Thermodynamic modeling of aluminum-magnesium-rare earth systems

The main objective of this thesis is to build a thermodynamic database for Al–Mg–La–Ce–Pr–Nd–Sm–Gd–Tb–Dy–Ho–Er systems. To this end, thermodynamic models of liquid and solid solutions and stoichiometric compounds have been proposed; phase diagrams and thermodynamic properties of binary Al–RE, Mg–RE, RE'–RE" systems, ternary Mg–Al–RE and Al– RE'– RE" systems have been optimized based on experimental and/or theoretical data.;First, Al–RE (RE= La, Ce, Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er) binary systems were optimized, based on literature review for the solids (intermetallic compounds, their crystal structures, melting points, enthalpy of formation, transformation temperature, etc), liquid properties (integral enthalpy of mixing, partial enthalpy of mixing, activities of the components, heat capacities, solid solubility and so on) and experimental phase diagram data. A systematic approach has been used in the course of thermodynamic optimizations. Since rare earths are chemically similar, comparision with neighboring rare earth elements and systematic analysis of melting point of compounds helped in the optimization of the Al–Tb system, in which few experimental data are available.;Second, for the Mg–RE systems in which the experimental data were scarce or contradictory, First-Principles were employed to calculate the enthalpy of formation of certain compounds and enthalpy of mixing for the FCC and HCP phases using the supercell technique. Meantime, estimations from the semi-empirical Miedema model were made and compared to First-Principles calculations. It was suggested that the semi-empirical Miedema model can provide a good initial guess for the enthalpy of formation of compounds and the enthalpy of mixing for solutions for thermodynamic optimizations, without high computing cost. Therefore, the Miedema model was used to estimate the enthalpy of formation of the ternary compounds and metastable phases in Mg–Al–RE ternary systems.;Third, as for Mg–Al–RE ternary systems, less research has been done on the thermodynamic properties and phase equilibria of these systems compared to other metallic systems. Thermodynamic evaluations and optimizations of the Al–Mg–RE (RE= La, Ce, Pr, Nd, Sm) systems have been systematically carried out on the basis of literature information and our experimental results. In this work, key experiments were performed for the Mg–Al–La, Mg–Al–Ce, Mg–Al–Pr, and Mg–Al–Nd systems to check phase equilibria in the Mg-rich corners and solid solubilities. The estimations of enthalpy of formation for the ternary compounds from Miedema model were used for thermodynamic optimizations of all the Mg–Al–rare earth systems. In the Mg–Al–Gd (Dy, Ho) systems, careful weighting was made between the experimental data (liquidus temperature) and optimization results, considering the restricted accuracy of experimental results at high temperatures (1200 K and more). It was judged unnecessary to use many excess ternary parameters for the liquid to fit satisfactorily the experimental data. The Mg–Al–Tb system was estimated by assuming similarities to the Mg–Al–Gd (Dy) systems and is presented for the first time in the present study. The Kohler-type interpolation method was used, seeing that the enthalpies of mixing of the liquid in the Al–Mg, Al–RE, and Mg–RE systems differ significantly.;Fourth, RE'–RE" (La–Ce, La–Pr, La–Nd, Ce–Pr, Ce–Nd, Pr–Nd) binary systems were optimized based on the available experimental information. Except for the La–Nd and Ce–Pr systems, no excess parameters were used for the liquid. For the Al–RE'–RE" systems, experimental data are available only for the Al–Ce–Nd system. Other Al– RE'–RE” systems were optimized assuming thermodynamic properties similar to those of the Al–Ce–Nd system. (Abstract shortened by UMI.).