First-Principles Study on Silicon Stabilization of the Cubic α- and Hexagonal α’-FeAl Phases

Commercial aluminum (Al) metals contain unavoidable impurities, such as iron (Fe) and silicon (Si). Due to its low solubility and high chemical affinity to Al, Fe exists in the form of Fe-containing intermetallic compounds (Fe-IMCs), which are crucial in solidification processes, determining the micro-structure and consequently the mechanical performance of the cast parts. Meanwhile, Si, as an impurity or addition, may join the binary Fe-IMCs. Here, we investigate the Si stabilization effects on the frequently observed Al-rich Fe-IMCs in a comprehensive and systematic way using a first-principles density-functional theory (DFT) approach. The study reveals different Si stabilization effects on the cubic α- and hexagonal α’-phase, as well as other binaries: Al12Fe, η-Al6Fe, τ4-, β-, and θ-phases. The enhancement of stability for the α-phase is moderate, while it is strong for the α’-phase. For the stability series (from higher to lower) is θ-Al13Fe4 > η-Al6Fe > α-Al4.75Fe in the binary system, while it becomes τ4-(Al,Si)5Fe > β-Al4.5SiFe > α’-(Al,Si)4.174Fe for the ternary Fe-IMCs. The information obtained here helps understand the formation of Fe-IMCs particles during casting of Al-Si alloys, and the design of novel Al alloys of fine micro-structures and desired mechanical performances of the products from the primary Al and the scraps and wastes.