Restraining effect of strontium on the crystallization of Mg2Si phase during solidification in Al–Si–Mg casting alloys and mechanisms

The effect of strontium on the crystallization of Mg2Si phase in Al–11.6%Si–0.4%Mg alloys was investigated with optical microscope and SEM. In the partially modified alloys, the Mg2Si phase grows as network or bamboo-shoot shape. However, very few and fine Mg2Si particles are isolated at the boundaries of the eutectic cells in the fully modified alloys. Addition of strontium restrains the crystallization of Mg2Si phase. The Mg2Si phase in Al–Si–Mg casting alloy is thought to nucleate on the surfaces of the eutectic silicon flakes. This hypothesis was analyzed from the point of view of the lattice match between the Mg2Si and Si crystals. The restraining effect of strontium is thought to be related to the combination of two effects of strontium, the increase of the amount of dendritic α phase and the change of the surface characteristic of silicon crystal caused by the addition of strontium.