Keonwook Kang

Yonsei University, Sŏul, Seoul, South Korea

Are you Keonwook Kang?

Claim your profile

Publications (13)50.39 Total impact

  • Keonwook Kang, Jie Yin, Wei Cai
    [Show abstract] [Hide abstract]
    ABSTRACT: The energy barrier for the cross slip of screw dislocations in face-centered cubic (FCC) nickel as a function of multiple stress components is predicted by both continuum line tension and discrete atomistic models. Contrary to Escaig's claim that the Schmid stress component has a negligible effect on the energy barrier, we find that the line tension model, when solved numerically, predicts comparable effects from the Schmid stress and the Escaig stress on the cross slip plane. When the line tension model is compared against an atomistic model for FCC nickel, a good agreement is found for the effect of the Escaig stress on the glide plane. However, the atomistic model predicts a stronger effect than the line tension model for the two stress components on the cross slip plane. This discrepancy is larger at higher stresses and is also more severe for the Escaig stress component than for the Schmid stress component.
    Journal of the Mechanics and Physics of Solids 01/2014; 62:181–193. · 4.29 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: We examine the development of stable bimetal interfaces in nanolayered composites in severe plastic deformation. Copper-niobium multilayers of varying layer thicknesses from several micrometers to 10 nanometers (nm) were fabricated via accumulative roll bonding (ARB). Investigation of their 5-parameter character and atomic scale structure finds that when layer thicknesses refine well below one micrometer, the interfaces self-organize to a few interface orientation relationships. With atomic scale and crystal plasticity modeling, we identify that the two controlling factors that determine whether an interface is stable under high strain rolling are orientation stability of the bicrystal and interface formation energy. A figure-of-merit is introduced that not only predicts the development of the prevailing interfaces but also explains why other interfaces did not develop. Through a suite of nanomechanical and bulk test results, we show that ARB composites containing these stable interfaces are found to have exceptional hardness (∼4.5 GPa) and strength (∼2 GPa).
    Journal of Materials Research. 07/2013; 28(13).
  • [Show abstract] [Hide abstract]
    ABSTRACT: Bulk nanostructured metals can attribute both exceptional strength and poor thermal stability to high interfacial content, making it a challenge to utilize them in high-temperature environments. Here we report that a bulk two-phase bimetal nanocomposite synthesised via severe plastic deformation uniquely possesses simultaneous high-strength and high thermal stability. For a bimetal spacing of 10 nm, this composite achieves an order of magnitude increase in hardness of 4.13 GPa over its constituents and maintains it (4.07 GPa), even after annealing at 500 °C for 1 h. It owes this extraordinary property to an atomically well-ordered bimaterial interface that results from twin-induced crystal reorientation, persists after extreme strains and prevails over the entire bulk. This discovery proves that interfaces can be designed within bulk nanostructured composites to radically outperform previously prepared bulk nanocrystalline materials, with respect to both mechanical and thermal stability.
    Nature Communications 04/2013; 4:1696. · 10.02 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Iron undergoes a bcc to close-packed structural phase transition under pressure, at around 13 GPa, as shown by diamond anvil and shock experiments. Atomistic simulations have been able to provide insights into the transition, but without any plasticity occurring before the phase change, in single crystals, defective single crystals, or polycrystals. However, experiments in polycrystals do show clear evidence for plasticity. Here we study homogeneous uniaxial compression of polycrystalline Fe using several interatomic potentials: three embedded-atom-model potentials and one modified embedded-atom-model potential. We analyze grain-boundary rotation and dislocation activity, and find that the amount of dislocation activity as a function of strain depends greatly on the potential used. This variation can be explained in terms of the dislocation properties, calculated in this work for each of these potentials.
    Physical Review B. 10/2012; 86(14).
  • Source
    Keonwook Kang, Vasily V Bulatov, Wei Cai
    [Show abstract] [Hide abstract]
