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Now showing items 1 - 7 of 7

  • Irradiation resistance of nanostructured interfaces in Zr-Nb metallic multilayers

    Chen, Elton Y.   Deo, Chaitanya   Dingreville, Remi  

    Irradiation resistance of metallic nanostructured multilayers is determined by the interactions between defects and phase boundaries. However, the dose-dependent interfacial morphology evolution can greatly change the nature of the defect-boundary interaction mechanisms over time. In the present study, we used atomistic models combined with a novel technique based on the accumulation of Frenkel pairs to simulate irradiation processes. We examined dose effects on defect evolutions near zirconium-niobium multilayer phase boundaries. Our simulations enabled us to categorize defect evolution mechanisms in bulk phases into progressing stages of dislocation accumulation, saturation, and coalescence. In the metallic multilayers, we observed a phase boundary absorption mechanism early on during irradiation, while at higher damage levels, the increased irradiation intermixing triggered a phase transformation in the Zr-Nb mixture. This physical phenomenon resulted in the emission of a large quantity of small immobile dislocation loops from the phase boundaries.
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  • Irradiation resistance of nanostructured interfaces in Zr–Nb metallic multilayers

    Chen, Elton Y.   Deo, Chaitanya   Dingreville, Rémi  

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  • Synergistic effects in hydrogen-helium bubbles

    Hayward, Erin   Deo, Chaitanya  

    The detrimental effects of hydrogen and helium on structural materials undergoing irradiation are well documented, if not well understood. There is experimental evidence to suggest that a synergistic effect between the two elements exists, which results in increased damage when both are present. This situation is expected in the next generation of fusion and fission reactors, so a fundamental understanding of these synergistic interactions is needed to predict materials performance. We perform atomistic simulations of hydrogen and helium bubbles in body-centered cubic iron to determine the mechanism behind this effect. We first develop an interatomic potential suitable for describing the interactions between hydrogen and helium. Through analysis of the energetics and structure of these bubbles, we explain the observed synergy as a consequence of bubble growth through helium induced loop punching, aided by the presence of hydrogen, instead of as a direct interaction between hydrogen and helium. The hydrogen benefits from an increased area of free surface on which to bind.
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  • Misfit dislocation networks in semi-coherent miscible phase boundaries:An example for U-Zr interfaces

    Chen, Elton Y.   Dingreville, Remi   Deo, Chaitanya  

    Semi-coherent cube-on-cube miscible U-Zr interfaces were studied using molecular dynamics simulations. The misfit accommodation of such semi-coherent phase boundaries was characterized by a two-dimensional dislocation network model utilizing a combination of theoretical predictions and analysis of the atomic system. The dislocation networks were discussed for various stacking orientation of the adjoining phases in terms of the composition of the dislocation sets, the partitioning between edge and screw components and the associated residual elastic fields. These analyses showed that the patterning of the network of dislocations forming these phase boundaries results from the competition between a structurally-driven process (i.e., function of the lattice misfit) and a chemically-driven process (i.e., due to the miscibility between U and Zr).
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  • First principles calculations of the structure and elastic constants of alpha, beta and gamma uranium

    Beeler, Benjamin   Deo, Chaitanya   Baskes, Michael   Okuniewski, Maria  

    This study analyzes structural and elastic properties of five uranium crystal structures: the face centered orthorhombic A20 (alpha phase), the tetragonal D8(b) (beta phase), body centered tetragonal (bct), body centered cubic (gamma phase) and face centered cubic structures. Calculations are performed within the density functional theory framework employing the Projector Augmented Wave method and the Perdew-Burke-Ernzerhof generalized gradient approximation (PBE-GGA) of the exchange correlation. The elastic constants are used to compute polycrystalline elastic moduli, Poisson's ratio and the Debye temperature for all five structures. The alpha and gamma phase properties are compared with theoretical and experimental results. The complex tetragonal 30 atom beta phase is examined in detail. Representation of the beta phase by a bct structure is examined; we find that the structure of the beta phase is significantly different from the bct phase but exhibits similar elastic properties. This is the first comprehensive investigation into the elastic constants of uranium utilizing the PBE-GGA. (c) 2012 Elsevier B.V. All rights reserved.
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  • Scaling laws and stability of nano-sized defect clusters in niobium via atomistic simulations and statistical analysis

    Vizoso, Daniel   Deo, Chaitanya   Dingreville, Remi  

    The predictions of scaling laws for the structure and properties of defect clusters are generally limited to small defect clusters in their ground-state configurations. We investigated the size and geometrical configuration dependence of nano-sized defect clusters in niobium (Nb) using molecular dynamics. We studied the structure and stability of large clusters of size up to fifty defects for vacancies and one hundred defects for interstitials, as well as the role of helium and metastable configurations on the stability of these clusters. We compared three different interatomic potentials in order to determine the relative stability of these clusters as a function of their size and geometrical configurations. Additionally, we conducted a statistical analysis to predict the formation and binding energies of interstitial clusters as a function of both their size and configuration. We find that the size dependence of vacancy and interstitial clusters can be approximated by functional forms that account for bulk and surface effects as well as some considerations of elastic interactions. We also find that helium and metastable configurations can make vacancy and interstitial clusters thermally stable depending on the configuration. Our parameterized functional forms for the formation and binding energies are valid for a very broad range of defect sizes and configurations making it possible to be used directly in a coarse-grained modeling strategy such as Monte Carlo, cluster dynamics or dislocation dynamics which look at defect accumulation and evolution in microstructures.
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  • Modeling and simulation of irradiation hardening in structural ferritic steels for advanced nuclear reactors RID A-2494-2008

    Deo, Chaitanya   Tom, Carlos   Lebensohn, Ricardo   Maloy, Stuart  

    Hardening and embrittlement are controlled by interactions between dislocations and irradiation induced defect clusters. In this work we employ the visco plastic self consistent (VPSC) polycrystalline code in order to model the yield stress dependence in ferritic steels on the irradiation dose. We implement the dispersed barrier hardening model in the VPSC code by introducing a hardening law, function of the strain, to describe the threshold resolved shear stress required to activate dislocations. The size and number density of the defect clusters varies with the irradiation dose in the model. We find that VPSC calculations show excellent agreement with the experimental data set. Such modeling efforts can both reproduce experimental data and also guide future experiments of irradiation hardening. Published by Elsevier B.V.
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