Refinement of Mg2Si in Mg-Al-Si alloys through Ca and Y additions: A novel core-shell structure evolution mechanism and enhanced mechanical properties

• The optimal individual addition for Mg 2 Siis determined to be 0.1 wt.% Ca and 0.9 wt.% Y respectively. • The optimal composition for simultaneous addition is Mg-Al 5.5 -Si 9.5 -Ca 0.1 -Y 0.6 (wt.%). • A core-shell structure evolution mechanism is proposed for MgSi 2 Y 2 //Al 4 MgY//Mg 2 Si. • Refutes the literature view that particles in Mg 2 Si are due to liquid entrapment. Comprehensive experimental and theoretical investigations on microstructure and mechanical properties were conducted to explore the refinement of Mg 2 Si in Mg-Al-Si alloys through the addition of Ca and Y. Alloys of Mg-Al 5.5 -Si 9.5 -Ca x ( x = 0, 0.05, 0.1, 0.15, wt.%) and Mg-Al 5.5 -Si 9.5 -Y x ( x = 0, 0.3, 0.6, 0.9, wt.%) were designed based on the CALPHAD (CALculations of PHAse Diagram) calculations and literature data. With the experimentally determined optimal individual addition of 0.1 wt.% Ca and 0.9 wt.% Y respectively, a cross-experiment involving simultaneous addition of Ca and Y for the refinement of Mg 2 Si was carried out. The alloy composition with the optimal refinement effect and mechanical properties was identified as Mg-Al 5.5 -Si 9.5 -Ca 0.1 -Y 0.6 (wt.%). In Y-containing alloys, a novel core-shell structure evolution mechanism of MgSi 2 Y 2 //Al 4 MgY//Mg 2 Si was proposed based on experimental observation and first principles calculations, revealing MgSi 2 Y 2 as the core for the heterogeneous nucleation of Mg 2 Si.