THERMODYNAMIC PROPERTIES OF MG BASED ALLOYS BY CALPHAD APPROACH COUPLED WITH FIRST-PRINCIPLES: APPLICATION TO MG-AL-CA-CE-SI SYSTEM

Open Access
- Author:
- Zhang, Hui
- Graduate Program:
- Materials Science and Engineering
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- August 27, 2010
- Committee Members:
- Dr Zi Kui Liu And Dr Long Qing Chen, Dissertation Advisor/Co-Advisor
Zi Kui Liu, Committee Chair/Co-Chair
Long Qing Chen, Committee Chair/Co-Chair
Jorge Osvaldo Sofo, Committee Member
Digby D Macdonald, Committee Member - Keywords:
- Mg alloys
Intermetallics
First-principles
Calculation of phase diagram - Abstract:
- In the last few decades, first-principles quantum mechanics technique based on density functional theory has progressed significantly and demonstrated in many cases, e.g., the accurate predictions of thermodynamic properties. Within the similar time frame, computational thermodynamics based on CALPHAD (CALculation of PHAse Diagram) technique is emerging as a key technology, which is used to predict the phase equilibrium, phase stability, and phase transformation. In order to accelerate the understanding and design of magnesium alloys, an integrated first-principles calculations and CALPHAD modeling approach is used to study the elastic and thermodynamic properties of Mg-containning alloys. The present project predicted the thermodynamic properties of Mg-based binary alloys from first-principles calculations. The obtained enthalpies of formation at 0 K were discussed for the Mg-X systems with X being the elements of As, Ba, Ca, Cd, Cu, Dy, Ga, Ge, La, Lu, Ni, Pb, Sb, Si, Sn and Y. By considering the phonon and thermal electronic contributions, first-principles are extended to predict the properties at finite temperatures. In the preset thesis, the phonon properties and the temperature dependent elastic and thermodynamic properties are also presented for the technically important ã-Al12Mg17 phase. The thermodynamic description for the Ce-Mg binary system was updated using the CALPHAD approach by incorporating additional experimental data. The resulting thermodynamic description for the Mg-Ca-Ce system was then developed by combining the derived databases of the Ca-Ce and Ce-Mg systems from the present work with that of the Ca-Mg system in the literature. We obtained the equilibrium structural properties of Mg, Si, beta-Mg2Si, beta’-Mg18Si10 and beta’’-Mg5Si6 as well as the enthalpies of formation of beta, beta’ and beta’’ in Al-Mg-Si ternary system. Using frozen-phonon calculations, we also ascertained the vibrational entropies of formation for each phase. The phase boudaries of beta, beta’ and beta’’ were thus predicted for Al-Mg-Si ternary system. To better understand the alloying behavior of Ca on Mg-Al alloys, the thermochemical properties of the Laves phases in the Mg-Al-Ca system are evaluated from both first-principles and CALPHAD modeling. First-principles provided insight into the understanding of phase stabilities of Laves phases, and herein developing self-consistent thermodynamic database of this ternay system by CALPHAD modeling.