Microstractural evolution of cast Mg-Al-Zn during friction stir processing and subsequent aging

摘要

Mg-Al-Zn casting was subjected to two friction stir processing (FSP) protocols: single-pass with a pre-solution treatment (Pre-ST) and two-pass. Both the two-pass FSP and Pre-ST FSP produced uniform recrystallized microstructures with similar grain sizes of 7.5-8.7 mu m and high ratios of high-angle grain boundaries of 95.9-97.5 percent; however, the boundary misorientation distributions were quite different for the two samples. For the Pre-ST FSP sample, a significant proportion of {1012} twins were operative, with a complex and significantly weakened texture component. The mechanism of dynamic recrystallization (DRX) was mainly twinning DRX and continuous DRX (CDRX). However, for the two-pass FSP sample, the twins were significantly reduced, with a simple (0002) texture component, and DRX was mainly associated with CDRX and discontinuous DRX. A post-FSP aging resulted in the precipitation of p-Mg_(17)Al_(12) phase in continuous and discontinuous forms. Most of the continuous precipitates had a Burgers orientation relationship, namely: (0001)_(Mg)(110)_(Mg_(17)Al_(12)), 1210_(Mg)111_(Mg_(17)Al_(12). The origin of the different microstractural characteristics in the two samples was discussed.