The microstructure and the precipitation behavior of 25wt% $SiC_p$/6061Al composites were investigated using primarily thermal analysis and transmission election microscopy, in close comparison with those of 6061Al alloy. Optical microscopy and microhardness testing were conducted as additional aids.
Observation showed the presence of high density of dislocation near the interface of SiC particulates which is usually severely curved or highly tangled. This is believed to be due to the large difference of thermal expansion coefficient between SiC and metal matrix. Although there appears no significant difference in the precipitation sequence between $SiC_p$/6061Al composite and 6061Al alloy, the overall precipitation kinetics, in particular precipitation kinetics of β', were greatly accelerated in the case of composite. This is believed to be due to the influence of dislocation on the β' precipitation by acting as a preferential nucleation site or high diffusional path. Slow quenching rate greatly retarded the precipitation kinetics of β" phase in the case of composite, while no such effect was observed in the 6061Al case. We believed that this is because the precipitation of β' phase occurs at dislocation during quenching in the composite case and therefore reduces the solute supersaturation. Results also showed that cold rolling of 6061Al alloy tends to accelerate the precipitation kinetics. This partly explains the reason for the accelerating aging kinetics in the composite as compared to the matrix alloy.
