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Planar defects such as free surfaces and grain boundaries in metals and alloys play important roles affecting many material properties such as fracture toughness, corrosion resistance, wetting, and catalysis. Their interactions with point defects and solute elements also play critical roles on governing the microstructural evolution and associated property changes in materials. This work seeks to use atomistic modeling to obtain a fundamental understanding of many surface and interface related properties and phenomena, namely: orientation-dependent surface energy of elemental metals and alloys, segregation of solute elements at grain boundaries and their impact on grain boundary cohesive strength, and the controversial sluggish diffusion in both the bulk and grain boundaries of high entropy alloys. First, an analytical formula is derived, which can predict the surface energy of any arbitrary (h k l) crystallographic orientation in both body-centered-cubic (BCC) and face-centered-cubic (FCC) pure metals, using only two or three low-index (e.g., (100), (110), (111)) surface energies as input. This analytical formula is validated against 4357 independent single element surface energies reported in literature or calculated by the present author, and it proves to be highly accurate but easy to use. This formula is then expanded to include the simple-cubic (SC) structure and tested against 4542 surface energies of metallic alloys of different cubic structures, and good agreement is achieved for most cases. Second, the effect of segregation of substitutional solute elements on grain boundary cohesive strength in BCC Fe is studied. It is found that the bulk substitution energy can be used as an effective indicator to predict the embrittlement or strengthening potency induced by the solute segregation at grain boundaries. Third, the controversial vacancy-mediated sluggish diffusion in an equiatomic FeNiCrCoCu FCC high entropy alloy is studied. Many literature studies have postulated that the compositional complexity in high entropy alloys could lead to sluggish diffusion. To test this hypothesis, this work compares the vacancy-mediated self-diffusion in this model high entropy alloy with a hypothetical single-element material (called average-atom material) that has similar average properties as the high entropy alloy but without the compositional complexity. The results show that the self-diffusivities in the two bulk systems are very similar, suggesting that the compositional complexity in the high entropy alloy may not be sufficient to induce sluggish diffusion in bulk high entropy alloys. Based on the knowledge learned from the bulk alloy, the exploration of the possible sluggish diffusion has been extended to grain boundaries, using a similar approach as in the study of self-diffusion in bulk. Interestingly, the results show that sluggish diffusion is evident at a Σ5(210) grain boundary in the high entropy alloy due to the compositional complexity, especially in the low temperature regime, which is different from the bulk diffusion. The underlying mechanisms for the sluggish diffusion at this grain boundary is discussed. / Doctor of Philosophy / Human beings have utilized metals and alloys for over ten millennia and learned much from them. Based on the accumulated knowledge, they have countless applications in our current daily life. However, there is still much to learn for improving our current technology and even opening new opportunities. Throughout most of history, our understanding of these materials was largely obtained through empirical experimentation and refining them into theories and scientific laws. Nowadays, due to the advancements in computer simulations, we can learn more by modeling the behaviors of metals and alloys at the length and time scales that are either be too arduous, costly, or currently impossible experimentally. This work aims at using computer modeling to study some important surface/interface related physical behaviors and properties in metals and alloys at the atomistic scale. First, this work intends to develop a robust surface energy model in an analytical form for any crystallographic orientation. Surface energy is an important material property for many surface-related processes such as fracturing, wetting, sintering, catalysis, and crystalline particle shape. Surface energy is different at different surface orientations, and predicting this difference is important for understanding these surface phenomena. Second, the effect of solute segregation on grain boundary cohesive strength is studied. Most commonly used metallic materials consist of many small crystalline grains and the borders between them are called grain boundaries, which are weak spots for fracture. The minimum energy required to split a boundary is called the grain boundary cohesive strength. The presence of solutes or impurities at grain boundaries can further alter the cohesive strength. A better understanding of this phenomena will eventually help us develop more fracture-resistant materials. The third project deals with the possible sluggish/retarded diffusion in high entropy alloys, which contain five or more principal alloying elements and have many unique mechanical, radiation-resistant, and corrosion-resistant properties. Many researchers attribute these unique properties to the slow species diffusion in these alloys, but its existence is still controversial. This work studies the atomic-level diffusion mechanisms in an FeNiCrCoCu high entropy alloy both in bulk (grain interior) and at grain boundaries in order to determine if sluggish diffusion is present and its causes.

