Effects of excess Mg and Si on the isothermal ageing behaviours in the Al-Mg2Si alloys

The effects of excess Mg and Si contents on the isothermal ageing processes of Al-Mg2Si alloys have been investigated by means of transmission electron microscopy and the following results were obtained. After the formation of beta" needles inducing large age hardening, cuboid beta particles precipitate in both the excess Mg and the quasi-binary alloys, but fine Si particles nucleate in the excess Si alloys. In the quasi-binary alloy, the following reaction is the precipitation of beta' rods and then beta plates form. In the excess Si alloys, various rodlike precipitates form after the precipitation of beta" needles and Si particles in the sequence: Type-A rods-->Type-B rods --> beta' rods. This precipitation sequence can be understood by considering the chemical compositions of them determined by Matsuda et al.