Microstructural characterisation of sand cast aluminium alloy A356–SiC particle metal matrix composite

Abstract

The microstructure of a cast aluminium alloy A356 reinforced with 15 vol.-%SiC particles has been investigated using analytical microscopy. It is shown that the morphology of the silicon phase, as well as that of the Al–Si eutectic structure, which are the features of this alloy system, are dramatically changed by the presence of the SiC particles. Significant effects of grain refinement werefound to occur not only on the primary aluminium grains, but also on the primary and eutectic silicon phases. Twinning and dislocations were often observed within that silicon phase which was situated adjacent to the SiC particles. Microcracks were also observed at the SiC/silicon interface. The presence of such microcracks suggests that a stress concentration had developed at the silicon/SiC particle interfaces, probably as a result of the thermal expansion mismatch occurring between the silicon and SiC particles. The microcracks which consequently develop are formed as a result of the poor silicon–SiC particle bonding. Other intermetallics, notably Mg₂Si and FeSiAl₅, which exhibit cubic and tetragonal symmetries respectively, were also identified as being present in the microstructure.

MST/1460