Microstructural evolution in P/M hypereutectic Al-Si alloys containing Fe and Ni

Hypereutectic Al-Si alloys usually form a coarse, brittle Si phase when fabricated by conventional casting. In the present study, hypereutectic Al-Si based alloys containing Fe and Ni were produced via a powder metallurgy technique employing rapid solidification resulting in powders with refined microstructures. The effect of powder size on the primary silicon size, and microstructural evolution during degassing were studied. Microstructural investigations were carried out by means of optical microscopy, field emission scanning electron microscopy, x-ray diffraction and transmission electron microscopy. The formation of coarse silicon blocks was suppressed, and the size of the primary silicon particles decreased linearly with decreasing powder size. The eutectic silicon was modified from a plate-like morphology, characteristic of conventional cast alloys, to fine silicon crystals distributed on the aluminum dendrites. During degassing, the silicon crystals change to a particulate morphology which is characteristic of the final extruded alloys. Additionally, in Fe-containing alloys, needle-like Al9FeSi was identified, which changes to equilibrium Al5FeSi during degassing. In Ni-containing alloys, the Al3Ni and Al3Ni2 phases dissolve substantial levels of Cu and Mg.