2002 Volume 43 Issue 2 Pages 246-255
The precipitation behaviours during continuous heating in the Al–Mg2Si alloys with excess Mg and Si contents have been investigated by means of Vickers hardness measurements, differential scanning calorimetry and transmission electron microscopy. The first exothermic reaction occurring at the lowest temperatures is due to the annihilation of quenched-in vacancies by the collapse of vacancy clusters and the migration of vacancies to various sinks such as grain boundaries. The prismatic dislocation loops formed by this collapse are frequently observed. The second reaction detected as broad hardening increases with increasing excess Si content and can be interpreted as the formation of solute atom clusters. The sharp and large exothermic reaction inducing the largest hardening corresponds to the precipitation of β′′ needles. The following exothermic reaction arises from the precipitation of β-Mg2Si particles. The formation of β′ and Type-B rods can be recognised in the quasi-binary and the excess Si at slightly higher temperatures.