Hot deformation behaviour, constitutive equations and processing map of Cu-Zn-Al-Ni shape memory alloy

Abstract

The hot workability and deformation behaviour of Cu-Zn-Al-Ni based shape memory alloy (SMA) was investigated. The alloy was isothermal compression tested at temperatures of 250–550 $°C$, strain rates of 0.1–5 s⁻¹, and a constant total strain of 0.5, using a thermomechanical Gleeble-3500 simulator. The results show that positive strain rate sensitivity characterized the plastic flow behaviour of the SMA. The hyperbolic-sine constitutive equation - determined activation energy for the hot deformation of Cu-Zn-Al-Ni SMA (154.34 kJ/mol) is about 24 % lower than the activation energy for self-diffusion of copper, and that of the stress exponent value (n) which was less than 5, both point to dynamic recrystallization to be the dominant dynamic softening mechanism. Furthermore, the processing map indicated that flow instability occurs in the low temperature and strain rate regions (250 - 350 $°C$, 0.1 – 5s⁻¹) with characteristic shear bands, dendritic structures, and micro-cracks in their microstructure. The temperature of 550 $°C$ and strain rates of between 0.1 and 2.5 s⁻¹, was established to be the optimal condition for hot deformation of the alloy. These conditions result in stable flow with microstructures consisting of fine dynamically recrystallized grains.