The influence of Nd on the microstructures,tensile properties and fracture behavior of cast Al-18 wt.%Mg2Si in situ metal matrix composite was investigated.Experimental results showed that,after introducing a proper amount of Nd,both primary and eutectic Mg2Si in the Al-18 wt.%Mg2Si composite were well modified.The morphology of primary Mg2Si was changed from irregular or dendritic to polyhedral shape,and its average particle size was significantly decreased from 47.5 to 13.0 μm.Moreover,the morphology of the eutectic Mg2Si phase was altered from flake-like to a thin laminar,short fibrous or dot-like structure.Tensile tests revealed that Nd addition improved the tensile strength and ductility of the material.Compared with those of unmodified composite,the ultimate tensile strength and percentage elongation with 0.5% Nd were increased by 32.4% and 200%,respectively.At the same time,Nd addition changed the fracture behavior from brittle to ductile. The influence of Nd on the microstructures, tensile properties and fracture behavior of cast Al-18 wt.% Mg2 Si in situ metal matrix composite was investigated. Experimental results showed that after introducing a proper amount of Nd, both primary and eutectic Mg2Si in the Al-18 wt.% Mg2Si composite were well modified. The morphology of primary Mg2Si was changed from irregular or deritic to polyhedral shape, and its average particle size was significantly decreased from 47.5 to 13.0 μm. Moreover, the morphology of the eutectic Mg2Si phase was altered from flake-like to a thin laminar, short fibrous or dot-like structure. Sensile tests revealed that Nd addition improved the tensile strength and ductility of the material. Compared with those of unmodified composite, the ultimate tensile strength and percentage elongation with 0.5% Nd were increased by 32.4% and 200% respectively. At the same time, Nd addition changed the fracture behavior from brittle to ductile.