Excellent superplastic elongations (in excess of 1,200%) were achieved in a commercial cast AZ31 alloy processed by low temperature equal-channel angular pressing (ECAP) with a back-pressure to produce a bimodal grain structure. In contrast, AZ31 alloy processed by ECAP at temperatures higher than 200 °C showed a reasonably uniform grain structure and relatively low ductility. It is suggested that a bimodal grain structure is advantageous because the larger grains contribute to strain hardening thus delaying the onset of necking, while grain boundary sliding associated with small grains provides a stabilizing effect due to enhanced strain rate sensitivity.
Lapovok, R. Y., Estrin, Y., Popov, M. V., Rundell, S., & Williams, T. B. (2008). Enhanced superplasticity of magnesium alloy AZ31 obtained through equal-channel angular pressing with back-pressure. Journal of Materials Science, 43(23-24), 7372 - 7378. https://doi.org/10.1007/s10853-008-2685-z