HRTEM observation of metastable phase in Al-Mg₂Si alloys

High resolution electron microscopy (HRTEM) was performed for an aged Al-1.0 mass%Mg₂Si alloy to clarify the morphology and crystal system of the precipitates that occur prior to the β′ intermediate phase. At the early stage of aging, there observed a number of precipitates with irregular arrangement of bright dots in their cross-sections of the HRTEM images. With increasing aging time, the precipitates with the irregular arrangement of bright dots (the random type) decrease and the precipitates with the parallelogram network arrangement of bright dots in their cross-sections (the parallelogram type) begin to increase. The parallelograms have interior angles between 62° and 89°. The 30% of them had a fixed angle of 75° (the 75° parallelogram type). In the over aging condition, the number density of the parallelogram type precipitates decrease, and the β′ intermediate phase becomes predominant. There are three types of precursory precipitates, the random, the parallelogram and the 75° parallelogram types, prior to the β′ phase at all aging temperatures investigated in Al-0.6 mass%Mg₂Si, Al-1.0 mass%Mg₂Si and Al-1.6 mass%Mg₂Si alloys. The compositional region, where the above three types of precipitates are observed, is in good agreement with the previously reported region for G.P. (2) zone in the equilibrium phase diagram of the Al-Mg₂Si pseudo-binary alloy.