On the evaluation of parameters of the constitutive equation for 7475 Al alloy

Abstract

From the mechanical data on 7475 Al alloy, it is evident that flow stress is significantly dependent on the strain during superplastic flow. This is due to its ability to strain-harden during superplasticity. The rate of increase in the flow stress is much higher at 457° C than at 517° C. This gives rise to non-unique values for the parameters of the constitutive equation. At 457° C, whereas the stress exponent (n) and activation energy for superplastic flow at 1 × 10⁻⁴ sec⁻¹ increase only slightly with strain, the grain size sensitivity parameter (p) and structure parameter (A) decrease significantly with strain. These changes in the constitutive parameters are associated with dislocation activity occurring within the grain interior, leading to grain elongation without significant changes in the grain size, through the parameter, (b/d)^p, of the constitutive equation.