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Now showing items 1 - 16 of 67

  • Room-Temperature Solid Solution Softening in Fe-V Binary System

    Park, Jihye   Shim, Jae-Hyeok   Lee, Young-Kook  

    Molecular dynamics simulations of the glide of an edge dislocation in the bcc matrix of Fe-V alloys were performed to investigate the room-temperature solid solution softening by V atoms. For this purpose, the glide velocity of an edge dislocation was calculated as a function of V concentration under the shear stresses of 100-300 MPa using the Fe-V cross-potential constructed newly in the present study. Whereas the solid solution hardening occurred as the V concentration is less than 0.13 at% or more than 0.5 at%, the room-temperature solid solution softening was observed in Fe-(0.13-0.5) at% V alloys. The solid solution softening occurring in Fe-(0.13-0.5) at% V alloys was caused by the accelerated growth velocity of kinks by solute V atoms. The increase in kink velocity happened when the interatomic distance between solute V atoms was similar to the length of dislocation kinks.
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  • Prediction of hydrogen permeability in V–Al and V–Ni alloys

    Shim, Jae-Hyeok   Ko, Won-Seok   Kim, Ki-Hyun   Lee, Heung-Soon   Lee, Young-Su   Suh, Jin-Yoo   Cho, Young Whan   Lee, Byeong-Joo  

    A semi-empirical methodology for predicting the permeability of hydrogen in metallic alloys is proposed by combining an atomistic simulation and a thermodynamic calculation. An atomistic simulation based on a modified embedded-atom method interatomic potential and a CALPHAD-type thermodynamic calculation technique was used to predict the diffusivity and solubility of hydrogen, respectively. The approach was applied to the prediction of the hydrogen permeability in V-Al and V-Ni alloys that are promising for non-Pd hydrogen separation membranes. The predicted permeability of hydrogen decreases, as Al or Ni concentration increases in the alloys. The predicted permeability is in quite good agreement with experimental data available in literature, successfully reproducing the overall trend for the effect of alloying elements, which enables an alloy design of metallic hydrogen permeable membranes. [All rights reserved Elsevier].
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  • Atomistic simulation of hydrogen diffusion at tilt grain boundaries in vanadium

    Shim, Jae-Hyeok   Ko, Won-Seok   Suh, Jin-Yoo   Lee, Young-Su   Lee, Byeong-Joo  

    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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  • Pressure-enhanced dehydrogenation reaction of the LiBH(4)-YH(3) composite

    Kim, Kee-Bum   Shim, Jae-Hyeok   Cho, Young Whan   Oh, Kyu Hwan  

    The increase in hydrogen back pressure unexpectedly enhances the overall dehydrogenation reaction rate of the 4LiBH(4) + YH(3) composite significantly. Also, argon back pressure has a similar influence on the composite. Gas back pressure seems to enhance the dehydrogenation reaction by kinetically suppressing the formation of the diborane by-product.
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  • A modified embedded-atom method interatomic potential for the V-H system

    Shim, Jae-Hyeok   Lee, Young-Su   Fleury, Eric   Cho, Young Whan   Ko, Won-Seok   Lee, Byeong-Joo  

    An interatomic potential for the vanadium-hydrogen binary system has been developed based on the second nearest-neighbor modified embedded-atom method (2NN MEAM) potential formalism, in combination with the previously developed potentials for V and H. Also, first-principles calculation has been carried out to provide data on the physical properties of this system, which are necessary for the optimization of the potential parameters. The developed potential reasonably reproduces the fundamental physical properties (thermodynamic, diffusion, elastic and volumetric properties) of V-rich bcc solid solution and some of the vanadium hydride phases. The applicability of this potential to the development of V-based alloys for hydrogen applications is discussed. (C) 2011 Elsevier Ltd. All rights reserved.
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  • Hydrogenation reaction of CaH(2)-CaB(6)-Mg mixture RID C-3834-2012

    Kim, Yoonyoung   Reed, Daniel   Lee, Young-Su   Shim, Jae-Hyeok   Han, Heung Nam   Book, David   Cho, Young Whan  

    Hydrogen desorption/absorption cycling properties of Ca(BH(4))(2)+MgH(2) mixture having 9.1 wt% of theoretical hydrogen capacity have been investigated. The hydrogenation reaction sequence starting from the dehydrogenated product CaH(2)+CaB(6)+Mg was thoroughly followed using X-ray diffraction, and that can be summarized as (i) Mg+H(2) -> MgH(2), (ii) 4CaH(2)+3MgH(2 ->)Ca(4)Mg(3)H(14), (iii) Ca(4)Mg(3)H(14)+2CaB(6)+20H(2)-> 6Ca(BH(4))(2)+3MgH(2). The steps (i) and (ii) proceeded rather fast, and the total amount of reabsorbed hydrogen was governed by the step (iii). The kinetics of the step (iii) was critically affected by the temperature of dehydrogenation/hydrogenation reaction, which we mainly attributed to the microstructure of the dehydrogenated product such as particle size. We indirectly proved our hypothesis by showing that inserting high-energy ball-milling process during hydrogenation is more effective than simply increasing the reaction time. Optimal reversibility was achieved from the sample which was dehydrogenated at 673 K and hydrogenated at 623 K. (c) 2009 Elsevier B.V. All rights reserved.
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  • Polymorphism and Thermodynamics of Y(BH(4))(3) from First Principles

