction of γ phase were shown to have strong influence on transformation thermal hysteresis. The most cyclically stable compositions with narrow hysteresis (<40 °C) were identified. In these compositions, a room-temperature aging phenomenon, possibly mediated by point defects, was discovered, which recovers the transformation temperature changes upon thermal cycling. They also demonstrate reversible martensitic transformation in constant-stress thermal cycling experiments. However, their crystallographic texture should be engineered to increase the transformation strain, and ductile γ-phase content should be reduced to improve cyclic reversibility.