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Now showing items 1 - 16 of 144

  • HIGH-ENTROPY ALLOY FOR ULTRA-LOW TEMPERATURE

    The present invention relates to a high-entropy alloy especially having excellent low-temperature tensile strength and elongation by means of having configured, through thermodynamic calculations, an alloy composition region having a FCC single-phase microstructure at 700°C or higher, and enabling the FCC single-phase microstructure at room temperature and at an ultra-low temperature. The high-entropy alloy, according to the present invention, comprises: Co: 3-12 at%; Cr: 3-18 at%; Fe: 3-50 at%; Mn: 3-20 at%; Ni: 17-45 at%; V: 3-12 at%; and inevitable impurities, wherein the ratio of the V content to the Ni content (V/Ni) is 0.5 or less, and the sum of the V content and the Co content is 22 at% or less.
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  • CR-FE-MN-NI-V-BASED HIGH-ENTROPY ALLOY

    The present invention relates to a high-entropy alloy especially having excellent low-temperature tensile strength and elongation by means of having configured, through thermodynamic calculations, an alloy composition region having a FCC single-phase microstructure at 700°C or higher, and enabling the FCC single-phase microstructure at room temperature and at an ultra-low temperature. The high-entropy alloy, according to the present invention, comprises: Cr: 3-18 at%; Fe: 3-60 at%; Mn: 3-40 at%; Ni: 20-80 at%; V: 3-12 at%; and inevitable impurities, wherein the ratio of the V content to the Ni content (V/Ni) is 0.5 or less.
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  • Novel Co-rich high entropy alloys with superior tensile properties

    Wei, Daixiu   Li, Xiaoqing   Heng, Weicheng   Koizumi, Yuichiro   He, Feng   Choi, Won-Mi   Lee, Byeong-Joo   Kim, Hyoung Seop   Kato, Hidemi   Chiba, Akihiko  

    We developed a series of Co-rich CoxCr25(FeNi)(75-x) (x =3D 35, 45, 55, 65) high entropy alloys with improved strength and/or ductility, derived from lowering the stacking fault energy (SFE) and reducing the fcc phase stability of the equiatomic CoCrFeNi alloy. Thermodynamics and ab initio calculations demonstrated that increasing Co while decreasing Fe and Ni concentrations lower the SFE and reduce the fcc phase stability. The Co35Cr25Fe20Ni20 and Co45Cr25Fe15Ni15 alloys with single fcc phase, exhibit superior tensile properties, contributing to the twinning and fcc -> hcp martensitic transformation. The present study offers a guideline for designing high-performance high entropy alloys. [GRAPHICS] IMPACT STATEMENT A series of novel Co-rich non-equiatomic high entropy alloys with enhanced tensile properties were developed by lowering the stacking fault energy and reducing the phase stability of equiatomic CoCrFeNi alloy.
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  • Oxidation-resistant titanium carbide MXene films

    Lee, Yonghee   Kim, Seon Joon   Kim, Yong-Jae   Lim, Younghwan   Chae, Yoonjeong   Lee, Byeong-Joo   Kim, Young-Tae   Han, Hee   Gogotsi, Yury   Ahn, Chi Won  

    Two-dimensional transition metal carbides (MXenes) have attracted much attention due to their excellent electrical conductivity and outstanding performances in energy storage, telecommunication, and sensing applications. It is known that MXene flakes are readily oxidized in either humid air or aqueous environments. While the chemical instability of MXenes may limit their use in applications involving ambient environments and long-term operation, oxidation behaviour of MXene films has not been addressed. In this work, we demonstrate a hydrogen annealing method to increase the oxidation stability of Ti3C2 MXene in two different aspects: (1) dramatic improvement in the oxidation stability of pristine MXene films against harsh conditions (100% relative humidity, 70 degrees C), and (2) large recovery in the electrical conductivity of previously oxidized Ti3C2 MXene films. We also demonstrate an electric-field-induced heater capable of stable operation under highly oxidizing conditions, based on the oxidation-resistant MXene film. A total loss of heat generation ability was observed for the as-prepared MXene film, while the hydrogen-annealed one maintained its bright infrared radiation, under the highly oxidizing conditions. This work offers a solution to industrial applications of unprotected MXene films, securing their stable and long-term operation in humid conditions.
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  • Novel Co-rich high entropy alloys with superior tensile properties

    Wei, Daixiu   Li, Xiaoqing   Heng, Weicheng   Koizumi, Yuichiro   He, Feng   Choi, Won-Mi   Lee, Byeong-Joo   Kim, Hyoung Seop   Kato, Hidemi   Chiba, Akihiko  

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  • An intermediate temperature creep model for Ni-based superalloys

