| The thermodynamic database for the Mg-Al-Ca-Sr-Zn quinary system was constructed with the combined CALPHAD, first-principles calculations, and experiments approach. There are ten binary systems, six of which were investigated in detail to develop their thermodynamic descriptions. There are ten ternary systems, in which Mg-Al-Ca ternary system was investigated. They were evaluated using Thermo-Calc, the software developed at The Royal Institute of Technology, Sweden.; The Ca-Sr system was modeled by using random solution models. Al-Mg binary system was remodeled with the help of the first-principles calculation results for gamma-Al12Mg17, epsilon-Al30Mg 23, and three laves phases at the Al2Mg composition. In modeling of the Al-Sr system, both random solution and associate models were applied to the liquid phase. It was also demonstrated for the Al-Sr system that the first-principles calculations provide reliable enthalpies of formation for stoichiometric compounds. Al3Sr8 phase is predicted and added into the Al-Sr system. The Ca-Mg system were modified with two-sublattice model (Mg,Ca)2(Mg,Ca) for the Mg2Ca laves-C14 phase. First-principles calculations provide reliable enthalpies of formation for non-stable end-members of the Mg2Ca phase. All compounds in the Mg-Sr binary system were calculated with first-principles calculations. The Mg-Sr thermodynamic database was constructed with the prediction that Mg 38Sr9 may be not stable at low temperature. The Sr-Zn system was modeled using the random solution model for liquid.; The constructed quinary database was used to calculate the liquidus projections of the ternary systems, i.e. Mg-Al-Ca, Mg-Al-Sr, Mg-Al-Zn, Mg-Ca-Sr, Mg-Ca-Zn, Mg-Sr-Zn, Al-Ca-Sr, Al-Ca-Zn, Al-Sr-Zn, and Ca-Sr-Zn. Their primary crystallization fields, invariant reaction points, and possible ternary laves phases were predicted. First-principles calculations were performed for ternary laves phases in the Mg-Al-Ca system with the help from Special Quasirandom Structures (SQS's). The existence of (Mg,Al)2Ca C36 phase is predicted by first-principles calculations and added into the Mg-Al-Ca database, which shows a good agreement with existing experimental data. The existence of the ternary Al2(Mg,Ca) phase predicted by first-principles calculations was verified by diffusion couples between pure Al and a Mg-Ca alloy. (Abstract shortened by UMI.)... |
