Multi-stage martensitic transformation behaviors and microstructural characteristics of Ti-Ni-Hf high temperature shape memory alloy powders

An abnormal multi-stage martensitic transformation was observed in the Ti-49Ni-15Hf (at.%) alloy powders fabricated by the plasma rotating electrode process, which were largely dependent on the particle size. Upon heating and cooling process, the Ti-Ni-Hf alloy powders less than 75 μm and with the size of 75∼150 μm exhibited the two-stage and the three-stage martensitic transformation, respectively. During the rapid solidification, the significant variation of the Hf concentrations existed at the different distances from the (Ti,Hf)₂Ni precipitates as a consequence of the larger cooling rate and the struggle diffusion rate of Hf, which was the main reason for the two-step martensite transformation in the Ti-Ni-Hf alloy powders smaller than 75 μm. In addition to the variation of the chemical composition at the micro-levels, the distinctive cooling rate between the outer layer and interior for the larger alloy powders led to the difference in the amounts of (Ti,Hf)₂Ni precipitates, further resulting in the distinguishing chemical composition and the appearance of an additional martensitic transformation.