Effect of particle volume fraction on intermediate phase precipitates in SiC particle dispersed Al-Mg-Si alloy composites

Aging process of Al-1.0 mass%Mg₂Si alloy composite materials with 4 vol% and 8 vol%SiC particles was investigated by micro-Vickers hardness measurement and transmission electron microscope observation. When SiC particles are dispersed into the Al-Mg-Si alloy, aging time to reach the maximum hardness is shortened at 473 K. The over-aged composite material with 4 vol%SiC particles includes various kinds of metastable phase precipitates; namely β′ phase, the TYPE-B and TYPE-C precipitates of which crystal structures are different to each other. Fractions of these three kinds of precipitates in the composites are the same as in the mother alloy containing 0.2 mass% excess Si. The composite material with 8 vol%SiC particles contains the TYPE-A, TYPE-B and TYPE-C precipitates, but is free from β′ phase precipitates. The existence of those precipitates has correspondingly been observed in the mother alloy containing 0.4 mass% excess Si. The estimation that the oxide (SiO₂) layer must have formed on the surface of each SiC particle with the maximum thickness of about 17 nm can be deduced from the assumption that a considerable portion of excess Si originates from the oxide layer on the surface of SiC particles in the composite materials.