Ultrasound alters the nucleation pathway of primary Mg₂Si in a chemically modified multicomponent Al–Mg₂Si alloy

We demonstrate the origin of the modification to the primary Mg₂Si phase induced by ultrasonic treatment (UST) of a P-modified Al–Mg₂Si alloy through combined experimental and theoretical investigations. Multicomponent Al–Mg₂Si alloys contain various secondary phases including primary Mg₂Si, Q, Si, θ, and Zn phases. Regardless of the cooling rate, applying UST transforms the coarse, irregular dendritic Mg₂Si phase into a fine, compact phase. High-resolution transmission electron microscopy revealed that in the absence of UST, the Mg₃P₂ phase acted as a preferential nucleation site for the primary Mg₂Si phase. By contrast, when UST was applied, the MgO particles were the main nucleation sites for this phase. First-principles calculations confirmed the higher nucleation potency of the MgO phase compared with that of the Mg₃P₂ phase. UST resulted in the deagglomeration of MgO particles while improving their wettability to molten Al, allowing the more potent MgO particles to act as nucleation sites for the primary Mg₂Si phase. Our experimental and theoretical results show that UST alters the primary Mg₂Si nucleation pathway from the Mg₃P₂ phase to the more potent MgO particles activated by UST, resulting in a significant modification of the primary Mg₂Si phase in chemically modified Al–Mg₂Si alloys.