Grain-Refinement by MDFing under Decreasing-Temperature Conditions Followed by Room-Temperature MDFing of AZX611Mg Alloy and Change in Mechanical Properties

Hot-extruded AZX611Mg alloy was multi-directionally forged under decreasing-temperature conditions (dMDFed) and followed by room-temperature MDFing (rMDFing). Coarse initial grains were gradually fragmented by dynamic recrystallization during dMDFing and ultrafine-grained structure was homogeneously developed over the cumulative strain of ΣΔε = 4.8. Additional rMDFing contributed to further grain fragmentation due to kinking and mechanical twinning. Nevertheless, coarse precipitates in the as-hot-extruded samples and newly formed fine precipitates during dMDFing impeded twinning. The achieved grain size appeared rather larger than that of the dMDFed AZ61Mg alloy. While mechanical properties were drastically improved by simple dMDFing or rMDFing, the combined processes of dMDFing and rMDFing were not so effective to induce additional strengthening. This was attributed to complicatedly combined effects of impediment of twinning by precipitates to grain fragmentation and fracture caused by stress concentration at around precipitates/matrices.