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Abstract

Density functional theory calculations have been used with spin-orbit coupling and onsite Coulomb correction (GGA+U) methods to investigate the U-Si system. Structural prediction methods were employed to identify alternate stable structures. Convex hulls of the U-Si system were constructed for each of the methods to highlight the competing energetics of various phases. For GGA calculations, new structures are predicted to be dynamically stable, but these have not been experimentally observed. When the GGA+U (U eff > 1.3 eV) method is considered, the experimentally observed structures are predicted to be energetically preferred. Phonon calculations were used to investigate the energy predictions and showed that the use of the GGA+U method removes the significant imaginary frequencies observed for U 3 Si 2 when the correction is not considered. Total and partial electron density of states calculations were also

2016-11-03T14:11:40

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a b s t r a c tMolecular dynamics simulation is used to characterize the deformation behavior of polycrystalline Zr. The predictions of two different potentials, an embedded atom method potential and a charge optimized many body potential are compared. The experimentally observed prismatic dislocations, pyramidal dislocations and twinning behaviors are produced in the simulations of ½1 1 2 0 and [0 0 0 1] textured structures and in fully 3D structure simulations. The relationship between the generalized stacking fault energy and the mechanical properties is discussed. In particular we find that the different shapes of the generalized stacking-fault energy curve for the two different interatomic descriptions of Zr have a significant effect on the deformation mechanisms. The deformation behavior of Zr is compared with analogous simulations of deformation of polycrystalline Mg.

This work uses density functional theory calculations to analyze the energy barriers for oxygen migration into the basal and prismatic surfaces of zirconium. Specifically, the migration energy barriers between each octahedral site and tetrahedral site in the basal surface, prism surface, and the bulk are determined. The possible oxygen migration paths in each system are also analyzed. Oxygen has higher energy barriers to penetrating the basal surface than the prism surface. It also has a lower energy barrier to escape from basal surface than from the prism surface. This is consistent with the experimental observation that the prism plane of zirconium oxidizes more quickly than the basal plane.