Heat treatment of hot-isostatic-pressed 60NiTi shape memory alloy: Microstructure, phase transformation and mechanical properties

Abstract 60NiTi alloy is considered to be a promising material for specialized bearing and gear applications due to its high hardness, strength, and low modulus. However, fabricating 60NiTi through conventional processing methods is challenging due to the brittleness and poor workability. In this study, 60NiTi with high relative density was successfully fabricated directly from pre-alloyed powder through hot isostatic pressing. The effects of solution and aging treatments on microstructure and mechanical properties were systematically studied by advanced characterization techniques. The hot-isostatic-pressed 60NiTi showed low average hardness and elastic strain due to the formation of a soft Ni3Ti phase and B2 NiTi matrix. Solution treatment above 1000 °C dissolved the Ni3Ti phase and promoted the formation of nanoscale Ni4Ti3 precipitates, which significantly improved the hardness, strength, and elastic strain of 60NiTi. The formation of the Ni4Ti3 phase can be mainly attributed to the driving forces induced by the chemical supersaturation and mechanical stress concentration. Finally, the phase transformation mechanisms during heat treatment and compression test were discussed.