Superplastic flow at elevated temperatures in As-rolled AZ91 sheet

Abstract

An alternative processing method for superplastic magnesium alloys has been studied on the form of sheet of Mg-9Al-1Zn (AZ91) via large strain by ingot metallurgy with an isothermal-rolling. Constant initial strain rate tensile tests have been conducted at elevated temperatures for AZ91 sheets as rolled with reduction in area of 90%. The values of total elongation exceeded over 50% at all strain rates and at all testing temperatures except 673K, and the maximum total elongation exhibited 210%. The total elongation had a tendency to increase at low temperature side. The strain rate sensitivity, m-values, exceeded over 0.5 at strain rates from 1.0×10-4 to 2.5×10-4s-1. The values of flow stress were lower than those of completely recrystallized AZ91 and did not depend so much on the tensile temperatures from 573 to 648 K at high strain rates. The activation energy required for superplastic flow in the high-m-deformation was calculated to be 80-83 kJ · mol-1. This value was close to the grain boundary self-diffusion energy of Mg. Anisotropic textures were no seen in X-ray (0002) pole figure analysis (XPFA) by Schulz's reflection method. It was suggested that the stress concentrations through grain boundaries or in grains could be relaxed during superplastic deformation process and the superplastic flow was more easily associated with boundary diffusion. © 2007 ISIJ.