Effect of Al addition on solidification microstructure and mechanical properties of flame-retardant Mg-5Ca-1.5Sm alloy

This study investigates the influence of 1-4 wt% Al additions on the microstructure of a flame-retardant Mg-5Ca-1.5Sm alloy, revealing a distinct threshold effect of Al on grain refinement. With 1-2 wt% Al, the Al 2 Sm phase forms but fails to refine the grains significantly. In contrast, 3 wt% Al addition triggers a remarkable refinement, reducing the grain size from 1510 μm of Mg-5Ca-1.5Sm alloy to 30 μm. This sharp transition is elucidated by coupling experimental observations with Thermo-Calc simulations of solidification sequences. The analysis establishes a critical criterion: Al 2 Sm acts as an effective grain refiner only when it precipitates before α-Mg solidification. At lower Al contents (1 and 2 wt%), α-Mg precipitates as the primary phase at 613 °C (for 1 wt% Al) and 608 °C (for 2 wt% Al), respectively, followed by the nucleation of the Al 2 Sm phase in dendritic arm regions of α-Mg grains. This delayed precipitation of Al 2 Sm phase inhibits its effective grain refinement and results in random orientation relationships (ORs) of the α-Mg grains. At 3 wt% Al, primary Al 2 Sm precipitation at ∼645 °C provides abundant potent nucleation sites, establishing consistent formation of two specific ORs between Al 2 Sm and the α-Mg matrix. This provides definitive crystallographic evidence for a heteroepitaxial nucleation mechanism. Furthermore, the Mg-5Ca-1.5Sm-3Al alloy demonstrates optimal mechanical performance, with YS, UTS and El reaching 167 MPa, 199 MPa and 1.8%, respectively. This work clarifies the composition-dependent grain refinement mechanism mediated by Al 2 Sm in Mg-Ca-Sm-Al alloys.