Influence of excess si on the morphology and thermal stability of metastable precipitates formed in an Al-Mg-Si alloy

The influence of excess Si on the precipitation behaviour in an Al-Mg-Si alloy was studied by means of Vickers microhardness tests, high-resolution transmission electron microscopy (HRTEM) and differential scanning calorimetry (DSC). The metastable-phase precipitates formed during isothermal ageing up to the time to peak hardness were investigated from the viewpoints of the morphology, the density of precipitates and the change in the exothermic heat caused by the formation of precipitates in the DSC measurements. The changes in the density of precipitates and the exothermic heat with excess Si at peak ageing time showed a similar tendency to that of the peak hardness curves. Before the formation of the beta', three exothermic peaks of the metastable-phase precipitates appeared in the DSC curves of the alloy specimens isothermally aged for a short time. It was revealed that the formation of precipitates, which were named as Type-1, -2 and -3 in this work, corresponded to the three exothermic peaks.