Mg 2Si/ZM5 Mg composites were prepared by vacuum induction furnace under the protection of Ar gas atmosphere. The effects of Si on as cast microstructure and properties of in situ Mg 2Si/ZM5 composites were observed and analyzed by means of OM

SEM

EDAX and XRD as well as tensile testing. The results showed that Mg 2Si intermetallic compound with high melting point and high hardness formed by additive reactant Si reacting on ZM5 base alloy can greatly improve strength of the composites at both ambient and elevated temperatures. The tensile strength and yield strength at ambient temperature increased by 20.1% and 61.5% respectively with 1.5% Si addition

and at elevated temperature increased by 14.9% and 25.7% respectively with 0.5% Si addition. Morphology of Mg 2Si phase varied with Si content. It is fine short rod like or flake eutectic Mg 2Si with adding 0.5% Si

and become bulk or Chinese script primary Mg 2Si plus flake or short rod like eutectic Mg 2Si with increasing more than 1.0%Si addition. Owing to Mg 2Si brittle phase

the Mg 2Si/ZM5 matrix composite presented the cleavage fracture

resulting in decreasing its plasticity.