Our study is focused on the behavior of grain boundaries in uranium dioxide system under irradiation conditions. The research can be seen as two parts: the study of interaction of the grain boundary and the damage cascade, and the calculation of Kapitza resistance of grain boundaries. The connection between these two parts lies in that damage cascades bring in changes in the structure and other properties of grain boundaries, and inevitably the Kapitza resistance of the grain boundary changes as well. For the first part, we studied interactions of grain boundaries and damage cascades in uranium dioxide system by simulating two types of bombardments: one direct bombardment into a grain boundary leading to ballistic-collision-mediated interface mixing; the other bombardment is in the close vicinity of a grain boundary causing interface biased defect migration. We found that more defects are trapped by the grain boundary followed by the first type of bombardment, resulting in enhanced grain boundary energy. By comparing with the second type of bombardment, we are able to reveal the mechanisms of the interaction between defects and grain boundaries. For the second part, we employed the non-equilibrium molecular dynamics method to calculate the Kapitza resistance of different coincident site lattice boundaries with or without defects loaded, and later we found that a universal positive correlation between the Kapitza resistance and the grain boundary energy can be well established, regardless of the cause of boundary energy changes. Our study provides a deeper understanding of the Kapitza resistance of the grain boundary and its evolutions under irradiation, which benefits multi-scale modeling of uranium dioxide thermal properties under extreme radiation conditions as well as experimental studies of fuel material thermal properties.

abstract: The initial microstructure of oxide fuel pellets can play a key role in their performance. At low burnups, the transport of fission products has a strong dependence on oxygen content, grain size distribution, porosity and grain boundary (GB) characteristics (crystallography, geometry and topology), all of which, in turn depend on processing conditions. These microstructural features can also affect the fuel densification, thermal conductivity and microstructure evolution inside the reactor. Understanding these effects can provide insight into microstructure evolution of fuels in-pile. In this work, mechanical and ion beam serial sectioning techniques were developed to obtain Electron Backscatter Diffraction (EBSD) data, both in 2-D and 3-D, for depleted UO2+X pellets manufactured under different conditions. The EBSD maps were used to relate processing conditions to microstructural features, with emphasis on special GBs according to the Coincident Site Lattice (CSL) model, as well as correlations between pore size and location in the microstructure. Furthermore, larger grains (at least 2.5 times the average grain size) were observed in all the samples and studied. Results indicate that larger grains, in samples manufactured under different conditions, dominate the overall crystallographic texture and have a fairly strong GB texture. Moreover, it seems that the preferential misorientation axis for these GBs, regardless of the O/M, is {001}. These results might be related to GB energy and structure and, suggest that the mechanism that controls grain growth seems to be independent of both processing conditions and stoichiometry. Additionally, a sample was heat treated to relate grain growth and crystallography. The results indicate that at least two mechanisms were involved. Lengthening of GBs was observed for larger grains. Another mechanism of grain growth was observed, in this case, grains rotate to match a neighboring grain forming a larger grain. In the new grain, the misorientation between the two neighboring grains decreases to less than 5 degrees, forming a new larger grain. The results presented in this work indicate that detailed studies of the initial microstructure of the fuel, with emphasis on the crystallography of grains and GBs could help to give insights on the in-pile microstructural evolution of the fuel. / Dissertation/Thesis / Ph.D. Materials Science and Engineering 2014

No description available.

