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

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 similar to 2.7 mu m. 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 finegrained structure but the creep rates decrease with increasing deformation due to grain growth. The stress exponent for flow in the early stages is similar to 2 and the activation energy is similar to 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 similar to 50 mu m 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.