Effects of scandium addition on iron-bearing phases and tensile properties of Al-7Si-0.6Mg alloys

Iron is the most deleterious impurity in aluminum alloys and can easily combine with aluminum to form an acicular β-Al₅FeSi phase that reduces ductility during the solidification of the molten metal. Adding scandium (Sc) to Al-7Si-0.6Mg alloys can transform the acicular β-Al₅FeSi phase into a comparatively harmless nodular Sc-Fe phase (Al₁₂Si₆Fe₂(Mg,Sc)₅). This Sc-Fe phase has a lower hardness and elastic modulus than the β-Al₅FeSi phase; it is thus less likely to initiate cracks in the Al matrix. Moreover, the nodular Sc-Fe phase can improve the fluidity of Al during solidification, reducing interdendritic shrinkage. Tensile testing measurements showed that the elongation of Al-7Si-0.6Mg alloys with 0.04 and 0.12wt% Sc can be respectively increased by 115% and 110% compared to Al-7Si-0.6Mg without Sc. The corresponding quality indices are increased by 17% and 19%, respectively, suggesting that the tensile properties of Al-7Si-0.6Mg alloys can be enhanced by adding scandium.