Phase equilibria and thermodynamics of the Mg-Si-Li system and remodeling of the Mg-Si system

Abstract

The ternary Li-Mg-Si phase relations were established using high-purity samples prepared by levitation melting. Analysis was done using x-ray powder diffraction (XRD), optical metallography, and differential thermal analysis (DTA) in customized sealed tantalum crucibles. Three ternary intermetallic phases were confirmed in this system: τ ₁ (Li₈MgSi₆), τ ₂ (Li₁₂Mg₃Si₄), and τ ₃ (Li₂MgSi). The LiMg₂Si phase reported previously is not a separate phase but the interstitial solid solution of Li in the binary Mg₂Si phase, Li _x Mg₂Si, at the maximum x ≈ 0.91 ± 0.05. Ternary liquidus data and phase transformations were also measured. A thermodynamic assessment of the ternary Li-Mg-Si system was worked out in parallel to the experimental study and used to select the relatively small number of our key experiments. The final consistent thermodynamic description is well supported by the present experimental data. It was found that the published thermodynamic parameter sets for the binary Mg-Si subsystem produce an artificial inverted liquid miscibility gap at higher temperatures. This system was remodeled to obtain data that could be reliably extrapolated to various temperatures and the ternary system. The experimentally supported calculated phase equilibria in the entire composition and temperature range of the Li-Mg-Si system are presented, including the liquidus surface and invariant reactions. Gradual changes in a monovariant eutectic/peritectic reaction type occur in this system. The intricacies involved due to solid solutions and the failure of the classic tangent criterion are discussed for some ternary alloys.