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Now showing items 1 - 3 of 3

  • Phase field modeling of sintering:Role of grain orientation and anisotropic properties

    Biswas, Sudipta   Schwen, Daniel   Wang, Hao   Okuniewski, Maria   Tomar, Vikas  

    The influence of anisotropic properties of powder particles on microstructural evolution during solid-state sintering processes is analyzed. Two types of anisotropy studied in the current work are direction-dependent interface diffusion anisotropy, and grain orientation dependent grain boundary energy anisotropy. A phase field modeling approach is utilized to assess how the individual anisotropic characteristics influence morphological changes during sintering. In addition, a novel approach for updating grain orientation after rigid-body rotation of particles during powder compaction accompanying the sintering process is developed. It is observed that conventional isotropic microstructural analysis over-simplifies the material behavior and demonstrates faster microstructural evolution. Direction-dependent diffusion produces gradual shape change of the particles by mass transfer from high curvature region to low curvature region along the particle surface, and delays the onset of the grain growth process. Depending on the grain boundary misorientation and inclination, anisotropic grain boundary energy may induce faster or slower grain boundary migration rate. Orientation update during the process plays a critical role in the consolidation kinetics, as well as in the final microstructural configuration. Variation in grain orientation and its evolution during the sintering process can produce different grain morphologies for the same initial condition.
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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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  • A nanomechanical Raman spectroscopy based assessment of stress distribution in irradiated and corroded SiC

    Mohanty, Debapriya Pinaki   Wang, Hao   Okuniewski, Maria   Tomar, Vikas  

    Silicon carbide (SiC) composites are under consideration for cladding and structural materials in various types of reactors. The effects of ion irradiation and corrosion on stress distribution due to mechanical loading on chemical vapor deposited (CVD) SiC were investigated in this paper by using nanomechanical Raman spectroscopy (NMRS). The stress distribution was analyzed as a function of the oxide formation on a corroded specimen and as a function of ion-induced irradiation damage in an irradiated specimen. A finite element method (FEM) based model was developed based on local mechanical properties measured using nanoindentation to predict the NMRS measured stress distribution. The stress distribution was also predicted theoretically by using a stress concentration factor, which is a function of sample geometry and boundary conditions. The maximum stress obtained theoretically was in good agreement with the FEM model and NMRS based measurements. FEM results captured the stress variation trends and maximum stress value in the analyzed samples. NMRS measurements predicted that corrosion had a greater influence on increasing the maximum value of stress in comparison to ion irradiation. The increase in stress attributed to corrosion in comparison to ion irradiated samples was approximately 10%-20%. (C) 2017 Elsevier B.V. All rights reserved.
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