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abstract: Uranium Dioxide (UO2) is a significant nuclear fission fuel, which is widely used in nuclear reactors. Understanding the influence of microstructure on thermo-mechanical behavior of UO2 is extremely important to predict its performance. In particular, evaluating mechanical properties, such as elasticity, plasticity and creep at sub-grain length scales is key to developing this understanding as well as building multi-scale models of fuel behavior with predicting capabilities. In this work, modeling techniques were developed to study effects of microstructure on Young’s modulus, which was selected as a key representative property that affects overall mechanical behavior, using experimental data obtained from micro-cantilever bending testing as benchmarks. Beam theory was firstly introduced to calculate Young's modulus of UO2 from the experimental data and then three-dimensional finite element models of the micro-cantilever beams were constructed to simulate bending tests in UO2 at room temperature. The influence of the pore distribution was studied to explain the discrepancy between predicted values and experimental results. Results indicate that results of tests are significantly affected by porosity given that both pore size and spacing in the samples are of the order of the micro-beam dimensions. Microstructure reconstruction was conducted with images collected from three-dimensional serial sectioning using focused ion beam (FIB) and electron backscattering diffraction (EBSD) and pore clusters were placed at different locations along the length of the beam. Results indicate that the presence of pore clusters close to the substrate, i.e., the clamp of the micro-cantilever beam, has the strongest effect on load-deflection behavior, leading to a reduction of stiffness that is the largest for any location of the pore cluster. Furthermore, it was also found from both numerical and i analytical models that pore clusters located towards the middle of the span and close to the end of the beam only have a very small effect on the load-deflection behavior, and it is concluded that better estimates of Young's modulus can be obtained from micro- cantilever experiments by using microstructurally explicit models that account for porosity in about one half of the beam length close to the clamp. This, in turn, provides an avenue to simplify micro-scale experiments and their analysis. / Dissertation/Thesis / Masters Thesis Mechanical Engineering 2015

A specialty class of alloys known as tungsten heavy alloys (WHAs) possess extremely desirable qualities for adoption in nuclear fusion reactors. Their high temperature stability, improvement in fracture toughness over other brittle candidates, and promising performance in initial experimental trials have demonstrated their utility, and recent advancements have been made in understanding and applying these multiphase materials systems. To that end, Pacific Northwest National Laboratory in collaboration with Virginia Tech have sought to understand and tailor the structure and properties of these materials to optimize them for service in fusion reactor interiors; thereby improving the robustness, efficiency, and longevity of structural materials selected for service in an extremely hostile environment. In this analysis of material viability, a multiscale investigation of the connections between structure-property relationships in these multiphase composite microstructures has been undertaken, employing advanced characterization techniques to bridge the macro, micro, and nanoscales for the purpose of generating a framework for the understanding of the ductile phase toughening effect in these systems. This analysis has yielded evidence suggesting the effectiveness of WHA microstructures in the simultaneous expression of high strength and toughness owes to the intimately bonded nature of the boundary which exists between the dissimilar phases in these bi-phase microstructures. Analytical techniques have been employed to provide added dimensionality to traditional materials characterization techniques, providing the first three-dimensional microstructure reconstructions exhibiting the effects of thermomechanical processing on these dual-phase microstructures, and the first time-resolved approach to the observation of WHA deformation through in-situ uniaxial tension testing. The contributions of purposefully introduced microstructural anisotropy and its contribution to texturing and boundary conformations is discussed, and an emphasis has been placed on the study of the interface between the dissimilar phases and its role in the overall expression of ductile phase toughening. In short, this collective work utilizes multiscale and multidimensional characterization techniques in the in-depth analysis and discussion of WHA systems to connect their structure to the properties which make them excellent candidates for fusion reactor systems. / Doctor of Philosophy / In the ongoing effort to realize nuclear fusion for commercial energy generation, there are numerous hurdles which must be overcome. A primary issue in the creation of these reactors is the implementation of materials which interface with the superheated plasma in the reactor interior, called plasma facing materials and components (PFMCs). These PFMCs must be able to withstand environmental conditions which will melt, irradiate, embrittle, and fracture a majority of common structural materials. Therefore these materials must exhibit unparalleled robustness in the form of high thermal and irradiation resistance. One class of alloys which is currently being considered for this purpose is tungsten heavy alloys (WHAs). These materials have exhibited excellent viability in early-stage experimental trials, and have necessarily become the subject of extended examination as PFMC candidates. In a joint collaboration between Pacific Northwest National Laboratory and Virginia Tech, these materials have been subjected to rigorous experimental testing and analysis to determine what underlying physics are responsible for their excellent properties. Advanced analytical techniques have been applied to observe the connections which exist between the atomic structure of boundaries and have been connected to the expression of observable properties on the macroscale. This work has provided the first available data on the full three-dimensional approach to the study of WHAs as well as the first dynamic observation of how the materials deform, leading to the conclusion that the two-phase composite-like structure of these alloys owe their combination of strength and ductility to the strong bond which exists between the two phases. This information on how material structure influences properties can be used to improve alloy design and produce even more effective WHA materials going forward.

Microstructure reconstruction is a necessary tool for use in multi-scale modeling, as it allows for the analysis of the microstructure of a material without the cost of measuring all of the required data for the analysis. For microstructure reconstruction to be effective, the synthetic microstructure needs to predict what a small sample of measured data would look like on a larger domain. The Markov Random Field (MRF) algorithm is a method of generating statistically similar microstructures for this process. In this work, two key factors of the MRF algorithm are analyzed. The first factor explored is how the base features of the microstructure related to orientation and grain/phase topology information influence the selection of the MRF parameters to perform the reconstruction. The second focus is on the analysis of the numerical uncertainty (epistemic uncertainty) that arises from the use of the MRF algorithm. This is done by first removing the material uncertainty (aleatoric uncertainty), which is the noise that is inherent in the original image representing the experimental data. The epistemic uncertainty that arises from the MRF algorithm is analyzed through the study of the percentage of isolated pixels and the difference in average grain sizes between the initial image and the reconstructed image. This research mainly focuses on two different microstructures, B4C-TiB2 and Ti-7Al, which are a ceramic composite and a metallic alloy, respectively. Both of them are candidate materials for many aerospace systems owing to their desirable mechanical performance under large thermo-mechanical stresses. / Master of Science / Microstructure reconstruction is a necessary tool for use in multi-scale modeling, as it allows for the analysis of the microstructure of a material without the cost of measuring all of the required data for the analysis. For microstructure reconstruction to be effective, the synthetic microstructure needs to predict what a small sample of measured data would look like on a larger domain. The Markov Random Field (MRF) algorithm is a method of generating statistically similar microstructures for this process. In this work, two key factors of the MRF algorithm are analyzed. The first factor explored is how the base features of the microstructures related to orientation and grain/phase topology information influence the selection of the MRF parameters to perform the reconstruction. The second focus is on the analysis of the numerical uncertainty that arises from the use of the MRF algorithm. This is done by first removing the material uncertainty, which is the noise that is inherent in the original image representing the experimental data. This research mainly focuses on two different microstructures, B4C-TiB2 and Ti-7Al, which are a ceramic composite and a metallic alloy, respectively. Both of them are candidate materials for many aerospace systems owing to their desirable mechanical performance under large thermo-mechanical stresses.

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