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 mu m 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. (c) 2008 Published by Elsevier B.V.