Effects of Si Solution on Stability of Early 3d Transition-Metal Tri-Aluminides, Al₃T (T = Sc, Ti and V)

Addition of the early 3d transition metals results in formation of primary Al₃T (T = Sc, Ti and V) phases in Al alloys during casting. The newly formed Al₃T particles not only improve the mechanical performance of the products but also act as grain-refiners in the solidification processes. Meanwhile, experiments found impacts of impurities, such as Si, on the formation of the Al₃T phases; the mechanism is not fully understood. We here investigate effects of Si solution on the stability and crystal chemistry of the Al₃T phases using first-principles density-functional theory. The study has revealed a rich variety of effects of Si solution on the Al₃T phases. Si solution stabilizes the D0₂₂-Al₃Ti structure so that it becomes the ground state, taking over the binary D0₂₃ phase. Si solution in D0₂₂-V occurs only at elevated temperature. Si solution has little impact on the Al₃Sc phase relationship. The obtained information helps characterize the (Al, Si)₃ T particles in Al products, understand their role in solidification and further design new Al alloys of desirable properties.