Refinement of Mg2Si reinforcement in a commercial Al-20%Mg2Si in-situ composite with bismuth, antimony and strontium

Refinement by addition elements of Al-Mg2Si alloys is known to result in a change of primary Mg2Si morphology. In this paper, the effects of Bi, Sb and Sr on the characteristic parameters of Al-20%Mg2Si in-situ composite have been investigated by computer aided cooling curve thermal analysis and microstructural inspection. Size, density and aspect ratio measurements showed that additions of 0.4 wt.% Bi, 0.8 wt.% Sb and 0.01 wt.% Sr refined the Mg2Si reinforcement. Exceeding these concentrations, however, resulted in coarsening of Mg2Si particles with no change in the morphology. The results also showed that addition elements caused a decrease in the nucleation and growth temperatures of Mg2Si particles. The refining effect of Bi, Sb and Sr is likely to be related to the effect of oxide bifilms suspended in the composite melt as favored nucleation substrates for Mg2Si particles.