The effect of temperature and grain size upon superplastic flow was investigated by using relatively coarse-grained (20μm) alloy samples. Tensile tests revealed that the strain rate was inversely proportional to the square of the grain size, and to the second power of the stress. The activation energy was close to that for grain boundary diffusion at 523 to 573K, and was close to that for lattice diffusion at 598 to 673K. On the basis of an analysis of the stress exponent, grain-size exponent and activation energy, it was suggested that the predominant diffusion process was affected by temperature and grain size. It was demonstrated that the concept of effective diffusivity could explain the experimental results.

Effect of Temperature and Grain Size on the Dominant Diffusion Process for Superplastic Flow in an AZ61 Magnesium Alloy. H.Watanabe, T.Mukai, M.Kohzu, S.Tanabe, K.Higashi: Acta Materialia, 1999, 47[14], 3753-8