Effect of Sr Microalloying on the Microstructure, Solution-Treatment Response, and Age-Hardening Behavior of Mg-Nd-Gd-Zn-Zr Alloy

Mg-Nd-Gd-Zn-Zr alloys containing 0, 0.20, and 0.75 wt.% Sr were prepared by gravity casting to investigate the effects of Sr microalloying on as-cast microstructure, solution-treatment response, and age-hardening behavior. The alloys were characterized by XRD, OM, SEM/EDS, DSC, and Brinell hardness testing. Sr was mainly enriched at grain boundaries and in secondary-phase regions, and Mg17Sr2-related diffraction peaks appeared in the Sr-containing alloys. The as-cast grain size varied non-monotonically among the three compositions investigated, with the 0.20 wt.% Sr alloy exhibiting the finest grains, whereas the area fraction of secondary phases increased continuously with Sr content. Under the selected solution-treatment conditions, the Sr-containing alloys retained smaller grains but contained larger amounts of residual secondary phases. Under the respective selected solution-treated starting states, the 0Sr alloy achieved the highest measured hardness at 200 °C, while the 0.20Sr alloy showed an earlier hardening response and a more gradual subsequent hardness evolution; the 0.75Sr alloy showed a more pronounced post-peak hardness decrease. The retained Sr-containing secondary phases may indicate incomplete dissolution of part of the Nd, Gd, and Zn, potentially reducing the effective solute reservoir in α-Mg available for subsequent aging. Within the three compositions investigated, the results indicate a trade-off between grain refinement/retention and age-hardening potential.