    Lee, Young-Su   Shim, Jae-Hyeok   Cho, Young Whan  

    Structure and thermodynamics of the recently discovered two polymorphs of Y(BH(4))(3) are investigated by first-principles calculation. Simulated X-ray and neutron diffraction patterns combined with structure analysis from first principles enable us to assign a space group Fm (3) over barc to the high-temperature polymorph among the several proposed space groups. An orientational disorder of [BH(4)](-) groups is considered and compared with NaBH(4), which has a disordered [BH(4)](-) arrangement at room temperature. In the case of NaBH(4), the structure stays in a local energy minimum irrespective of the [BH(4)](-) orientation, but in Y(BH(4))(3), [BH(4)](-) reorientation is suppressed by a strong repulsive force created by close H-H contacts and the structure becomes unstable, thus favoring an ordered [BH(4)](-) arrangement. The calculated high energy barrier for the [BH(4)](-) reorientation partly accounts for the slow phase transition observed in Y(BH(4))(3), again making a good contrast with the facile [BH(4)](-) flipping in NaBH(4). The thermodynamics of Y(BH(4))(3) appears quite attractive, exhibiting a lower dissociation temperature than Mg(BH(4))(2) under 1 bar of H(2).
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  • New Equation for Prediction of Martensite Start Temperature in High Carbon Ferrous Alloys

    Park, Jihye   Shim, Jae-Hyeok   Lee, Seok-Jae  

    Since previous equations fail to predict M (S) temperature of high carbon ferrous alloys, we first propose an equation for prediction of M (S) temperature of ferrous alloys containing > 2 wt pct C. The presence of carbides (Fe3C and Cr-rich M C-7(3)) is thermodynamically considered to estimate the C concentration in austenite. Especially, equations individually specialized for lean and high Cr alloys very accurately reproduce experimental results. The chemical driving force for martensitic transformation is quantitatively analyzed based on the calculation of T (0) temperature.
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  • Cyclic oxidation of yttria dispersed austenitic stainless steels RID D-1605-2011

    Phaniraj, M. P.   Kim, Dong-Ik   Shim, Jae-Hyeok   Cho, Young Whan  

    Cyclic oxidation of austenitic steels (Fe-20Ni-14Cr-2.5Mo-2Mn-2.5Al-wt.%) dispersed with 0, 0.5 and 5 wt.% yttria was carried out at 800 degrees C in air. The scale surface and cross-section were characterized using X-ray diffraction, scanning electron microscopy (SEM), electron probe microanalysis (EPMA) and X-ray photoelectron spectroscopy (XPS). Yttria additions improve the resistance to spallation. The increase in the resistance to spallation appears to be related to the presence of mixed oxides between yttria and base metal oxides. (C) 2010 Elsevier Ltd. All rights reserved.
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  • Identification of the Dehydrogenated Product of Ca(BH(4))(2)

    Kim, Yoonyoung   Reed, Daniel   Lee, Young-Su   Lee, Ji Youn   Shim, Jae-Hyeok   Book, David   Cho, Young Whan  

    We have investigated the decomposition path and reversibility of Ca(BH(4))(2) and Ca(BH(4))(2) + MgH(2) composite using X-ray diffraction, thermogravimetric analysis, differential scanning calorimetry, and Raman spectroscopy. Formation of CaB(6) during dehydrogenation of both systems was confirmed for the first time. CaB(6) appears as broad peaks in X-ray diffraction data, but Raman spectroscopy unambiguously captures the existence of CaB(6). Reversibility of catalyzed Ca(BH(4))(2) was previously reported, and here we demonstrate reversibility of Ca(BH(4))(2) + MgH(2) composite. Dehydrogenated product of Ca(BH(4))(2) + MgH(2) is composed of CaH(2), CaB(6), and Mg. About 60% reversibility was achieved after rehydrogenation for 24 h under 90 bar of hydrogen pressure at 350 degrees C even without the help of catalysts, which makes a good contrast with the case of pure Ca(BH(4))(2) where almost negligible rehydrogenation occurs under the same conditions. To understand the difference, the role of Mg in rehydrogenation is worth further investigation. Formation of CaB(6) seems critical in the reversibility of Ca(BH(4))(2) containing systems; the case of other borohydrides is compared.
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  • Multiscale modeling of point defect interactions in Fe-Cr alloys

    Wong, Kwan L.   Lee, Hyon-Jee   Shim, Jae-Hyeok   Sadigh, Babak   Wirth, Brian D.  

