Phase transformation of NiTi shape memory alloy powders prepared by ball milling

Ni50.8Ti49.2 alloy powders of different particle shapes and sizes have been prepared by ball milling. The powder milled for 1 h has large flaky particles and are partially amorphous. The powder crystallized through a single-stage exothermic transformation upon heating. Powders milled for longer durations consisted of partially amorphous Ni–Ti, [Fe,Ni] and Ni–Cr–Fe phases, indicating heavy contamination from the milling vessel. Upon heating, the milled powders exhibited an endothermic event at ∼400 °C. After annealing at 500 °C, the milled powders were fully crystallized. The crystallized powders exhibited martensitic transformation with reduced heat effect. The critical temperatures of the transformation were found to decrease with increasing ball milling time.