Effect of silver addition on formation of secondary phases in squeeze cast Al - Cu - Mg alloys

摘要

The effect of silver addition on the formation of secondary phases in squeeze cast Al - 4 centre dot 0Cu - 1 centre dot 5Mg and Al -4 centre dot 0Cu -1 centre dot 5Mg - 0 centre dot 7Ag (all wt- percent) 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. Al_2Cu, Al_2CuMg, 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-Al_2Cu-Al_2CuMg eutectic in Al-4 centre dot 0Cu-1 centre dot 5Mg and L->L' +Al_2Cu->#alpha#-Al-Al_2Cu-Al(Cu_(0 centre dot 75Ag_(0 centre dot 25))Mg eutectic in Al - 4 centre dot 0Cu - 1 centre dot 5Mg - 0 centre dot 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, Al_2Cu, 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 approx 28)Cu - (9 approx 10)Fe and Al - (26 approx 27)Cu - (7 approx 9)Fe (all at.- percent) respectively.