2005 Volume 46 Issue 4 Pages 880-884
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 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.