One-Step Synthesis of Spherical Primary Mg₂Si in Ca/Sb-Modified Al-Mg₂Si Alloy

One-step synthesis of spherical primary Mg₂Si is achieved in 0.5 wt.% Ca/Sb-modified Al-12Mg₂Si alloy by gravity casting for the first time. The addition of 0.5 wt.% Ca/Sb results in large number density of CaSb₂ compounds for the nucleation of primary Mg₂Si. Embryos of primary Mg₂Si can be formed on these CaSb₂ compounds in the hypereutectic areas caused by the composition fluctuations. Low content of Mg/Si reduces the Mg/Si concentration for the growth of primary Mg₂Si, which results in the final particle size which is less than the critical size of primary Mg₂Si isotropic growth; the spherical primary Mg₂Si is formed. Fracture behavior changes from the transgranular fracture of eutectic Mg₂Si to intergranular fracture of primary Mg₂Si with the formation of primary Mg₂Si in Al-12Mg₂Si alloy. Formation of spherical primary Mg₂Si decreases the stress concentration, while the bonding strength between the particle and matrix is enhanced. Ultimate tensile strength of 0.5 wt.% Ca/Sb-modified Al-12Mg₂Si alloy is improved significantly (from ~ 168 to ~ 230 MPa). The study provides a simple methodology to prepare spherical primary Mg₂Si, which is beneficial to improving the mechanical property of Al-Mg₂Si alloy.