    Kim, Young-Kwang   Kim, Donghoon   Kim, Hong-Kyu   Oh, Chang-Seok   Lee, Byeong-Joo  

    A mathematical model to predict creep properties of Ni-based superalloys at intermediate temperature ranges based on microstructure, process and creep condition, and material properties has been developed by modifying and combining previous models. Modifications are made so that as many necessary model parameters as possible, whose values have been obtained from experimental measurement of creep properties, are determined from CALPHAD (CALculation of PHAse Diagrams) thermodynamic calculations. The model can reproduce minimum creep rate, creep rupture time, and time to specific strain for a wide range of Ni-based superalloys, correlating well with experimental data without any further creep experiments. The applicability and limitation of the model as an alloy design tool for new Ni-based superalloys with improved creep properties are discussed in this paper. (c) 2015 Elsevier Ltd. All rights reserved.
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  • {100} texture evolution in bcc Fe sheets - Computational design and experiments

    Kim, Kyeong-Min   Kim, Hyun-Kyu   Park, Jun Young   Lee, Jae Sang   Kim, Seong Gyoon   Kim, Nack Joon   Lee, Byeong-Joo  

    A computational and experimental study has been carried out to produce highly value-added {100} textured steel sheets. This study is based on an idea that an anisotropic surface segregation tendency of impurity atoms would cause an inversion of relative surface energy between {110} and {100} surfaces in bcc Fe and may induce the formation of {100} texture in steel sheets during grain coarsening. A phase field model that can consider surface segregation of impurity atoms (and resultant decrease of surface energy) and grain growth simultaneously is newly developed. The phase field model and surface property data obtained from an atomistic approach are used to simulate grain coarsening behavior of phosphorus-containing iron and to find an optimum process condition that can induce the {100} texture in steel sheets. The phase field simulation indicates that the surface segregation and resultant change in the relative difference in surface energy should be accomplished before the start of grain coarsening for an effective formation of the {100} texture. A strong {100} texture is indeed generated experimentally in phosphorus-containing bcc Fe-3.5wt% Si steel sheets through a two-step annealing process, one at a relatively low temperature where only surface segregation can occur but not the grain coarsening and the other at a relatively high temperature where the grain coarsening can also occur. (C) 2016 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
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  • Theory for plasticity of face-centered cubic metals

    Jo, Minho   Koo, Yang Mo   Lee, Byeong-Joo   Johansson, Borje  

    The activation of plastic deformation mechanisms determines the mechanical behavior of crystalline materials. However, the complexity of plastic deformation and the lack of a unified theory of plasticity have seriously limited the exploration of the full capacity of metals. Current efforts to design high-strength structural materials in terms of stacking fault energy have not significantly reduced the laborious trial and error works on basic deformation properties. To remedy this situation, here we put forward a comprehensive and transparent theory for plastic deformation of face-centered cubic metals. This is based on a microscopic analysis that, without ambiguity, reveals the various deformation phenomena and elucidates the physical fundaments of the currently used phenomenological correlations. We identify an easily accessible single parameter derived from the intrinsic energy barriers, which fully specifies the potential diversity of metals. Based entirely on this parameter, a simple deformation mode diagram is shown to delineate a series of convenient design criteria, which clarifies a wide area of material functionality by texture control.
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  • METHOD FOR IMPROVING PROCESSABILITY OF HIGH-ENTROPY ALLOY TO WHICH AL IS ADDED

    The purpose of the present invention relating to a method for improving the processability of a high-entropy alloy to which AL is added is to resolve the problem of limitations of enlargement and industrial application according to the deterioration of processability caused by the brittleness of an high-entropy alloy to which AL is added and which has a BCC phase formed at a low temperature. The method according to the present invention performs treatment before processing a high-entropy alloy having a BCC phase formed by the addition of Al in an FCC single phase, and then performs processing, the treatment comprising the steps of: heating the high-entropy alloy at at least a temperature at which the phase fraction of the BCC phase thereof is 6% or less, and maintaining the same until the phase fraction is reached; and performing cooling such that the BCC phase fraction of the heated high-entropy alloy is maintained. The method enables enlargement of an alloy and processing thereof into a desired shape by resolving weak processability of the high-entropy alloy to which AL is added and which is effective for ensuring price competitiveness and reducing weight, and thus can increase the industrial applicability of the high-entropy alloy to which AL is added.
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  • Plasma oxidation of thermally grown graphenes and their characterization

