降温MDF後の冷間MDFによるAZX611Mg合金の結晶粒超微細化と機械的特性調査

抄録

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.

Fig. 9　Change in hardness during dMDFing to various cumulative strains and followed by rMDFing of a fixed cumulative strain of ΣΔε = 1.0 (i.e., 10 passes of rMDFing). The arrows indicate the change in the hardness of the dMDFed and additionally rMDFed samples.