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 fab ricated by conventional casting. In the present study, hypereutectic Al-Si based alloys containing Fe and Ni were produced via a powder metallurgy tec hnique employing rapid solidification resulting in powders with refined mic rostructures. The effect of powder size on the primary silicon size, and mi crostructural evolution during degassing were studied. Microstructural inve stigations were carried out by means of optical microscopy, field emission scanning electron microscopy, x-ray diffraction and transmission electron m icroscopy. The formation of coarse silicon blocks was suppressed, and the s ize of the primary silicon particles decreased linearly with decreasing pow der size. The eutectic silicon was modified from a plate-like morphology, c haracteristic of conventional cast alloys, to fine silicon crystals distrib uted on the aluminum dendrites. During degassing, the silicon crystals chan ge to a particulate morphology which is characteristic of the final extrude d alloys. Additionally, in Fe-containing alloys, needle-like Al9FeSi was id entified, which changes to equilibrium Al5FeSi during degassing. In Ni-cont aining alloys, the Al3Ni and Al3Ni2 phases dissolve substantial levels of C u and Mg.