Effects of compositions on the two-step ageing of AI-Mg-Si alloys

Abstract

Effects of Mg (0.20.9 at.%) and Si (0.10.6 at.%) contents and trace of additional elements (0.010.2 at.%) on the two-step ageing phenomena of AI-Mg-Si alloys were studied by the measurements of hardness and electric resistivity and also by electron microscopy.The phenomena of two-step ageing of AI-Mg-Si alloys highly depended upon the contents of Mg2Si and additional elements as well as the conditions of pre-ageing. The pre-ageing led to either refining or coarsening of acicular precipitates in finally aged condition, according to the composition of alloys. Consequently, it resulted in either increase or decrease (negative effect) in strength of the alloys.For AI-Mg-Si alloys containing more than about 1.1 at.% of Mg2Si the pre-ageing at room temperature caused coarsening of finally aged structure but it caused refining of precipitates for the alloys containing less than about 0.9 at.% of Mg2Si. The negative effect of pre-ageing is interpretated as follows. It is based on the deleterions effect of impoverishment of solute atoms in matrix resulting from the clustering at room temperature, forming small clusters which modestly contribute to the strength.The negative effect of pre-ageing of Al-0.70 at.% Mg-0.35 at.% Si (1.05 at.% Mg2Si) alloy was decreased by the addition of transition elements such as Mn, Cr, Zr, V, and Fe, but increased by the addition of Ag, Cu, Be, Cd, and Zn. This may be caused by fast that the apparent supersaturation of Mg2Si in the alloys is increased by the latter elements, but decreased by the former elements which easily form insoluble compounds with Si and Al atoms.