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 mu m 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 {1 0 (1) over bar 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 beta-Mg17Al12 phase in continuous and discontinuous forms. Most of the continuous precipitates had a Burgers orientation relationship, namely: (0001)(Mg)parallel to(1 1 0)Mg17Al12, [1 (2) over bar 1 0]Mg parallel to [1 (1) over bar 1]Mg17Al12. The origin of the different microstructural characteristics in the two samples was discussed. (C) 2009 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.