    ABSTRACT: Dislocation mobility is a fundamental material property that controls strength and ductility of crystals. An important measure of dislocation mobility is its Peierls stress, i.e., the minimal stress required to move a dislocation at zero temperature. Here we report that, in the body-centered cubic metal tantalum, the Peierls stress as a function of dislocation orientation exhibits fine structure with several singular orientations of high Peierls stress-stress spikes-surrounded by vicinal plateau regions. While the classical Peierls-Nabarro model captures the high Peierls stress of singular orientations, an extension that allows dislocations to bend is necessary to account for the plateau regions. Our results clarify the notion of dislocation kinks as meaningful only for orientations within the plateau regions vicinal to the Peierls stress spikes. These observations lead us to propose a Read-Shockley type classification of dislocation orientations into three distinct classes-special, vicinal, and general-with respect to their Peierls stress and motion mechanisms. We predict that dislocation loops expanding under stress at sufficiently low temperatures, should develop well defined facets corresponding to two special orientations of highest Peierls stress, the screw and the M111 orientations, both moving by kink mechanism. We propose that both the screw and the M111 dislocations are jointly responsible for the yield behavior of BCC metals at low temperatures.
    Proceedings of the National Academy of Sciences 09/2012; 109(38):15174-8. · 9.81 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: A unique size-dependent strain hardening mechanism, that achieves both high strength and ductility, is demonstrated for penta-twinned Ag nanowires (NWs) through a combined experimental-computational approach. Thin Ag NWs are found to deform via the surface nucleation of stacking fault decahedrons (SFDs) in multiple plastic zones distributed along the NW. Twin boundaries lead to the formation of SFD chains that locally harden the NW and promote subsequent nucleation of SFDs at other locations. Due to surface undulations, chain reactions of SFD arrays are activated at stress concentrations and terminated as local stress decreases, revealing insensitivity to defects imparted by the twin structures. Thick NWs exhibit lower flow stress and number of distributed plastic zones due to the onset of necking accompanied by more complex dislocation structures.
    Small 07/2012; 8(19):2986-93. · 7.82 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: The strength of true metallic nanowires and nanopillars (diameters below 100 nm) is known to be higher than the strength of bulk metals and is most likely controlled by dislocation nucleation from free surfaces. Dislocation nucleation is a thermally activated process that is sensitive to both temperature and strain rate. However, most simulations rely on high strain rate molecular dynamics to investigate strength and nucleation, which is limited by short molecular dynamics time scales. In this work, the energetics of dislocation nucleation in gold nanowires are computed using atomistic simulations, and transition state theory is used to estimate the strength at experimental strain rates revealing detailed information outside the realm accessible to molecular dynamics simulations. This allows investigation into the competition between thermally activated dislocation nucleation and other failure mechanisms such as elastic and structural instabilities. Additionally, the mechanisms of dislocation nucleation are compared against analytical continuum models which allow a better understanding of the nucleation process including the effects of the wire surfaces. This study helps clarify and consolidate our understanding of the nature of dislocation nucleation in small structures.
    Journal of the Mechanics and Physics of Solids 01/2012; 60(1):84–103. · 4.29 Impact Factor
  • Source
    Seunghwa Ryu, Keonwook Kang, Wei Cai
    [Show abstract] [Hide abstract]
    ABSTRACT: Predicting the dislocation nucleation rate as a function of temperature and stress is crucial for understanding the plastic deformation of nanoscale crystalline materials. However, the limited time scale of molecular dynamics simulations makes it very difficult to predict the dislocation nucleation rate at experimentally relevant conditions. We recently develop an approach to predict the dislocation nucleation rate based on the Becker–Döring theory of nucleation and umbrella sampling simulations. The results reveal very large activation entropies, which originated from the anharmonic effects, that can alter the nucleation rate by many orders of magnitude. Here we discuss the thermodynamics and algorithms underlying these calculations in greater detail. In particular, we prove that the activation Helmholtz free energy equals the activation Gibbs free energy in the thermodynamic limit and explain the large difference in the activation entropies in the constant stress and constant strain ensembles. We also discuss the origin of the large activation entropies for dislocation nucleation, along with previous theoretical estimates of the activation entropy.
    Journal of Materials Research. 09/2011; 26(18):2335 - 2354.
  • Source
    Seunghwa Ryu, Keonwook Kang, Wei Cai
    [Show abstract] [Hide abstract]
