Influence of Al2O3 particulate on the aging response of A356 Al-based composites

A356 Al alloy reinforced with Al2O3 particulates of different sizes (25 μm angular or 40 μm microsphere) and volume fractions (5, 10 and 20 vol.%) was prepared using vortex method. The influence of Al2O3 particulates on the precipitation and hardening behavior of the A356 Al–Al2O3 composites was studied. It was found that the MgAl2O4 spinel formed at the interface led to Mg depletion in the matrix and subsequently to lesser age hardening in the composites. Therefore, it was necessary for the composite matrix to have a higher Mg concentration prior to casting to achieve the same level of hardening in the composite as in the unreinforced. Different amounts of Mg, depending upon the volume fraction of the Al2O3 particulates, were added into the composite melts. The results showed that the hardening kinetics was enhanced by Al2O3 particulate additions because hardening precipitates preferentially developed on the dislocation lines that increased due to coefficient of thermal expansion mismatch between the matrix and reinforcement. However, the presence of reinforcement did not alter precipitation sequence of the investigated composites.