Thermodynamic modeling of the magnesium-aluminum-bismuth and magnesium-aluminum-antimony systems

In recent years the magnesium application in automobile sector has been increasing. Recent research and development studies of magnesium and its alloys have focused on weight reduction, energy saving and limiting environmental impact. In this regard researchers have identified incorporating certain key alloying elements like Al, Zn, Si, Pb, Ge, Bi, Sb, Sr, etc. to the magnesium alloys. These alloying elements are seen as potential elements for novel magnesium alloy design with superior structural properties to the existing alloys used for automotive applications.;The present work is the part of wide thermodynamic database development project for Mg-Sn-Al-Zn-Si-Mn-Pb-Ge-Sb-Bi-Ge-etc. system. In this regard, for the database development four binaries: Mg-Bi, Mg-Sb, Al-Bi and Al-Sb were 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 binary systems to estimate the thermodynamic properties of ternary solutions in the Mg-Al-Bi and Mg-Al-Sb 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 continued developments in CALPHAD (Chapter 2) assessment of alloys (and other materials) brought about the development of thermodynamic databases. These databases contain the Gibbs free energy description of all the phases present in a system with respect to temperature and composition. With the use of Gibbs energy minimization software such as FactSage one can have access to these databases to calculate the amounts and compositions of all phases at equilibrium at any temperature and composition in multicomponent alloys, the amount of phases appearing during equilibrium or non-equilibrium cooling can be calculated too. 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 can be stored as set of model parameters in databases which can be stored in computers.