Observation of age precipitates in Al₂O₃ particle dispersed Al-Mg₂Si alloy composite materials

Aging process of Al-1.0 mass% Mg₂Si alloy composite materials with 4 vol% and 8 vol% Al₂O₃ particles was investigated by Vickers micro-hardness measurement and transmission electron microscope observation. When Al₂O₃ particles are dispersed into an Al-Mg-Si alloy, the age-hardenability was obviously lowered above at 473 K aging. The rectangular shaped spinels were observed at the interface between Al₂O₃ particles and the matrix. The over-aged composite material with 4 vol% Al₂O₃ particles includes various kinds of metastable phase precipitates: namely β′ phase, the TYPE-B and TYPE-C precipitates of which crystal structures are different each other. Fraction of the three kinds of precipitates are the same as in the mother alloy containing 0.2 mass% excess Si. The composite material with 8 vol% Al₂O₃ 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.