Effect of aging treatment on the precipitation transformation and age hardening of Mg-Gd-Y-Zn-Zr alloy

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.