Microstructural evolution of cast Mg–Al–Zn during friction stir processing and subsequent aging

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 μm and high ratios of high-angle grain boundaries of 95.9–97.5%; however, the boundary misorientation distributions were quite different for the two samples. For the Pre-ST FSP sample, a significant proportion of {101¯2} 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 〈0 0 0 2〉 texture component, and DRX was mainly associated with CDRX and discontinuous DRX. A post-FSP aging resulted in the precipitation of β-Mg17Al12 phase in continuous and discontinuous forms. Most of the continuous precipitates had a Burgers orientation relationship, namely: (0001)Mg∥(110)Mg17Al12,[12¯10]Mg∥[11¯1]Mg17Al12. The origin of the different microstructural characteristics in the two samples was discussed.