Age-induced multi-stage transformation in a Ni-rich NiTiHf alloy

Abnormal multi-stage transformations have already been studied in binary Ni-rich NiTi alloys. In this research, this kind of transformation was investigated in a low supersaturated Ni-rich ternary NiTiHf high-temperature shape memory alloy by aging at intermediate temperatures for various durations. Meticulous examinations of the results of differential scanning calorimetry tests demonstrated the heterogeneous precipitation of (Ti,Hf)3Ni4 particles and the three-stage transformation (one-stage R and two-stage B19′) in the aged alloy. Aging provided a significant rise in transformation temperatures (TTs) until they reached their equilibrium states, corresponding to the equilibrium Ni content at each aging temperature. Equilibrium TTs were higher when aging was performed at a lower temperature. The remarkable increase in TTs was compared with those in aged Ni-rich NiTi alloys, and discussed based on the variation in valence electron concentration. A model was also proposed for the microstructural evolution during aging. Furthermore, aging provided enhanced hardness and strain recovery for the alloy. In particular, aging at a lower temperature resulted in a considerable improvement in hardness and shape recovery, which was discussed based on the microstructural changes in the aged alloy. Equilibrium Ni content at each aging temperature proved to be a crucial parameter in controlling the alloy properties, even in heterogeneous precipitations.