A computational study of the effects of alloying elements on the thermodynamic and diffusion properties of Mg alloys

Open Access
Author:
Zhou, Bicheng
Graduate Program:
Materials Science and Engineering
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
October 13, 2015
Committee Members:
  • Zi Kui Liu, Dissertation Advisor
  • Zi Kui Liu, Committee Chair
  • Long Qing Chen, Committee Member
  • Jorge Osvaldo Sofo, Committee Member
  • Tarasankar Debroy, Committee Member
Keywords:
  • Mg alloys
  • thermodynamics
  • diffusion
  • thermodynamic modeling
  • CALPHAD
  • first-principles calculations
  • DFT
  • kinetics
Abstract:
In recent years, magnesium (Mg) alloys have received an increasing interest due to their low density, earth abundance, high specific strength, and good castability. These properties make Mg alloys attractive for automotive, aerospace, and other light-weight structural applications. The majority of Mg alloys derives their mechanical properties from precipitation hardening, while the study of precipitation process demands accurate thermodynamic and kinetic (diffusion) properties. In this dissertation, two computational techniques, the CALculation of PHAse Diagram (CALPHAD) modeling and first-principles calculations, have been employed to understand the effects of various alloying elements on the thermodynamic and diffusion properties of Mg alloys. Thermodynamics and phase stability of two Mg ternary alloy systems, Mg-Sn-Sr and Mg-Ce-Sn, have been investigated through use of the CALPHAD modeling technique. They have the potential to be used for high-temperature applications due to the highly stable Mg2Sn as the main precipitate phase. The thermodynamic modeling is supplemented by finite temperature first-principles calculations based on density functional theory (DFT) using the quasi-harmonic phonon calculations and the Debye model with inputs from first-principles calculations. The associate solution model is used to describe the short-range ordering behavior in the liquid phases of these two alloy systems. To better understand the diffusion properties of Mg alloys, the self-diffusion and solute (impurity) diffusion coefficients of 61 alloying elements in hcp Mg are calculated from first-principles by combining transition state theory and an 8-frequency model. The minimum energy pathways and the saddle point configurations during solute migration are calculated with the climbing image nudged elastic band method. Vibrational properties are obtained using the quasi-harmonic Debye model with inputs from first-principles calculations. An improved generalized gradient approximation of PBEsol is used in the present first-principles calculations, which is able to well describe both vacancy formation energies and vibrational properties. It is found that the solute diffusion coefficients in dilute hcp Mg are roughly inversely proportional to bulk modulus of the dilute alloys, which reflects the solutes’ bonding to Mg. Transition metal elements with d electrons show strong interactions with Mg and have large diffusion activation energies. Correlation effects are not negligible for solutes Ca, Na, Sr, Se, Te, Y, and early rare earths La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, in which the direct solute migration barriers are much smaller than the solvent (Mg) migration barriers. Solutes with large atomic size have lower migration barriers due to large local strain in the Mg matrix. Calculated diffusion coefficients are in remarkable agreement with available experimental data in the literature. The calculated diffusion coefficients can be used as the input in mesoscale simulations like phase field and finite element simulations or be used to develop CALPHAD-type multi-component mobility databases for Mg alloys.