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Now showing items 1 - 16 of 26

  • Ductility enhancement of tungsten after plastic deformation

    Oh, Yeonju   Kwak, Nojun   Lee, Keunho   Ko, Won-Seok   Han, Heung Nam  

    An unusual room temperature mechanical behavior of pure tungsten is investigated by focusing on three specimens prepared with different microstructures: as-received (hot-rolled), recrystallized, and cold-rolled specimens. Contrary to ordinary expectations in metallic materials, only the cold-rolled specimen exhibits significant plastic deformation during tensile testing, with improved strength and ductility, while the recrystallized and the as-received specimens fail in the elastic region. We further provide an explanation of such characteristics by systematically utilizing experimental and theoretical analysis at a small scale: nano-indentation tests clarify the inherent mechanical response inside grains and provide a statistical distribution of the maximum shear stress during pop-in events, corresponding to onset of plastic yielding; atomistic simulations provide information on the overall fracture strength of various grain boundaries. By comparing the maximum shear stress and the grain boundary fracture stress, we could explain that the distinctive plasticity in the cold-rolled specimen is caused by the movement of pre-existing dislocations before grain boundary fracture. This contrasts the recrystallized and as-received specimens, where grain boundary fracture occurs prior to the nucleation of dislocations or activation of dislocation sources. (C) 2019 Elsevier B.V. All rights reserved.
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  • Atomistic simulations of PdTi high-temperature shape-memory alloys

    Ko, Won-Seok   Jeon, Jong Bae  

    Martensitic phase transformations in palladium-titanium high-temperature shape-memory alloys (HTSMA) are studied using molecular dynamics simulations. On the basis of the second nearest-neighbor modified embedded atom method formalism, an interatomic potential for the binary palladium-titanium system is determined by improving the unary descriptions of pure palladium. The developed interatomic potential accurately reproduces physical properties at the equiatomic composition and the resultant temperature- and stress-induced phase transformations between B2 austenite and B19 martensite structures. Subsequent large-scale molecular dynamics simulations demonstrate that the developed potential can be successfully utilized to investigate atomic details of phase transformations in nanocrystalline palladium-titanium alloys.
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  • Ductility enhancement of tungsten after plastic deformation

    Oh, Yeonju   Kwak, Nojun   Lee, Keunho   Ko, Won-Seok   Han, Heung Nam  

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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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  • 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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  • Atomistic modeling of the Al–H and Ni–H systems

    Ko, Won-Seok   Shim, Jae-Hyeok   Lee, Byeong-Joo  

    Second nearest-neighbor modified embedded-atom method (MEAM) interatomic potentials for the Al-H and Ni-H binary systems have been developed on the basis of previously developed MEAM potentials of pure Al, Ni, and H. The potentials can describe various fundamental physical properties of the relevant binary alloys (structural, thermodynamic, defect, and dynamic properties of metastable hydrides or hydrogen in face-centered cubic solid solutions) in good agreement with experiments or first-principles calculations. The applicability of the present potentials to atomic level investigations of dynamic behavior of hydrogen atoms in metal membranes is also discussed.
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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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  • Interatomic potential that describes martensitic phase transformations in pure lithium

    Ko, Won-Seok   Jeon, Jong Bae  

    An interatomic potential for the pure lithium system is developed on the basis of the second nearest neighbor modified embedded-atom method formalism, utilizing the force-matching method with a DFT database of various atomic configurations. The developed potential accurately reproduces fundamental physical properties including an unusual order of surface energies of the bcc lithium, (100) < (110) <(111). Subsequent molecular dynamics simulations verify that the present potential can be successfully applied to study martensitic phase transformations of pure lithium at low temperatures. The present results provide detailed insights into the formation of a disordered polytype structure consisting of short-ranged fcc- and hcp-type stacking sequences supporting the experimental observation of this structure in high-purity lithium. (C) 2016 Elsevier E.V. All rights reserved.
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  • Interatomic potential that describes martensitic phase transformations in pure lithium

    Ko, Won-Seok   Jeon, Jong Bae  

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  • Intergranular embrittlement of iron by phosphorus segregation: an atomistic simulation

    Ko, Won-Seok   Jeon, Jong Bae   Lee, Chang-Hoon   Lee, Jae-Kon   Lee, Byeong-Joo  

    The intergranular embrittlement in bcc iron by the grain boundary (GB) segregation of phosphorus is investigated using an atomistic simulation. The inhibition of the nucleation of dislocations near the crack tip is found to be the governing mechanism of the intergranular embrittlement in phosphorus-containing iron, in contrast to the conventional reasoning that focuses on the GB decohesion. The correlation between the nucleation of dislocations and dislocation transfer across a GB (GB strengthening) is discussed. Experimental evidence and supplementary simulation results that support the new finding in terms of the GB strengthening are also demonstrated.
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  • Temperature dependence of NiTi martensite structures:Density functional theory calculations

