Bi has a good modification effect on the hypoeutectic Al-Si alloy,and the morphology of eutectic Si changes from coarse acicular to fine fibrous.Based on the similarity between Mg2Si and Si phases in crystalline structure and crystallization process,the present study investigated the effects of different concentrations of Bi on the microstructure,tensile properties,and fracture behavior of cast Al-15wt.%Mg2Si in-situ metal matrix composite.The results show that the addition of the proper amount of Bi has a significant modification effect on both primary and eutectic Mg2Si in the Al-15wt.%Mg2Si composite.With an increase in Bi content from 0 to 1wt.%,the morphology of the primary Mg2Si is changed from irregular or dendritic to polyhedral shape;and its average particle size is significantly decreased from 70 to 6 μm.Moreover,the morphology of the eutectic Mg2Si phase is altered from flake-like to very short fibrous or dot-like.When the Bi addition exceeds 4.0wt.%,the primary Mg2Si becomes coarse again.However,the eutectic Mg2Si still exhibits the modified morphology.Tensile tests reveal that the Bi addition can improve the tensile strength and ductility of the material.Compared with those of the unmodified composite,the ultimate tensile strength and percentage elongation after fracture with 1.0wt.% Bi increase 51.2% and 100%,respectively.At the same time,the Bi addition changes the fracture behavior from brittle to ductile. Bi has a good modification effect on the hypoeutectic Al-Si alloy, and the morphology of eutectic Si changes from coarse acicular to fine fibrous. Based on the similarity between Mg2Si and Si phases in crystalline structure and crystallization process, the present study investigated the effects of different concentrations of Bi on the microstructure, tensile properties, and fracture behavior of cast Al-15 wt.% Mg2 Si in-situ metal matrix composite. results show that the addition of the proper amount of Bi has a significant modification effect on both primary and eutectic Mg2Si in the Al-15 wt.% Mg2Si composite. With an increase in Bi content from 0 to 1 wt.%, the morphology of the primary Mg2Si is changed from irregular or deritic to polyhedral shape; and its average particle size is significantly decreased from 70 to 6 μm. Moreover, the morphology of the eutectic Mg2Si phase is altered from flake-like to very short fibrous or dot-like. What the other addition exceeds 4.0 wt.%, the primary Mg2Si becom es coarse again. However, the eutectic Mg2Si still exhibits the modified morphology. Sensile tests reveal that the Bi addition can improve the tensile strength and ductility of the material. Compared with those of the unmodified composite, the ultimate tensile strength and percentage elongation after fracture with 1.0 wt.% Bi increase 51.2% and 100% respectively. At the same time, the Bi addition changes the fracture behavior from brittle to ductile.