The structure and spatial arrangement of grain boundaries strongly affect the properties of polycrystalline materials such as corrosion, creep, weldability, superconductivity, and diffusivity. However, constructing predictive grain boundary structure-property models is taxing, both experimentally and computationally due to the high dimensionality of the grain boundary character space. The purpose of this work is to develop an effective method to infer grain boundary structure-property models from measurement of the effective properties of polycrystals by utilizing the inverse problem theory. This study presents an idealized case in which structure-property models for grain boundary diffusivity are inferred from a noisy simulation. The method presented in this study is derived from a general mathematical expression of inverse problem theory. The derivation of the method is carried step by step by considering diffusivity as the property of interest. The use of the Bayesian probability approach in the inference method makes the uncertainty quantification possible to perform. This study demonstrates how uncertainty quantification for the inferred structure-property models is easily performed within the idealized case framework. The method of quantifying the uncertainty is carried by utilizing the Metropolis-Hastings algorithm and Kernel Density Estimation method. The validation of the method is carried out by considering structure-property models with one, three, and five degrees of freedom. Two- and three-dimensional simulated polycrystals are used in this study to obtain the simulation data. The two-dimensional simulated polycrystals used in this study are generated using grain growth simulation performed using a front-tracking algorithm. The three-dimensional polycrystals used in this study are generated using Neper software resulting in a real-like polycrystals. The structure-property models used in the validation are picked by considering the qualitative features that reflect trends observed in literature. The inference method is performed by ignoring any knowledge about the structure-property model in the process.

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.

QC 20100927

Diffusion controlled phase transformations are studied bymeans of the phase-field method. Morphological evolution ofdendrites, grains and Widmanst\"atten plates is modeled andsimulated. Growth of dendrites into highly supersaturated liquids ismodeled for binary alloy solidification. Phase-field equationsthat involve both temperature and solute redistribution areformulated. It is demonstrated that while at low undercoolingheat diffusion does not affect the growth of dendrites, i.e.solidification is nearly isothermal, at high cooling rates thesupersaturation is replaced by the thermal undercooling as thedriving force for growth. In experiments many crystals with different orientationsnucleate. The growth of randomly oriented dendrites, theirsubsequent impingement ant formation of grain boundaries arestudied in two dimensions using the FEM on adaptive grids. The structure of dendrites is determined by growthconditions and physical parameters of the solidifying material.Effects of the undercooling and anisotropic surface energy onthe crystal morphology are investigated. Transition betweenseaweeds, doublons and dendrites solidifying out of puresubstance is studied and compared to experimental data. Two-and three-dimensional simulations are performed in parallel onadaptive and uniform meshes. A phase-field method based on the Gibbs energy functional isformulated for ferrite to austenite phase transformation inFe-C. In combination with the solute drag model, transitionbetween diffusion controlled and massive transformations as afunction of C concentration and temperature is established byperforming a large number of one dimensional calculations withreal physical parameters. In two dimensions, growth ofWidmanstaetten plates is governed by the highly anisotropicsurface energy. It is found that the plate tip can beapproximated as sharp, in agreement with experiments. Keywords:heat and solute diffusion, solidification,solid-solid phase transformation, microstructure, crystalgrowth, dendrite, grain boundary, Widmanstaetten plate,phase-field, adaptive mesh generation, FEM. / <p>NR 20140805</p>

Student Number : 0405740V - MSc (Eng) dissertation - School of Chemical and Metallurgical Engineering - Faculty of Engineering and the Built Environment / Silicon carbide ceramics have many attractive properties, one of which is their high degree of corrosion resistance. Even though corrosion is slow, it does occur. Standard procedures for corrosion testing such as the immersion method is limited due to the low corrosion rates of most of these materials: it does not elucidate the mechanism of corrosion, but only gives the rate and degree of dissolution. Electrochemical techniques offer the possibility to further elucidate corrosion mechanisms and establish the resistance stability of conducting or partially-conducting ceramic materials, thus enhancing the understanding of ceramic material behaviour. In conjuction with microstructural changes, the electrochemical corrosion behaviour of solid state sintered silicon carbide (SSiC) and liquid phase sintered silicon carbide (LPSSiC) have successfully been studied at room temperature in acidic and alkaline environments by using potentiodynamic polarisation measurements. Several hypotheses were proposed to assist in establishing the effect of silicon and carbon on the corrosion mechanisms of these materials. The effect of the secondary phase on the electrochemical corrosion of the LPSSiC was also investigated. Corrosion current densities of the LPSSiC materials were much lower than the SSiC materials in all test solutions. The SSiC materials showed pseudo-passive behaviour in HCl and HNO3, due to the formation of thin layer of SiO2 on the surface. The carbon in the SiC compound increased the corrosion current densities in all test solutions for SSiC materials. The electrochemical corrosion of LPSSiC is due to the dissolution of SSiC and not the oxides; the chemcial attack on the oxide phases is mainly by acid-base type of reactions, rather than electrochemical corrosion involving redox reactions.