<p>A framework is described using the phase-field crystal model for the study of premelting in binary alloys through short-range interfacial interactions that arise from the structure of grain boundaries. A nonconserved model A formulation of PFC was used to model grain boundaries in two dimensions for several different angles of misorientation: 27.8, 21.8, 17.8, 13.2, and 5 degrees. The character of the premelting transition, whereby a liquid-like film develops at a defect at temperatures below the melting point, changed with misorientation angle. An excess mass over the grain boundary can be defined as an analog to the liquid layer thickness due to premelting. It is found that low-angle grain boundaries remain at a relatively constant value of excess mass, and indeed can remain solid above the melting point. High-angle grain boundaries have a logarithmically increasing width that diverges at the melting point. A width-dependent energy can be defined called the disjoining potential that takes into account structure, interfacial and bulk energies to describe the liquid-layer width. The form of this disjoinging potential was found to be exponential and monotonically decreased as width increased for high angles and produced an attractive minimum for low angles. The results of this work were compared to a pure material from a previous study.</p> / Master of Applied Science (MASc)

Nuclear energy is a proven large-scale, emission-free, around-the-clock energy source. As part of improving the nuclear energy efficiency and safety, a significant amount of effort is being expended to understand how the microstructural evolution of nuclear fuels affects the overall fuel performance. Grain growth is an important aspect of microstructural evolution in nuclear fuels because grain size can affect many fuel performance properties. In this work, the anisotropy of grain boundary energy and mobility, which are two important properties for grain growth, is examined for the light water reactor fuel uranium dioxide (UO₂) by molecular dynamics simulations. The dependence of these properties on both misorientation angle and rotation axis is studied. The anisotropy in grain boundary energy is found to be insignificant in UO₂. However, grain boundary mobility shows significant anisotropy. For both 20º and 45º misorientation angles, the anisotropy in grain boundary mobility follows a trend of M₁₁₁>M₁₀₀>M₁₁₀, consistent with previous experimental results of face-centered-cubic metals. Evidences of grain rotation during grain growth are presented. The rotation behavior is found to be very complex: counterclockwise, clockwise, and no rotation are all observed. / M.S. / Energy needs in the world increase year after year. As part of the effort to address these increasing needs, an increasing effort is needed to study each aspect of energy generation. For energy generated via nuclear fission, i.e., nuclear energy, many things need to be understood to gain maximum efficiency with maximum safety. At the core of a nuclear reactor, transport of energy generated by nuclear fission is heavily dependent on the microscopic structure (microstructure) of the materials being used as fuel. Thus, this work examines the microstructure of the most common nuclear fuel, uranium dioxide (UO₂). The microstructure changes based on at least two properties: grain boundary energy, and grain boundary mobility. This work examines how these properties change based on the orientation of individual crystallites within the polycrystalline material. An additional aspect of microstructural evolution, namely grain rotation, is briefly discussed.

L'objectif de cette thèse est de mettre en évidence l'influence de la microstructure initiale sur le glissement intergranulaire lors du fluage à haute température d'un superalliage polycristallin à base de nickel. Dans ce but, plusieurs microstructures sont obtenues à partir de la microstructure de référence de l'alliage NR6, par application de traitements thermiques spécifiques. L'influence des paramètres microstructuraux sur les déformations locales est ensuite étudiée à l'aide d'une technique de microextensométrie couplée à une analyse par diffraction des électrons rétrodiffusés. Il est ainsi possible de relier microstructure, déformations locales et comportement macroscopique en fluage. Pour la microstructure de référence de l'alliage NR6, la déformation opère principalement par cisaillement des phases γ et γ'. Ce mécanisme est favorable au glissement intergranulaire. L'absence de précipités tertiaires de phase γ' favorise le contournement des précipités secondaires par les dislocations. Ceci permet de réduire le glissement intergranulaire mais est également néfaste pour la résistance à la déformation de l'alliage. La présence de joints de grains dentelés augmente la résistance au glissement intergranulaire mais diminue la résistance à la déformation intragranulaire en favorisant le contournement des précipités. Ainsi la résistance globale à la déformation n'est pas affectée. Enfin, l'augmentation de la taille de grains n'a d'influence ni sur les mécanismes de déformation mis en jeu ni sur l'amplitude du glissement. Cependant, la fraction moins élevée de joints de grains induit une diminution de la contribution du glissement intergranulaire à la déformation globale. / The aim of this study is to highlight the influence of initial microstructure on grain boundary sliding during high-temperature creep of a polycrystalline nickel-based superalloy. To reach this goal, several microstructures are produced from the reference microstructure of NR6 alloy by adequate heat treatments. The influence of microstructural parameters on local deformations is then studied thanks to a microextensometry technique coupled with an electron back-scattered diffraction analysis. It thereby enables linking microstructure, local deformations and macroscopic creep behaviour. In the case of NR6 alloy reference microstructure, deformation occurs mainly by γ and γ' phases cutting by dislocations. This mechanism is grain boundary sliding-favourable. The absence of tertiary γ' phase precipitates promotes secondary precipitates bypassing by dislocations. This results in a reduction of grain boundary sliding but is also harmful to the alloy creep resistance. Grain boundary serration improves grain boundary sliding resistance but diminishes intragranular deformation resistance by favouring precipitate bypassing. Then global deformation resistance is not changed. Finally, grain size increase has influence neither on activated deformation mechanisms nor on sliding amplitude. However, the decrease of grain boundary fraction leads to a reduction of grain boundary sliding contribution to overall strain.