Evolution of long-period stacking ordered phases and their effect on recrystallization in extruded Mg-Gd-Y-Zn-Zr alloy during annealing

In this study, the extruded Mg-6.9Gd-3.2Y-1.5Zn-0.5Zr (wt%) alloy was annealed at 693 K for 1-120 h, evolution of the long-period stacking ordered (LPSO) phases under elevated temperature and their effect on the recrystallization behavior were investigated. Initial microstructure of the extruded alloy consists of coarse deformed grains containing thin plate-shaped 14H-LPSO phase and fine recrystallized grains containing lamellar metastable phase. During annealing, deformed grains are gradually consumed by growing recrystallized grains, accompanied by dissolution of the LPSO phase in the former. Meanwhile, lamellar metastable phase within recrystallized grains transforms into thin plate-shaped 14H-LPSO phase, and simultaneously grows along the basal plane and thickens along the c-axis direction. Migration of recrystallized grain boundary is hindered by the LPSO phase in deformed grains. The hindrance is more significant along the c-axis of LPSO phase due to the remaining Mg matrix between LPSO phase and recrystallized grain boundary. While parallel to the basal plane of LPSO phase, the LPSO phase directly contacts recrystallized grain boundary, and the hindrance is weakened. Besides, the LPSO phase formed in recrystallized grains can also restrict the migration of grain boundary.