Optimization of hot working parameters of as-forged Nitinol 60 shape memory alloy using processing maps

Abstract

The hot deformation behavior of as-forged Nitinol 60 alloy (60 wt% Ni, 40 wt% Ti) was studied over the ranges of temperature, 650–850 °C, and strain rate, 0.01–1 s⁻¹, using isothermal constant strain rate compression tests in a Gleeble-3500 simulator. The processing maps, based on the dynamic materials model, were developed to optimize the hot working parameters. The results show that the deformation parameters have a marked effect on the power dissipation efficiency and the instability parameter. A single unstable region (650–775 °C, 0.037–1 s⁻¹), associated with flow localization and/or adiabatic shear, is detected from the processing map. This should be avoided in hot working process. The optimized hot working conditions correspond to 680–790 °C, 0.01–0.025 s⁻¹ with peak efficiency of 0.45 at 720 °C, 0.01 s⁻¹, and 820–850 °C, 0.1–1 s⁻¹ with peak efficiency of 0.5 at 850 °C, 1 s⁻¹. Microstructure observations indicate that the main deformation mechanism of optimized domains involves dynamic recrystallization.