Influence of the holding temperature on the kinetics of the isothermal B2 → B19′ transformation in TiNi-based shape memory alloy

Abstract

The aim of the present paper was to study the kinetics of the isothermal B2 → B19′ transformation in the quaternary Ti40.7Hf9.5Ni44.8Cu5 shape memory alloy in a wide temperature range to find the common features of this phenomenon in NiTi-based alloys. It was found that the isothermal B2 → B19′ martensitic transformation occurred during holding at a constant temperature both within and outside of the temperature range of the forward transformation. The holding temperature of T* influenced the maximum value of the martensite phase (ΦM) that appeared under isothermal condition and the rate of the martensite volume fraction variation. The maximum ΦM value of 80% was found when holding was at T* = Ms (the start temperature of the forward martensitic transformation on cooling), and it was two times larger than in quenched Ni51Ti49 alloy. An increase in holding temperature larger than Ms decreased the maximum ΦM value and the slope of the ΦM(t) curve. A decrease in the holding temperature less than Ms decreased the ΦM value and hardly influenced the dΦM/dt value. It was found that the variation in volume fraction of the martensite phase that appeared under isothermal holding at different temperatures in the Ti40.7Hf9.5Ni44.8Cu5 shape memory alloy was close to the binary Ni51Ti49 alloy, and hence, it was concluded that the features of the kinetics of the isothermal B2 → B19′ transformation were common for this type of transformation in non-stoichiometric NiTi-based alloys.