Phase equilibria and thermodynamics of the Mg-Si-Li system and remodeling of the Mg-Si system
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Section I: Basic and Applied Research
Phase Equilibria and Thermodynamics of the Mg-Si-Li System and Remodeling of the Mg-Si System D. Kevorkov, R. Schmid-Fetzer, and F. Zhang
(Submitted 8 June 2003) 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: 1 (Li8MgSi6), 2 (Li12Mg3Si4), and 3 (Li2MgSi). The LiMg2Si phase reported previously is not a separate phase but the interstitial solid solution of Li in the binary Mg2Si phase, LixMg2Si, 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.
1. Introduction The search for new magnesium (Mg) alloys is primarily driven by the demand for further weight reduction in automotive industry. Present Mg die casting alloys are based on the Mg-Al system. To achieve a better creep resistance, the Mg-Al based alloys should usually have a low Al-content and show the formation of stable intermetallic particles. A well-known alloying addition is silicon. Commercial MgAl-Si die casting alloys as AS41 and AS21 show improved creep resistance. On the other hand, in researched Mg-Al-Li alloys, Li addition produces a further density reduction and increases ductility. Small amounts of Li (
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