Cavities were formed in the random boundaries of superplastic Mg-alloys due to the higher boundary energy. The cavity growth rate of fine-grained alloy (with the predominant diffusion process being grain-boundary diffusion) was lower than that of coarse-grained alloy (involving lattice diffusion) during superplastic flow.

Effect of Dominant Diffusion Process on Cavitation Behavior in Superplastic Mg–Al–Zn Alloy. H.Somekawa, T.Mukai: Scripta Materialia, 2007, 57[11], 1008-11