Development of a critically evaluated thermodynamic database of magnesium alloys

Although magnesium-based materials have a long history of important commercial applications, including automotive, there remains much to be learned about the basic properties of the metal and its alloys. With the recent renewed interest in lightweight wrought materials, including both sheet and tube applications, there has been an increased focus on developing a better understanding of novel magnesium alloys, including those that incorporate additions of elements such as Si, Ce, Y, Mn, Zn, and Al. These alloy systems, along with other potential candidates, are being actively pursued as possible routes to develop magnesium materials with improved ductility, or even practical room temperature formability.;The properties of cast or wrought material depend first and foremost upon the phases and microstructural constituents (eutectics, precipitates, solid solutions, etc.) which are present. In an alloy with several alloying elements, the phase relationships are very complex. In order to investigate and understand these complex phase relationships effectively, it is very useful do develop thermodynamic databases containing model parameters giving the thermodynamic properties of all phases as functions of temperature and composition. Using Gibbs free energy minimization software such as FactSage(TM), the automotive and aeronautical industries and their suppliers will be able to access the databases to calculate the amounts and compositions of all phases at equilibrium at any temperature and composition in multicomponent alloys, to follow the course of equilibrium or non-equilibrium cooling, to calculate corresponding heat effects, etc.;Such thermodynamic databases are prepared by critical evaluation, modeling, and optimization. In a thermodynamic "optimization" adjustable model parameters are calculated using, simultaneously, all available thermodynamic and phase-equilibrium data in order to obtain one set of model equations as functions of temperature and composition. Thermodynamic data, such as activities, can aid in the evaluation of the phase diagrams, and information on phase equilibria can be used to deduce thermodynamic properties. Thus, it is frequently possible to resolve discrepancies in the available data. From the model equations, all of the thermodynamic properties and phase diagrams can be back-calculated, and interpolations and extrapolations can be made in a thermodynamically correct manner. The data are thereby rendered self-consistent and consistent with thermodynamic principles, and the available data are distilled into a small set of model parameters, ideal for computer storage.;As part of a broader research project being conducted at the Centre de Recherche en Calcul Thermochimique (CRCT) at Ecole Polytechnique, Montreal to develop a comprehensive thermodynamic database for Mg-alloys with 25 potential alloying elements, the present work deals with the critical evaluation and optimization of the Si-Zn, Ce-Si, Y-Si, Mn-Si, Al-Mn, Mg-Si-Zn, Mg-Ce-Si, Mg-Y-Si, Mg-Mn-Si and Mg-Al-Mn systems.;All the binary systems have been critically evaluated and optimized based upon available phase-equilibrium and thermodynamic data. The model parameters obtained as a result of simultaneous optimization have been used to represent the Gibbs energies of all phases as functions of temperature and composition. Optimized binary model parameters were combined with those of previously optimized (at the CRCT, Ecole Polytechnique) binary systems to estimate the thermodynamic properties of ternary solutions in the Mg-Si-Zn, Mg-Ce-Si, Mg-Y-Si, Mg-Mn-Si and Mg-Al-Mn systems. Proper "geometric" models were used for these estimations. Ternary phase diagram were calculated and compared with experimental data where available. Usually, the available ternary data were well reproduced with only the binary model parameters. These phase diagrams predictions will be helpful in future planning of experiments for the detailed study of these ternary systems.;The liquid phase of the binary systems optimized in the present work exhibit considerable short-range ordering as evidenced by the very negative "V-shaped" enthalpy of mixing curves. Previous optimizations of these systems were based upon a Bragg-Williams random-mixing model for the liquid phase. The use of the Bragg-Williams model in liquids with a high degree of short-range ordering often results in unsatisfactory results and in poor predictions of ternary properties from binary model parameters. In the present work, the Modified Quasichemical Model which is capable of taking short-range ordering into account has been used. Furthermore, the present optimizations and evaluations take into account experimental data which were not considered in the previous optimizations.