Effect of Bi Modification on Mg_2Si Phase in Hypereutectic Al-15Mg_2Si Alloy
The influence of Bi additions on the microstructure of hypereutectic Al-15Mg2Si alloy was investigated by XRD,SEM and EDS,and the modification mechanism was analyzed.The results show that the proper amount of Bi has a significant modification effect on both primary and eutectic Mg2Si in the alloy.When 1.0%Bi is added,the morphology of primary Mg2Si changes from dendritic to polyhedral shape,its average grain size significantly decreases from 70 μm to 6 μm;at the same time,the eutectic Mg2Si from flake-like to very short fibrous or dot-like.The modification mechanism may be related to the decrease of the interfacial tension between the Mg2Si phase and Al liquid and the composition supercooling caused by the segregation of Bi at the front of the solid/liquid interface of Mg2Si during solidification.