Effect of thermomechanical treatment on the superplasticity of Zn-22%Al

Abstract

The effect of prior thermomechanical treatment on the mechanical behavior of superplastic Zn-22%Al eutectoid was studied at temperatures from 443 to 533 K. Constant-velocity tension testing revealed three regions of behavior over the strain rate range from 5 × 10 −7 s −1 to 2 × 10 0 s −1. At low strain rates a region I with n = 3.1 and Q = 123 ± 10 kJ mol −1 was evident after concurrent grain growth had been compensated for. At high strain rates, region III with n = 4.7 and Q = 100 ± 5 kJ mol −1 was observed, with no detectable dependence of flow stress on grain size in this region. At intermediate strain rates, region II with n values of 1.8 – 2.3 and activation energies of 73 – 94 kJ mol −1 was observed. The n value was independent of strain rate in region II, but both n and the flow stress level decreased as the amount of prior cold working increased. The activation energy increased as the prior cold working was increased. Variations in the calculated values of A for different amounts of prior cold working were considered insignificant because of other changes in the constitutive equation.