    ABSTRACT: Dislocation nucleation is essential to our understanding of plastic deformation, ductility, and mechanical strength of crystalline materials. Molecular dynamics simulation has played an important role in uncovering the fundamental mechanisms of dislocation nucleation, but its limited timescale remains a significant challenge for studying nucleation at experimentally relevant conditions. Here we show that dislocation nucleation rates can be accurately predicted over a wide range of conditions by determining the activation free energy from umbrella sampling. Our data reveal very large activation entropies, which contribute a multiplicative factor of many orders of magnitude to the nucleation rate. The activation entropy at constant strain is caused by thermal expansion, with negligible contribution from the vibrational entropy. The activation entropy at constant stress is significantly larger than that at constant strain, as a result of thermal softening. The large activation entropies are caused by anharmonic effects, showing the limitations of the harmonic approximation widely used for rate estimation in solids. Similar behaviors are expected to occur in other nucleation processes in solids.
    Proceedings of the National Academy of Sciences 03/2011; 108(13):5174-8. · 9.81 Impact Factor
  • Source
    Sylvie Aubry, Keonwook Kang, Seunghwa Ryu, Wei Cai
    [Show abstract] [Hide abstract]
    ABSTRACT: We compare the energy barriers predicted by continuum mechanics models for homogeneous dislocation nucleation in copper with explicit atomistic calculations. We find that a relatively simple continuum model can agree with full atomistic calculations if the dislocation Burgers vector is allowed to increase continuously during nucleation. The analysis identifies the significant effect of the applied shear stress on the generalized stacking fault energy and leads to a more physical definition of the ideal shear strength.Highlights► Energy barriers of homogeneous dislocation nucleation are predicted by simple continuum models. ► The Burgers vector needs to vary in the continuum models to agree with atomistic calculations. ► The effect of applied shear stress on the generalized stacking fault energy is also important.
    Scripta Materialia. 01/2011; 64(11):1043-1046.
  • Source
    Keonwook Kang, Wei Cai
    [Show abstract] [Hide abstract]
    ABSTRACT: We present molecular dynamics simulations of [1 1 0]-oriented Si nanowires (NWs) under a constant strain rate in tension until failure, using the modified embedded-atom-method (MEAM) potential. The fracture behavior of the NWs depends on both temperature and NW diameter. For NWs of diameter larger than 4 nm, cleavage fracture on the transverse (1 1 0) plane are predominantly observed at temperatures below 1000 K. At higher temperatures, the same NWs shear extensively on inclined {1 1 1} planes prior to fracture, analogous to the brittle-to-ductile transition (BDT) in bulk Si. Surprisingly, NWs with diameter less than 4 nm fail by shear regardless of temperature. Detailed analysis reveals that cleavage fracture is initiated by the nucleation of a crack, while shear failure is initiated by the nucleation of a dislocation, both from the surface. While dislocation mobility is believed to be the controlling factor of BDT in bulk Si, our analysis showed that the change of failure mechanism in Si NWs with decreasing diameters is nucleation controlled. Our results are compared with a recent in situ tensile experiment of Si NWs showing ductile failure at room temperature.
    International Journal of Plasticity 01/2010; · 4.36 Impact Factor
  • Source
    Keonwook Kang, Wei Cai
    [Show abstract] [Hide abstract]
    ABSTRACT: Fracture of silicon and germanium nanowires in tension at room temperature is studied by Molecular Dynamics simulations using sev- eral inter-atomic potential models. While some potentials predict brittle fracture initiated by crack nucleation from the surface, most potentials predict ductile fracture initiated by dislocation nucleation and slip. A simple parameter based on the ratio between the ideal tensile strength and the ideal shear strength is found to correlate very well with the observed brittle versus ductile behaviours for all the potentials used in this study. This parameter is then computed by ab initio methods, which predict brittle fracture at room tempera- ture. A brittle-to-ductile transition (BDT) is observed in MD simula- tions at higher temperature. The BDT mechanism in semiconductor nanowires is difierent from that in the bulk, due to the lack of a pre- existing macrocrack that is always assumed in bulk BDT models.
    Philosophical Magazine A 01/2007; 87(14):2169-2189.
  • Source
    Wei Cai, Jie Deng, Keonwook Kang

Publication Stats

78 Citations
50.39 Total Impact Points

Institutions

  • 2014
    • Yonsei University
      • Department of Mechanical Engineering
      Sŏul, Seoul, South Korea
  • 2011–2013
    • Los Alamos National Laboratory
      • • Materials Science and Technology Division
      • • Theoretical Division
      Los Alamos, California, United States
  • 2010–2012
    • Stanford University
      • • Department of Mechanical Engineering
      • • Department of Physics
      Stanford, CA, United States