Final PhD Oral Examination (Thesis Title: “Quantum Simulations of the Interaction between Solutes and Spatial Defects in Metals”)

Event Date and Time: 
Mon, 2015-11-02 09:00 - 11:00
Room 200, Graduate Student Centre
Local Contact: 
Physics and Astronomy, UBC
Intended Audience: 

This thesis presents a computational study of the interaction between solute atoms and defects in the crystal lattice of metals at low solute concentrations. To accurately capture the electronic behaviour of these solutes in complex environments, we use density functional theory calculations. We begin by examining the diffusion of technologically relevant rare earth solutes in Mg. The calculated activation energies agree qualitatively with experimental values. This work also demonstrates that the existing 8-frequency model for diffusion in hexagonally close-packed metals can be improved with the consideration of an additional vacancy jump frequency.
We calculate the binding energies of solute atoms to a Ʃ7 grain boundary in Mg. These energies are dominated by elastic effects for many important alloying species, and we develop a computationally efficient model for predicting the binding energies for these elastically dominated solutes. These results agree well with the findings of other experimental and computational studies of Mg boundaries. Using molecular statics calculations of general high-angle grain boundaries in combination with our binding energy model, we predict binding energies for boundaries not accessible by quantum calculations. In addition to providing a better understanding of solute-grain boundary interaction in Mg, binding energies for use in models at higher length scales, and a new, efficient model for predicting binding energy, we also show the importance of using segregation models which utilize multiple binding energies to predict segregation levels.
We develop an improved multiscale method for embedding quantum mechanical density functional theory calculations in much larger classical simulations. This method removes the constraints on structure size usually imposed by the high cost of quantum calculations by treating a small region of interest (e.g. a solute and its neighbourhood) with quantum accuracy while coupling the quantum domain to a much larger classical system.
The method is developed and tested using solute binding energies to a vacancy and to sites at a special, high-symmetry grain boundary in Al, for which purely quantum mechanical evaluations are also possible, providing a reliable benchmark. We then apply this method to find solute binding energy to a general, low-symmetry grain boundary in Al.

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