Effects of Mg content on primary Mg2Si phase in hypereutectic Al-Si alloys

Hypereutectic Al-Si alloy with variant Mg contents were fabricated by casting, and the effects of Mg content on the microstructure of primary Mg2Si particles in hypereutectic Al-Si alloys were investigated. The results show that the volume fraction of primary Mg2Si particles increases linearly with raising the Mg content, but the average size of Mg2Si particles does not exhibit a corresponding change. When the Mg content is 3%, <1 0 0> directions have the fastest growth velocity, so that Mg2Si particles are likely to form octahedron shape. When gradually increasing the Mg content, the distributions of Mg and Si atoms on the solid-liquid interface become inhomogeneous, which results in the formation of irregular octahedron structures. Finally, when the Mg content is about 10%, the morphology of primary Mg2Si particles changes from the octahedron shape into various complex structures with a large size.