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Morphology and thermal stability of metastable phase precipitates formed in a balanced (Mg/Si=2) Al-Mg-Si alloy were studied by means of high-resolution electron microscopy (HRTEM) observations and differential scanning calorimetry (DSC) measurements. The present study revealed that the four types of precipitate morphology were found in the HRTEM images, that the heat change of the exothermic peak P in DSC measurements was mainly caused by the formation of type-2 and 3 precipitates in morphlogy and that the heat change of the exothermic peak Q which was yielded after the peak P probably corresponded to the formation of type-4 precipitates .

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The influence of excess Si on the precipitation behaviour in an Al-Mg-Si alloy was studied by means of Vickers microhardness tests, highresolution 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 of 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. Four exothermic peaks of meatastable-phase precipitates appeared in the DSC curves of the alloy specimens isothermally aged for a short time. It was revealed that the precipitates named type-1, -2 and -3 in this work, correspond to the exothermic peaks (1), (2) and (3), respectively.