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.

In this study, the precipitation transformation and age hardening of solution-treated Mg-9Gd-4Y-2Zn-0.5Zr(wt.%) alloy were investigated at different aging treatment parameters. The precipitation sequences of the alloy aged at 200 ℃, 250 ℃ and 300 ℃ are β″(DO19) → β′(BCO) → β(FCC), β″(DO19) → β′(BCO) → β₁(FCC) → β(FCC) and β(FCC), respectively. The streaks sequences of the alloy aged at 200 ℃, 250 ℃ and 300 ℃ are SF, SF → 14H-LPSO and SF → 14H-LPSO, respectively. For the alloy aged at 200 ℃ and 250 ℃, the increase in hardness with increasing aging time is contributed from the increase in precipitate volume fraction and the transformation from β″ to β′ phase with basal → prismatic and spherical → spindle-like precipitate changes. The decrease in hardness after the peak-aging stage is attributed to the appearance of micro-sized β precipitates. Because of the smaller size of precipitates and the triangular arrangement of β′ precipitate, the hardness of the alloy aged at 200 ℃ is higher than that aged at 250 ℃. For the alloy aged at 300 ℃, the appearance of only micro-sized β precipitate and its coarsening with increasing aging time leads to the lowest hardness and an overall decrease in hardness with the aging time.