Investigation on the microstructure evolution of high strength and ductility as-cast Mg-9.5Gd-2.3Y-1Zn-0.5Zr alloy via double peak-aging

This article reports a systematic investigation on the relationship between the microstructure evolution and mechanical properties of as-cast Mg-9.5Gd-2.3Y-1Zn-0.5Zr (VW92, wt.%) alloy during aging treatment. The results indicate that the alloy exhibits obvious double peak-aging characteristics at 180 ​°C, 200 ​°C, and 220 ​°C; the first peak-aging appeared at 96 ​h, 48 ​h, and 48 ​h, respectively, while the second peak-aging occurred at 204 ​h, 180 ​h, and 180 ​h, respectively. Moreover, the strengths of the first peak-aging were higher than those of the second peak-aging. Consequently, the first peak-aging at 200 ​°C achieved the best mechanical properties, with ultimate tensile strength (UTS), yield strength (YS), and elongation (EL) of 380 (±2.0) MPa, 255 (±1.8) MPa, and 12.8 (±1.7) %, respectively. While the strength decreased in the second peak-aging, the elongation increased to 17.2 (±0.5)%. The first peak-aging strengthening is ascribed to the participation of the nano-β′ phases in the matrix and the long period stacking ordered (LPSO) phases at grain boundaries (GBs). Additionally, the second peak-aging strengthening is associated with the emergence of a relatively new 3D structure comprising long-chain-like structural phases β' ​+ ​β′ F ​+ ​β 1 , γ′ phases, and LPSO phases within the grain, combined with the fine and uniform LPSO phases at the GBs.