@article{jokisaari_mahbuba_wang_beeler_2022, title={The impact of anisotropic thermal expansion on the isothermal annealing of polycrystalline alpha-uranium}, volume={205}, ISSN={["1879-0801"]}, url={https://doi.org/10.1016/j.commatsci.2022.111217}, DOI={10.1016/j.commatsci.2022.111217}, abstractNote={Although grain growth impacts microstructural evolution in a wide variety of materials systems, the effect of anisotropic thermal expansion on grain boundary mobility and texture evolution has not been widely studied. Anisotropic thermal expansion occurs in multiple non-cubic metals, and the thermomechanical processing behavior of these materials can be better understood with further study into the impact of thermal expansion on grain boundary mobility and texture evolution. In this work, we develop a mesoscale phase field model of grain growth that includes the effect of anisotropic thermal expansion, which is applied to study polycrystalline α-uranium, a highly anisotropic metal. Three-dimensional simulations on polycrystalline α-uranium with and without thermal expansion eigenstrains are performed to study the grain boundary mobility and texture evolution as a function of temperature. A strain-free temperature of 933 K is selected, and the system is studied within the range of 873–933 K at intervals of ten degrees, resulting in increasing thermal eigenstrain with decreasing temperature. We also estimate a grain boundary mobility prefactor and activation energy based on existing experimental data of isothermal annealing of α-uranium. The grain boundary mobility is found to display significant deviation from Arrhenius behavior with the inclusion of thermal expansion eigenstrain as the amount of thermal eigenstrain (and thus elastic strain energy within the system) increases. This result explains an experimentally observed grain boundary mobility deviation from Arrhenius behavior. Furthermore, the texture evolution is affected, such that the grain orientations become less random with increasing thermal eigenstrain, which could explain experimentally observed texture behavior. These results indicate that the effect of thermal expansion should be considered when predicting the thermomechanical processing behavior of α-uranium and other materials with anisotropic thermal expansion.}, journal={COMPUTATIONAL MATERIALS SCIENCE}, publisher={Elsevier BV}, author={Jokisaari, Andrea M. and Mahbuba, Khadija and Wang, Yuhao and Beeler, Benjamin}, year={2022}, month={Apr} }
 @article{beeler_mahbuba_wang_jokisaari_2021, title={Determination of Thermal Expansion, Defect Formation Energy, and Defect-Induced Strain of alpha-U Via ab Initio Molecular Dynamics}, volume={8}, ISSN={["2296-8016"]}, DOI={10.3389/fmats.2021.661387}, abstractNote={Uranium (U) is often alloyed with molybdenum (Mo) or zirconium (Zr) in order to stabilize its high-temperature body-centered cubic phase for use in nuclear reactors. However, in all metallic fuel forms, the α phase of U remains in some fraction. This phase decomposition due to temperature or compositional variance can play an outsized role on fuel performance and microstructural evolution. Relatively little is known about fundamental point defect properties in α-U at non-zero temperatures, from either computational or experimental studies. This work performs the first thorough evaluation of the α phase of U via ab initio molecular dynamics (AIMD). A number of thermophysical properties are calculated as a function of temperature, including equilibrium lattice parameters, thermal expansion, and heat capacity. These results indicate a two-region behavior, with the transition at 400 K. The thermal expansion/contraction in the a/b direction occurs rapidly from 100 up to 400 K, after which a more linear and gradual change in the lattice constant takes place. The volumetric expansion matches experiments quantitatively, but the individual lattice constant expansion only matches experiments qualitatively. Point defect formation energies and induced lattice strains are also determined as a function of temperature, providing insight on defect populations and the fundamentals of irradiation growth in α-U. Interstitials induce significantly more strain than vacancies, and the nature of that strain is highly dependent on the individual lattice directions. The direction of point defect-induced lattice strain is contrary to the irradiation growth behavior of α-U. This work shows that isolated point defects cannot be the primary driving force responsible for the significant irradiation-induced growth of α-U observed experimentally.}, journal={FRONTIERS IN MATERIALS}, author={Beeler, Benjamin and Mahbuba, Khadija and Wang, Yuhao and Jokisaari, Andrea}, year={2021}, month={Jun} }
 @article{mahbuba_beeler_jokisaari_2021, title={Evaluation of the anisotropic grain boundaries and surfaces of alpha-U via molecular dynamics}, volume={554}, ISSN={["1873-4820"]}, url={https://doi.org/10.1016/j.jnucmat.2021.153072}, DOI={10.1016/j.jnucmat.2021.153072}, abstractNote={Alloys based on uranium-zirconium are gaining renewed interest as fuels for the Versatile Test Reactor and a number of microreactor designs. Implementing metallic fuel in reactors creates the need for robust descriptive and predictive fuel performance modeling. The current state of metallic fuel performance modeling relies on empirical equations derived from historical experiments, which may be unreliable when applied outside of their temperature, power, and composition phase space. One area where such data is lacking is the irradiation behavior of α-U, specifically tearing and porosity formation at the early stages of irradiation. While grain boundaries likely play a key role in this fuel behavior, relatively little is known about grain boundaries in α-U. Thus, we evaluate the grain boundary, surface energy, and work of adhesion of α-U utilizing molecular dynamics. Symmetric tilt grain boundaries (STGBs) are analyzed with the tilt plane oriented along each major crystallographic axis, for a total of eighty unique grain boundaries. The effect of temperature, tilt plane, and misorientation angle on interfacial energies are analyzed. The interfacial energies typically increase with temperature and there is significant variance as a function of misorientation angle, irrespective of the tilt plane. At 500 K, the average surface energy (1.23 J/m2) is approximately 1.5 times the grain boundary energy (0.79 J/m2), and the work of adhesion is approximately twice the grain boundary energy (1.68 J/m2). Orientations for the likely formation of twins and likely failure planes are identified.}, journal={JOURNAL OF NUCLEAR MATERIALS}, publisher={Elsevier BV}, author={Mahbuba, Khadija and Beeler, Benjamin and Jokisaari, Andrea}, year={2021}, month={Oct} }