To control the morphology and size of the primary and eutectic Mg2Si phases in in situ Mg2Si/Al-Si composite and achieve a feasible and reliable technique to produce appropriate feedstock for the thixo-casting and rheo-casting of this type of material,three Al-Si matrix composites reinforced by 5wt.%,9wt.%and 17wt.%Mg2Si with hypoeutectic,eutectic and hypereutectic compositions were prepared by the low superheat pouring（LSP）process.The effects of the pouring temperature（superheat）on the morphology and size distribution of primary phases（primaryα-Al and Mg2Si）,binary（α-Al+Mg2Si）eutectic cell and eutectic Mg2Si were investigated.The experimental results show that low pouring temperature（superheat）not only refines the grain structure of the primaryα-Al and binary（α-Al+Mg2Si）eutectic cell in three composites and promotes the formation of more nondendritic structural semi-solid metal（SSM）slurry of these phases;but also refines the primary and eutectic Mg2Si phases,which seems to be attributed to the creation of an ideal condition for the nucleation and the acquisition of a high survival of nuclei caused by the LSP process.
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