Effect of sliver addition on formation of secondary phases in squeeze cast Al-Cu-Mgalloys

Abstract

The effect of silver addition on the formation of secondary phases in squeeze cast Al-4.0Cu-1.5Mg and Al-4.0Cu-1.5Mg-0.7Ag (an wt-%) alloys has been investigated using optical microscopy, scanning electron microscopy, energy dispersive X-ray spectroscopy, X-ray diffractometry, and transmission electron microscopy. The as cast microstructure of both alloys consists of primary dendritic alpha -Al and various types of secondary solidification phase, e.g. Al2Cu, Al2CuMg, Al(Cu,Ag)Mg, and icosahedral (I) and decagonal (D) quasicrystalline phases. However, the solidification path in the interdendritic region during squeeze casting is different for each alloy, i.e. L-->ternary alpha -Al-Al2Cu-Al2CuMg eutectic in Al-4.0Cu-1.5Mg and L-->L' Al2Cu-->alpha -Al-Al2Cu-Al(Cu0.75Ag0.25) Mg eutectic in Al-4.0Cu-1.5Mg-0.7Ag. This indicates that silver acts as an alloying element stabilising the formation of Al(Cu,Ag)Mg Laves phase. The remaining copper and iron rich liquid in the interdendritic region at the final stage of solidification solidifies into a mixed structure of alpha -Al, Al2Cu, and AlCuFe I (or D) phases. The composition of the I and D phases, measured by energy dispersive X-ray spectroscopy, is in the range Al-(27 similar to 28)Cu-(9 similar to 10)Fe and Al-(26 similar to 27)Cu-(7 similar to9)Fe (all at.-%) respectively.