    Ko, Won-Seok  

    Temperature dependences of the stability and structural parameters of the martensite phase in nickel titanium shape-memory alloys are studied by applying a harmonic approximation within density functional theory. By analyzing detailed free energy landscapes of martensite structures with various monoclinic angles, the transformation barrier between the reported (B19') and the hypothetical (B33) martensite structures, the monoclinic angle, and the lattice constants of the martensite structure have been thoroughly investigated. The results show that the vibrational effect at finite temperatures plays a decisive role in resolving the reported discrepancies between experimental and theoretical results. (C) 2018 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
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  • Atomic scale processes of phase transformations in nanocrystalline NiTi shape-memory alloys

    Ko, Won-Seok   Maisel, Sascha B.   Grabowski, Blazej   Jeon, Jong Bae   Neugebauer, J?rg  

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  • Short-range order strengthening in boron-doped high-entropy alloys for cryogenic applications

    Seol, Jae Bok   Bae, Jae Wung   Kim, Jung Gi   Sung, Hyokyung   Li, Zhiming   Lee, Hyun Hwi   Shim, Sang Hun   Jang, Jae Hoon   Ko, Won-Seok   Hong, Sun Ig   Kim, Hyoung Seop  

    Boron doping with an adequate concentration is highly desirable for alloy development because it profoundly improves the material's interface cohesion via interfacial segregation. However, scientific and applicable potentials of soluble boron that resides at the alloy internal grains are generally overlooked. Here we report a strategy for overcoming the typically low strengths of face-centered cubic high-entropy alloys (HEAs) through exploiting soluble boron instead of the interfacial boron. We find that soluble boron increases stress strain field at the recrystallized HEA grain structure, leading to the generation of short-range order (SRO) in those deformation structure under load at 77 K. The highly increased degree of SRO at planar dislocation slip band that forms during straining, proved by electron microscopy and synchrotron X-ray diffraction, strengthens a typical non-equimolar Fe40Mn40Co10Cr10 (at%) HEA, particularly increasing yield strengths by similar to 32%, to similar to 1.1 GPa compared to those of boron-free reference materials with similar grain sizes. The advent of deformation-induced SRO domains causes severe lattice distortion (specifically, contraction), leading to the increased cryogenic yield strength of similar to 210 MPa, but generating micro-voids at grain boundaries. This study on deformation-induced SRO via boron advances the fundamental understanding of SRO impacts on HEA grains and mechanical properties at cryogenic temperatures, which may pave a general pathway for developing a wide range of ultrastrong alloys for cryogenic applications. (C) 2020 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
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  • Origin of unrealistic blunting during atomistic fracture simulations based on MEAM potentials

    Ko, Won-Seok   Lee, Byeong-Joo  

    Atomistic simulations based on interatomic potentials have frequently failed to correctly reproduce the brittle fracture of materials, showing an unrealistic blunting. We analyse the origin of the unrealistic blunting during atomistic simulations by modified embedded-atom method (MEAM) potentials for experimentally well-known brittle materials such as bcc tungsten and diamond silicon. The radial cut-off which has been thought to give no influence on MEAM calculations is found to have a decisive effect on the crack propagation behaviour. Extending both cut-off distance and truncation range can prevent the unrealistic blunting, reproducing many well-known fracture behaviour which have been difficult to reproduce. The result provides a guideline for future atomistic simulations that focus on various fracture-related phenomena including the failure of metallic-covalent bonding material systems using MEAM potentials.
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  • Modified embedded-atom method interatomic potentials for pure Y and the V–Pd–Y ternary system

    Ko, Won-Seok   Lee, Byeong-Joo  

    Interatomic potentials for pure Y and the V-Pd-Y ternary system have been developed on the basis of the second nearest-neighbor modified embedded-atom method (2NN MEAM) formalism, with a purpose of investigating the interdiffusion mechanism and the role of yttrium in the palladium-coated vanadium-based hydrogen separation membranes. The potentials can describe various fundamental physical properties of pure Y (the bulk, defect and thermal properties) and the alloy behaviors (structural, thermodynamic and defect properties of solid solutions and compounds) of constituent systems in reasonable agreement with experimental data or first-principles calculations.
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  • Atomistic modeling of an impurity element and a metal–impurity system: pure P and Fe–P system

    Ko, Won-Seok   Kim, Nack J   Lee, Byeong-Joo  

    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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