    Lee, Byeong-Joo   Jeong, Goo-Hwan  

    We report the plasma oxidation of thermally synthesized graphenes and their characterization using Raman spectroscopy and atomic force microscope (AFM). A graphene was synthesized by thermal chemical vapor deposition with methane and transferred onto trench substrate to make suspended configuration in order to exclude substrate effects. The air plasma treatment at 0.4 W for 5 min and property characterization were alternately performed to address the effect of oxidation. After the oxidation, a drastic change in Raman spectra was observed, which implies that considerable structural changes occurred in the graphene. Interestingly, we observed from the Raman and AFM analyses that the number of layers can be reduced by the controlled plasma oxidation treatment. The results may open the possibility of graphene formation from graphite sheets through the precise control of plasma treatment conditions. [All rights reserved Elsevier].
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  • Plasma oxidation of thermally grown graphenes and their characterization

    Lee, Byeong-Joo   Jeong, Goo-Hwan  

    We report the plasma oxidation of thermally synthesized graphenes and their characterization using Raman spectroscopy and atomic force microscope (AFM). A graphene was synthesized by thermal chemical vapor deposition with methane and transferred onto trench substrate to make suspended configuration in order to exclude substrate effects. The air plasma treatment at 0.4 W for 5 min and property characterization were alternately performed to address the effect of oxidation. After the oxidation, a drastic change in Raman spectra was observed, which implies that considerable structural changes occurred in the graphene. Interestingly, we observed from the Raman and AFM analyses that the number of layers can be reduced by the controlled plasma oxidation treatment. The results may open the possibility of graphene formation from graphite sheets through the precise control of plasma treatment conditions. (C) 2012 Elsevier Ltd. All rights reserved.
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  • IRON-BASED ALLOY FOR POWDER INJECTION MOLDING

    Disclosed is an iron-based alloy for powder injection molding. The iron-based alloy for powder injection molding is characterized by comprising 52.59-78.15 wt % of iron (Fe), 16.45-37.34 wt % of chromium (Cr), 3.42-7.76 wt % of boron (B), 1.64-1.92 wt % of silicon (Si), 0-0.21 wt % of sulfur (S), 0.16-0.18 wt % of carbon (C), and unavoidable impurities.
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  • Design of new face-centered cubic high entropy alloys by thermodynamic calculation

    Choi, Won-Mi   Jung, Seungmun   Jo, Yong Hee   Lee, Sunghak   Lee, Byeong-Joo  

    A new face-centered cubic (fcc) high entropy alloy system with non-equiatomic compositions has been designed by utilizing a CALculation of PHAse Diagram (CALPHAD) - type thermodynamic calculation technique. The new alloy system is based on the representative fcc high entropy alloy, the Cantor alloy which is an equiatomic Co- Cr-Fe-Mn-Ni five-component alloy, but fully or partly replace the cobalt by vanadium and is of non-equiatomic compositions. Alloy compositions expected to have an fcc single-phase structure between 700 A degrees C and melting temperatures are proposed. All the proposed alloys are experimentally confirmed to have the fcc single-phase during materials processes (> 800 A degrees C), through an X-ray diffraction analysis. It is shown that there are more chances to find fcc single-phase high entropy alloys if paying attention to non-equiatomic composition regions and that the CALPHAD thermodynamic calculation can be an efficient tool for it. An alloy design technique based on thermodynamic calculation is demonstrated and the applicability and limitation of the approach as a design tool for high entropy alloys is discussed.
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  • Atomistic modeling of pure Co and Co-Al system

    Dong, Wei-Ping   Kim, Hyun-Kyu   Ko, Won-Seok   Lee, Byeong-Moon   Lee, Byeong-Joo  

    Interatomic potentials for pure Co and the Co-Al binary system have been developed based on the second nearest-neighbor modified embedded-atom method (2NN MEAM) potential formalism. The potentials can describe various fundamental physical properties of the relevant materials in good agreement with experimental information. The potential is utilized to an atomistic computation of interfacial properties between fcc-Co (gamma) and Co3Al (gamma') phases. It is found that the anisotropy in the gamma/gamma' interfacial energy is relatively small and leaves a room for further modification by alloying other elements. The applicability of the atomistic approach to an elaborate alloy design of advanced Co-based superalloys through the investigation of the effect of alloying elements on interfacial and elastic properties is discussed. (C) 2012 Elsevier Ltd. All rights reserved.
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  • Atomistic modeling of pure Li and Mg-Li system

    Kim, Young-Min   Jung, In-Ho   Lee, Byeong-Joo  

    Interatomic potentials for pure Li and the Mg-Li binary system have been developed based on the second nearest-neighbor modified embedded-atom method formalism. The potentials can describe various fundamental physical properties of pure Li (bulk, point defect, planar defect and thermal properties) and alloy behaviors (thermodynamic, structural and elastic properties) in reasonable agreement with experimental data or higher-level calculations. The applicability of the potential to atomistic investigations on the deformation behavior of Mg alloys and the effect of Li is demonstrated.
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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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