Investigation in Mg-Al-Ca-Sr-Zn System by Computational Thermodynamics Approach Coupled with First-Principles Energetics and Experiments

Open Access
Author:
Zhong, Yu
Graduate Program:
Materials Science and Engineering
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
July 20, 2005
Committee Members:
  • Vincent Henry Crespi, Committee Member
  • Jorge Osvaldo Sofo, Committee Member
  • Long Qing Chen, Committee Member
  • Zi Kui Liu, Committee Chair
Keywords:
  • Computational Thermodynamics
  • Mg Alloy
  • Creep Resistance
  • First-Principles
Abstract:
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 g-Al12Mg17, e-Al30Mg23, 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 Mg38Sr9 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. The database was used to understand the creep resistance of Mg alloys at elevated temperatures by study phase stabilities through Scheil simulations and equilibrium calculations. The addition effect of Ca, Sr, and Zn on AM50 alloy is discussed and the available experimental observations in Mg-Al-Ca, Mg-Al-Sr, and Mg-Al-Zn alloys were successfully explained. The recent discovery of C36 phase in the as-cast samples of GM-C alloys was also explained. The combined effect of Ca, Sr, and Zn on AM50 alloy was predicted. The addition of Ca and Sr should be helpful to the creep resistance of Mg-Al-Zn alloys with the precipitation of Al2Ca-C15, C36, and Al4Sr.