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Al matrix composites reinforced with in situ ceramic phases by adding 20% SiO 2 were fabricated by powder metallurgy process. The interfacial microstructure of the composites was studied by means of SEM and HREM. It was shown that the ceramic phases mainly composed of spinel MgAl 2O 4 are formed in situ in the original SiO 2 particle and the size of small MgAl 2O 4 crystallites is about dozens of nanometers, which can adjacent to Al and Si. MgO could not found in original SiO 2 particle but a little in matrix and may exist with Si, Mg 2Si and Al. Si is mostly distributed in matrix and forms some segregation zones. The size of Mg 2Si is about 50～100 nm and can usually be seen in the matrix.
Al matrix composites reinforced with in situ ceramic phases by adding 20% SiO 2 were fabricated by powder metallurgy process. The interfacial microstructure of the composites was studied by means of SEM and HREM. It was shown that the ceramic phases mainly composed of spinel MgAl 2O 4 are formed in situ in the original SiO 2 particle and the size of small MgAl 2O 4 crystallites is about dozens of nanometers, which can adjacent to Al and Si. MgO could not found in original SiO 2 particle but a little in matrix and may exist with Si, Mg 2Si and Al. Si is mostly distributed in matrix and forms some segregation zones. The size of Mg 2Si is about 50 ~ 100 nm and can usually be seen in the matrix.