Effect of Sb on modification of primary Mg2Si crystal in hypereutectic Mg-Si alloy

The hypereutectic Mg-4.8%Si alloy was modified by using Mg-20%Sb master alloy. The effect of Sb addition on the modification of primary Mg2Si crystals and its modification mechanism were investigated. The results show that the Sb content is a significant factor that determines the modification effect of primary Mg2Si crystals. The primary Mg2Si crystals are mainly coarse dendritic for the unmodified hypereutectic Mg-4.8%Si alloy. The morphologies of primary Mg2Si crystals can not be effectively modified by Sb under the condition of Sb content less than 0.8%. The average sizes of primary Mg2Si crystals slightly decrease. With the Sb addition increasing to 1.2%, the primary Mg2Si crystals are effectively modified into small particles, which distribute uniformly. Compared with the unmodified hypereutectic Mg-4.8%Si alloy, the average sizes of the primary Mg2Si crystals for the modified Mg-4.8%Si alloy by 1.2%Sb addition drastically decrease. The modification mechanism should be associated with the Mg3Sb2 particles remained in the hypereutectic Mg-4.8%Si meld modified by Mg-20%Sb master alloy. The Mg3Sb2 particles should act as nucleating substrates of the primary Mg2Si crystals during solidification.