Precipitation characteristics and short-time aging treatment of Mg-Gd-Y-Zn-Zr alloy

Abstract

Currently, Mg-Gd-Y-Zn-Zr alloys face the issue of a long aging duration. To establish a short-time aging treatment route, the precipitation characteristics and their effects on mechanical properties during elevated-temperature heat treatment prior to low-temperature aging treatment, low-temperature single-stage aging treatment, and low-temperature two-stage aging treatment were studied. The following results were obtained: Wider intragranular lamellar phases, including 14-LPSO and γ′′ phases, are more easily obtained during long-term holding at heat treatment temperatures of 400 °C and 450 °C. Although these lamellar phases do not contribute to strengthening, they enhance ductility by hindering crack propagation. Micro-sized β phases precipitate more readily at heat treatment temperatures of 300 °C and 350 °C. Intragranular needle-like β phases are not effective strengthening phases, and β phase precipitating along grain boundaries form a networked distribution, which reduces ductility. The nano-sized β' phase, as the main strengthening phase, is more likely to precipitate during single-stage aging at temperatures of 200 °C and 250 °C. The β' phase formed at 200 °C is denser, leading to higher strength, but requiring a longer aging time. For two-stage aging, which involves a primary-stage at 200 °C for 8 to 12 h followed by a second-stage at 250 °C for 10 h, the aging time is reduced to at least one-quarter of that required for single-stage aging at 200 °C, ensuring strength while improving ductility. The formation of very dense nano-sized β' phases during the primary-stage aging facilitates the densification of β' phases during the subsequent second-stage aging. Additionally, the shortened aging time hinders the precipitation of β phase along the grain boundaries, thus improving ductility.