Modification of Mg2Si morphology in Mg–Si alloys with Bi

The effect of bismuth (Bi) addition on primary Mg2Si phases in Mg–Si alloys was investigated. It is found that proper amount of Bi in the alloys is efficient in refining the microstructure of Mg–Si alloys. According to the present experiment, the optimal modification effect is obtained when the Bi content in the alloy is 0.5%. The size of primary Mg2Si is significantly decreased to 15 μm or less, and its morphology is changed from coarse dendritic shape to polyhedral shape. However, when the Bi content is increased to or exceeds 0.8%, it is found that many primary Mg2Si become coarse again. Moreover, a small amount of Mg–Bi intermetallic compounds in the form of Mg3Bi2 are formed in the eutectic microstructure of the alloys with 0.5, 0.8 and 1%Bi.