AlSi2(Sc, Zr, Ti)2 phase formation in Sc(Zr)-containing Al–7Si–Mg alloy and their effects on microstructure and mechanical properties

To investigate the effects of AlSi 2 (Sc, Zr, Ti) 2 phase on the microstructure and mechanical properties of Al–7Si–Mg alloys, alloys with varying scandium (Sc) content (0–0.60 wt%) and constant Zr (0.20 wt%) additions were prepared through rapid solidification in a copper mold. XRD and TEM results reveal that the Sc-containing phase is AlSi 2 (Sc, Zr, Ti) 2 phase in the alloy, formed by the partial substitution of Sc with Zr and Ti elements in AlSi 2 Sc 2 phase. With increasing Sc addition from 0 to 0.60 wt%, both α-Al grains and eutectic Si phases exhibit progressive refinement. At 0.45 wt% Sc, the average size of α-Al grains and eutectic Si are minimized to 67.8 μm and 3.1 μm, respectively. The morphology of the AlSi 2 (Sc, Zr, Ti) 2 solidification phase transitions sequentially from rod-like (0.15 wt% Sc) to strip-like (0.30–0.45 wt% Sc), ultimately forming a network structure at 0.45 wt% Sc. The alloy achieves optimal mechanical properties at 0.15 wt% Sc, with yield strength, tensile strength, and elongation reaching 360 MPa, 412 MPa, and 7.0 %, respectively. The improvement of alloy properties is attributed to the combined effects of microstructure refinement strengthening, solute element solution strengthening and precipitation strengthening of AlSi 2 (Sc, Zr, Ti) 2 precipitates, β-Mg 2 Si and Si-containing dispersoids. Excessive Sc addition leads to coarsening of primary AlSi 2 (Sc, Zr, Ti) 2 , which acts as crack initiation sites, degrading ductility. These findings provide a pathway for designing high-performance cast Al–Si alloys.