    Predictive performance models of ferritic/martensitic alloys in fusion neutron irradiation environments require knowledge of point defect interactions with Cr, which can be investigated by a multiscale modeling approach. Molecular dynamics simulations, using Finnis-Sinclair-type potentials, have been used to investigate the interstitial diffusion and reveal that the extremes of attractive and repulsive binding between Cr and interstitials change the characteristics of interstitial migration and the Cr-to-Fe diffusivity ratio. Ab-initio calculations have been performed to determine the vacancy-Cr interactions, and these calculations reveal complex electronic and magnetic interactions between Cr and Fe. The ab-initio values have been used to calculate the Cr-to-Fe diffusivity ratio by a vacancy mechanism using the LeClaire multi-frequency model and a kinetic lattice Monte Carlo model, both of which indicate that Cr diffuses faster than Fe. The modeling results are discussed in the context of the radiation-induced segregation of Cr at grain boundaries in BCC Fe-Cr alloys. (C) 2009 Published by Elsevier B.V.
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  • Evaluation of W-Si-C thick coating as a plasma facing material

    Seok, Hyun Kwang   Jung, Kyung Ho   Kim, Yu Chan   Shim, Jae-Hyeok   Kim, Dong-Ik   Han, Seung-Hee   Baik, Kyeong Ho   Cha, Pil-Ryung  

    We present tungsten alloy coating of 150-200 pm thickness with improved plasma erosion resistance fabricated by plasma spraying of granular W-SiC composite powders. During increasing the SiC concentration to 8 wt%, we observed the increase in the hardness of the coating from 250 to 440 Hv. The plasma erosion depth of the coating decreased by 10 times compared with pure tungsten in the same erosion environment. (C) 2009 Elsevier B.V. All rights reserved.
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  • Temperature dependent dislocation bypass mechanism for coherent precipitates in Cu-Co alloys

    Shim, Jae-Hyeok   Voigt, Hyon-Jee Lee   Wirth, Brian D.  

    Molecular dynamics simulations of dislocation interaction with coherent cobalt precipitates embedded in Cu-Co alloys reveal a temperature dependent bypass mechanism. Below 300 K, the trailing partial dislocation clearly bypasses the coherent, face centered cubic (FCC) cobalt precipitate by Orowan looping, caused by a reversible structural transformation as the leading partial locally converts the precipitate to the lower-energy hexagonal close packed (HCP) structure. The FCC versus HCP energy difference of cobalt is temperature dependent, and the dislocation bypass mechanism becomes pure shear above 300 K. Based on a combination of inertial effects due to phonon drag and this observed bypass mechanism, we develop a temperature dependent critical resolved shear stress (CRSS) model, which is in excellent agreement with long-standing measurements of the CRSS temperature dependence of Cu-Co alloys, and those obtained from MD simulation. The model explains both the CRSS increase at low temperatures and the existence of a peak value around 200 K. (C) 2016 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
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  • Temperature dependent dislocation bypass mechanism for coherent precipitates in Cu–Co alloys

    Shim, Jae-Hyeok   Voigt, Hyon-Jee Lee   Wirth, Brian D.  

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  • Decomposition Reactions and Reversibility of the LiBH(4)-Ca(BH(4))(2) Composite

    Lee, Ji Youn   Ravnsbaek, Dorthe   Lee, Young-Su   Kim, Yoonyoung   Cerenius, Yngve   Shim, Jae-Hyeok   Jensen, Torben R.   Hur, Nam Hwi   Cho, Young Whan  

    LiBH(4) is one of the promising candidates for hydrogen storage materials because of its high gravimetric and volumetric hydrogen capacity. However, its high dehydrogenation temperature and limited reversibility has been a hurdle for its use in real applications. In an effort to overcome this barrier and to adjust the thermal stability, we make a composite system LiBH(4)-Ca(BH(4))(2). In order to fully characterize this composite system we study xLiBH(4) + (1 - x)Ca(BH(4))(2) for several x values between 0 and 1, using differential scanning calorimetry, in situ synchrotron X-ray diffraction, thermogravimetric analysis, and mass spectrometry. Interestingly, this composite undergoes a eutectic melting at ca. 200 degrees C in a wide composition range, and the eutectic composition lies between x = 0.6 and 0.8. The decomposition characteristics and the hydrogen capacity of this composite vary with x, and the decomposition temperature is lower than both the pure LiBH(4) and Ca(BH(4))(2) at intermediate conpositions, for example, for x approximate to 0.4, decomposition is finished below 400 degrees C releasing about 10 wt % of hydrogen. Partial reversibility of this system was also confirmed for the first time for the case of if mixed borohydride composite.
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  • Thermal decomposition behavior of calcium borohydride Ca(BH(4))(2)

    Kim, Jae-Hun   Jin, Seon-Ah   Shim, Jae-Hyeok   Cho, Young Whan  

    The thermal decomposition behavior of adduct-free Ca(BH(4))(2), prepared by heating Ca(BH4)(2)center dot 2THF powder under vacuum, was investigated by X-ray diffraction and thermal analyses. It has been found that Ca(BH(4))(2) undergoes a polymorphic transformation at 440 K and eventually decomposes in two steps between 620 and 770 K. CaH(2) and an unknown intermediate compound form after the first step, but CaH(2) is the only crystalline phase observed after the second step with a total weight loss of about 9.0 wt.%. (c) 2007 Elsevier B.V. All rights reserved.
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