Effect of temperature and grain size on the dominant diffusion process for superplastic flow in an AZ61 magnesium alloy

The effect of temperature and grain size on superplastic flow was investigated using a relatively coarse-grained (∼20 μm) Mg–Al–Zn alloy for the inclusive understanding of the dominant diffusion process. Tensile tests revealed that the strain rate was inversely proportional to the square of the grain size and to the second power of stress. The activation energy was close to that for grain boundary diffusion at 523–573 K, and was close to that for lattice diffusion at 598–673 K. From the analysis of the stress exponent, the grain size exponent and activation energy, it was suggested that the dominant diffusion process was influenced by temperature and grain size. It was demonstrated that the notion of effective diffusivity explained the experimental results.