Effects of friction stir process and subsequent aging treatment on the microstructure evolution and mechanical properties of Mg-Gd-Y-Zn-Zr alloy

Abstract

The friction stir process (FSP) was conducted on the as-cast and as-homogenized Mg-Gd-Y-Zn-Zr alloy to investigate the microstructure characteristic of the stirred zone (SZ) during FSP. The interdendritic long period-stacking order (LPSO) phases in the SZ were effectively broken into block LPSO phases and tiny LPSO particles during FSP. The lamellar 14H-LPSO phases precipitated across the dynamic recrystallization (DRX) grains and the equilibrium β phases with superior thermal stability precipitated at triangular DRX grain boundaries during FSP. The statistically average grain size decreased from ~120 μm to ~1–2 μm in the SZ. The hardness of the SZ were elevated to 134.9 and 136.2 for as-cast and as-homogenized samples after post-FSP aging treatment, which can be ascribed to the precipitation of the dense nano-sized β′ phase. The ellipsoid β′ phases precipitated along the prismatic planes across the DRX grains and also precipitated within stacking faults or lamellar 14H-LPSO phases. The grain boundary strengthening and precipitation strengthening effect predominately contribute to the improved tensile yield strength after post-FSP aging treatment.