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Now showing items 1 - 5 of 5

  • Atomistic simulation of hydrogen diffusion at tilt grain boundaries in vanadium

    Jae-Hyeok Shim   Won-Seok Ko   Jin-Yoo Suh…  

    Molecular dynamics simulations of hydrogen diffusion at I 3 pound and I 5 pound tilt grain boundaries in bcc vanadium (V) have been performed based on modified embedded-atom method interatomic potentials. The calculated diffusivity at the grain boundaries is lower than the calculated bulk diffusivity in a temperature range between 473 and 1473 K, although the difference between the grain boundary and bulk diffusivities decreases with increasing temperature. Compared with that of the other directions, the mean square displacement of an interstitial hydrogen atom at the I 3 pound boundary is relatively small in the direction normal to the boundary, leading to two dimensional motion. Molecular statics simulations show that there is strong attraction between the hydrogen atom and these grain boundaries in V, which implies that the role of grain boundaries is to act as trap sites rather than to provide fast diffusion paths of hydrogen atoms in V.
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  • Origin of hydrogen embrittlement in vanadium-based hydrogen separation membranes

    Won-Seok Ko   Jong Bae Jeon   Jae-Hyeok Shim   Byeong-Joo Lee  

    Hydrogen embrittlement in metals is a challenging technical issue in the proper use of hydrogen energy. Despite extensive investigations, the underlying mechanism has not been clearly understood. Using atomistic simulations, we focused on the hydrogen embrittlement in vanadium-based hydrogen separation membrane. We found that, contrary to the conventional reasoning for the embrittlement of vanadium, the hydrogen-enhanced localized plasticity (HELP) mechanism is the most promising mechanism. Hydrogen enhances the nucleation of dislocations near the crack tip, which leads to the localized plasticity, and eventually enhances the void nucleation that leads to the failure. Those results provide an insight into the complex atomic scale process of hydrogen embrittlement in vanadium and also help us design a new alloy for hydrogen separation membranes. [All rights reserved Elsevier].
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  • Atomic scale processes of phase transformations in nanocrystalline NiTi shape-memory alloys

    Won-Seok Ko   Sascha B. Maisel   Blazej Grabowski   Jong Bae Jeon   Jörg Neugebauer  

    Abstract Molecular dynamics simulations are performed to investigate temperature- and stress-induced phase transformations in nanocrystalline nickel-titanium shape-memory alloys. Our results provide detailed insights into the origins of the experimentally reported characteristics of phase transformations at the nanoscale, such as the decrease of the transformation temperature with grain size and the disappearance of the plateau in the stress-strain response. The relevant atomic scale processes, such as nucleation, growth, and twinning are analyzed and explained. We suggest that a single, unified mechanism—dominated by the contribution of a local transformation strain—explains the characteristics of both temperature- and stress-induced phase transformations in nanocrystalline nickel-titanium. Graphical abstract Image 1
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  • Atomistic modeling of an impurity element and a metal–impurity system: pure P and Fe–P system

    Won-Seok Ko   Nack J Kim   Byeong-Joo Lee  

    An interatomic potential for pure phosphorus, an element that has van der Waals, covalent and metallic bonding character, simultaneously, has been developed for the purpose of application to metal–phosphorus systems. As a simplification, the van der Waals interaction, which is less important in metal–phosphorus systems, was omitted in the parameterization process and potential formulation. On the basis of the second-nearest-neighbor modified embedded-atom method (2NN MEAM) interatomic potential formalism applicable to both covalent and metallic materials, a potential that can describe various fundamental physical properties of a wide range of allotropic or transformed crystalline structures of pure phosphorus could be developed. The potential was then extended to the Fe–P binary system describing various physical properties of intermetallic compounds, bcc and liquid alloys, and also the segregation tendency of phosphorus on grain boundaries of bcc iron, in good agreement with experimental information. The suitability of the present potential and the parameterization process for atomic scale investigations about the effects of various non-metallic impurity elements on metal properties is demonstrated.
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  • The role of metastable LPSO building block clusters in phase transformations of an Mg-Y-Zn alloy

    Jin-Kyung Kim   Won-Seok Ko   Stefanie Sandlöbes   Markus Heidelmann   Blazej Grabowski   Dierk Raabe  

    Abstract We present a systematic atomic scale analysis of the structural evolution of long-period-stacking-ordered (LPSO) structures in the (i) α-Mg matrix and in the (ii) interdendritic LPSO phase of an Mg 97 Y 2 Zn 1 (at. %) alloy annealed at 500 °C, using high resolution high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM). Various types of metastable LPSO building block clusters have been observed in both regions. The thermodynamic phase stabilities computed by density-functional-theory calculations explain the diversity of the LPSO structures which are distinguished by their different arrangements of the Y/Zn enriched LPSO building blocks that have a local fcc stacking sequence on the close packed planes. A direct evidence of the transformation from 18R to 14H is presented. This finding suggests that LPSO structures can change their separation distance — quantified by the number of α-Mg layers between them — at a low energy penalty by generating the necessary Shockley partial dislocation on a specific glide plane. Based on our results the most probable transformation sequence of LPSO precipitate plates in the α-Mg matrix is: single building block → various metastable LPSO building block clusters → 14H, and the most probable transformation sequence in the interdendritic LPSO phase is: 18R→ various metastable LPSO building block clusters → 14H. The thermodynamically most stable structures in both the α-Mg matrix and the interdendritic LPSO phase are a mixture of 14H and α-Mg. Graphical abstract
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