Analysis of the creep behavior of fine-grained AZ31 magnesium alloy

Abstract

Double-shear creep testing was used to evaluate the creep behavior of a magnesium AZ31 alloy processed by equal-channel angular pressing to produce an average grain size of ~2.7 μm. The results show that rapid creep rates are observed in the early stages of deformation due to the occurrence of grain boundary sliding in the fine-grained structure but the creep rates decrease with increasing deformation due to grain growth. The stress exponent for flow in the early stages is ~2 and the activation energy is ~92 kJ mol−1 where these values are consistent with the expectations for grain boundary sliding under superplastic conditions. Annealing the material for 24 h at 723 K before creep testing produces a significantly larger grain size of ~50 μm and this prevents grain boundary sliding and leads to an increasing stress exponent at higher stresses. Deformation mechanism maps are constructed incorporating both the present experimental results for a fine-grained magnesium alloy and results from published data for the AZ31 alloy. These maps provide a useful tool for evaluating the experimental conditions that are necessary for achieving superplastic forming operations.