Experimental determination of equilibrium stress in stress-induced martensitic transformation at low temperatures in Ni-rich TiNi

Abstract

The exceptional broadening of superelastic stress hysteresis at low temperatures in Ni-rich Ti–Ni shape-memory alloys impedes their cryogenic applications. This broadening arises from thermally activated habit plane glide. Traditionally, equilibrium stress has been assumed to lie at the midpoint between forward and reverse martensitic transformation (MT) stresses, assuming reciprocal kinetics. Here, we assess this assumption using strain-rate jump tests, a simple method that detects the magnitude of stress change in response to strain-rate variation. The observed stress change is consistently larger during the forward MT than the reverse MT, indicating an asymmetric thermal activation and a shift in equilibrium stress toward the reverse MT stress. This result deviates from the classical midpoint approximation in systems with significant hysteresis broadening. Strain-rate jump test is demonstrated to be a simple yet effective method for locating the equilibrium stress, even when it is bracketed deep within a broadened stress hysteresis.