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

摘要

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 mu m and with the size of 75 similar to 150 mu m 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)(2)Ni 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 mu m. 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)(2)Ni precipitates, further resulting in the distinguishing chemical composition and the appearance of an additional martensitic transformation. (C) 2018 Elsevier B.V. All rights reserved.