Nanocrystalline materials with grain size less than 100 nm have been receiving much attention because of their unparallel properties compared with their microcrystalline counterparts. Because of its high hardness, nanocrystalline nickel has been used for MEMS. Long term thermomechnical properties and deformation mechanism at both ambient and elevated temperatures need to be evaluated which is vital for reliability of its applications as structural material. In this thesis, nanoindentation creep of nanocrystalline nickel with an as-deposited grain size of 14 nm was characterized at elevated temperatures. The nanoindentation creep rate was observed to scale with temperature and applied load (or stress), and could be expressed by an empirical power-law equation for describing conventional crystalline solids. Creep activation energy was found to be close to that for grain boundary self-diffusion in nickel. The activation volume was also evaluated using a stress relaxation technique. The creep results were compared with those for fine-grained nickel in the literature. Possible mechanisms were discussed in light of the creep rate and temperature ranges. To provide a direct comparison, uniaxial creep tests were conducted on nanocrystalline nickel with an as-deposited grain size of 14 nm at 398 K. It was found that stress exponents under the two test conditions are almost the same, indicating a similar creep mechanism. However, the strain rate measured by nanoindentation creep was about 100 times faster than that by uniaxial creep. The rate difference was discussed in terms of stress states and the appropriate selection of Tabor factor. To further explore the time-dependent plastic behavior, multiple unload-reload tests were conducted on electrodeposited nanocrystalline nickel in both compression and tension. A hysteresis was observed during each unload-reload cycle, indicating irreversible energy dissipation. The dissipated energy was evaluated and the energy dissipation rate was found to increase with the flow stress to the third power and sensitive to the stress state (tension or compression). A mechanistic model based on grain boundary sliding was proposed to describe the unload-reload behavior. Experimental results were found to be in good agreement with the model predictions, suggesting the observed hysteresis was indeed caused by grain boundary sliding.

The effect of thermo-mechanical processing by deformation and annealing on the grain boundary configuration of commercially pure Ni-200 is reported in this thesis. Ni-200 is unalloyed, thus avoiding the complex effects associated with alloying elements on the formation and development of different types of grain boundaries. One step strain-recovery with strain levels in the range of 3% to 7.5% (with 1.5% intervals) and annealing temperatures in the range of 800ºC to 1000ºC (with 100ºC intervals) were used in processing. The effects of parameters such as strain level, annealing temperature, annealing time and grain growth on grain boundary configurations were studied. Using Orientation Image Microscopy (OIM) it was found that the Fsp (fraction of special grain boundaries) value of strained samples annealed in the range of 800ºC to 1000ºC began to increase after a critical length of time, after which the Fsp value increased quickly and becoming a maximum in 2~4 minutes. The length of the critical annealing time for the increase of Fsp was shorter in the material with the higher levels of strain at a constant annealing temperature. Also the critical annealing time was shorter when annealed at higher temperatures under a fixed level of strain. The Fsp value increased to 80% from an as received value of about 30% in the samples with varying strain levels. However, the Fsp values only increased from 30% to 45% in the material without strain. Due to grain boundary migration, the Fsp values increased with grain size and became a maximum during the heat treatment of the strained material. In the material without strain however even when grain growth occurred, limited improvement in Fsp values occurred showing that contribution of strain is very important to the formation of special boundaries. By varying the strain levels, annealing temperatures and times, material with high Fsp values in a wide range of grain size can be obtained. Under the present processing conditions used however, multi-cycle was not helpful to the improvement of Fsp. TEM observations indicated dislocation tangles occurred near the grain boundary of the 1x6% strained samples. These dislocation tangles decreased with time at 800˚C and were reduced considerably after 20 minutes. Thermodynamic and kinetic models were used in the calculations of twin density-grain size relationships. The results indicated that the contribution of strain is equivalent to the increase of grain boundary energy, which provided an extra driving force and improved probability of twin embryo formation. / February 2009