A study on the hot workability of wrought NiTi shape memory alloy

The hot workability of a wrought 49.8 Ni–50.2 Ti (at pct) alloy was assessed using the hot compression tests in temperature range of 700–1000 °C, strain rate of 0.001–1 s−1, and the total strain of 0.7. The constitutive equations of Arrhenius-type hyperbolic-sine function was used to describe the flow stress as a function of strain rate and temperature. The preferable regions for hot workability of the alloy were achieved at Z (Zener–Holloman parameter) values of about 109–1013 corresponding to the peak efficiency of 20–30% in the processing map. However, a narrow area in the processing map including the deformation temperature of 1000 °C and strain rate of 1 s−1 is inconsistent with the related Z values. A flow instability region was observed at high Z values. Further instability regions were found at low temperature of 700 °C and low strain rates of 0.01–0.001 s−1 as well as at high temperature of 1000 °C and high strain rate of 1 s−1. The apparent feature of flow curves, the low value of peak efficiency, the similarity between the estimated apparent activation energy of deformation and that of the self diffusion of Ti in Ni, and the stress exponent of higher than 5, suggested that dynamic recovery (DRV) is the dominant restoration phenomenon during the hot working of the alloy.