Microstructural evolution and grain boundary sliding in a superplastic magnesium AZ31 alloy

Tensile experiments on a fine-grained single-phase Mg-Zn-Al alloy (AZ31) at 673 K revealed superplastic behavior with an elongation to failure of 475% at 1 x 10(-4) s(-1) and non-superplastic behavior with an elongation to failure of 160% at 1 x 10(-2) s(-1); the corresponding sponding strain rate sensitivities under these conditions were similar to 0.5 and similar to 0.2. respectively. Measurements indicated that the grain boundary sliding (GBS) contribution to strain xi was similar to 30% under non-superplastic conditions: there was also a significant sharpening in texture during such deformation. Under superplastic conditions, xi was similar to 50% at both low and high elongations of similar to 20% and 120%; the initial texture became more random under such conditions. In non-superplastic conditions, deformation occurred under conditions without grain growth before significant flow localization whereas, under superplastic conditions, there was grain growth during the early stages of deformation, leading to strain hardening. The grains retained equiaxed shapes under all experimental conditions. Superplastic deformation is attributed to GBS, while non-superplastic deformation is attributed to intragranular dislocation creep with some contribution from GBS. The retention of equiaxed grain shapes during dislocation creep is consistent with a model based on local recovery related to the disturbance of triple junctions. (C) 2009 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.