Effect of annealing and thermal cycling on phase transformation behaviour of Ni - Mn - Ga alloy

The effect of annealing treatment and thermal cycling on the phase transformation behaviour of Ni - Mn - Ga alloy prepared by are melting technology has been investigated. The results show that the main transformation parameters decrease in a few initial thermal cycles, and then stabilise with further cycling. The transformation temperature interval decreases with an increase of annealing temperature. As cast material exhibits a columnar grain microstructure, while annealed material exhibits coarsened grains with clearly martensitic plates. When the alloy is annealed at 1073 K, both the martensite start temperature M_f and the austenite finish temperature A_f decrease, while the martensite finish temperature M_f and the austenite start temperature A_s increase with an increase in annealing time. This implies that the chemical order degree of the parent phase increases with increasing annealing time, which causes the chemical driving force for martensitic transformation to decrease.