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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. Elsevier 2016 Acta materialia Vol.110 No.-
<P>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.</P>
Shim, Jae-Hyeok,Povoden-Karadeniz, Erwin,Kozeschnik, Ernst,Wirth, Brian D. Elsevier 2015 JOURNAL OF NUCLEAR MATERIALS Vol.462 No.-
<P><B>Abstract</B></P> <P>The long-term evolution of precipitates in type 316 austenitic stainless steels at 400°C has been simulated using a numerical model based on classical nucleation theory and the thermodynamic extremum principle. Particular attention has been paid to the precipitation of radiation-induced phases such as γ′ and G phases. In addition to the original compositions, the compositions for radiation-induced segregation at a dose level of 5, 10 or 20dpa have been used in the simulation. In a 316 austenitic stainless steel, γ′ appears as the main precipitate with a small amount of G phase forming at 10 and 20dpa. On the other hand, G phase becomes relatively dominant over γ′ at the same dose levels in a Ti-stabilized 316 austenitic stainless steel, which tends to suppress the formation of γ′. Among the segregated alloying elements, the concentration of Si seems to be the most critical for the formation of radiation-induced phases. An increase in dislocation density as well as increased diffusivity of Mn and Si significantly enhances the precipitation kinetics of the radiation-induced phases within this model.</P> <P><B>Highlights</B></P> <P> <UL> <LI> We model the precipitation kinetics in irradiated 316 austenitic stainless steels. </LI> <LI> Radiation-induced phases are predicted to form at over 10dpa segregation conditions. </LI> <LI> The Si content is the most critical for the formation of radiation-induced phases. </LI> </UL> </P>
Strengthening of Nanosized bcc Cu Precipitate in bcc Fe: A Molecular Dynamics Study
Shim, Jae-Hyeok,Kim, Dong-Ik,Jung, Woo-Sang,Cho, Young Whan,Wirth, Brian D. The Japan Institute of Metals 2009 Materials transactions Vol.50 No.9
<P>The strengthening effect of nanosized Cu precipitates in bcc Fe has been studied by performing molecular dynamics simulations of the interaction between a screw dislocation and a coherent bcc Cu precipitate of 1–4 nm diameter in bcc Fe. The dislocation detachment mechanism changes from shear at a precipitate diameter of 4 and 2.5 nm in the twinning and anti-twinning directions, respectively, due to the coherency loss caused by the screw dislocation assisted martensitic transformation of the precipitate. The screw dislocation detachment mechanism with the larger, transformed precipitates involves annihilation-and-renucleation, or Orowan looping in the twinning vs. anti-twinning direction, respectively. The critical resolved shear stress (CRSS) of the screw dislocation-precipitate interaction increases with increasing precipitate size, and is strongly dependent on the precipitate structure and detachment mechanism. The CRSS is much larger in the anti-twinning direction.</P>
Shim, Jae-Hyeok,Hwang, Byoungchul,Lee, Myoung-Gyu,Lee, Joonho Elsevier 2018 CALPHAD, computer coupling of phase diagrams and t Vol.62 No.-
<P><B>Abstract</B></P> <P>In order to satisfy the demands for both safety and global warming reduction, a high-strength seismic reinforced steel bar is required in the structural steel market. Recent developments in computational thermodynamics and related application software have made it possible to design a suitable material as well as support engineers of steel manufacturing companies in the production of the designed material with minimum benchmarks in practical operations. This paper reports our recent success in developing grade 600 MPa reinforced steel bars for seismic safety in South Korea. First, conventional alloy design based on CALPHAD-type computational thermodynamics was carried out. For this purpose, a typical alloy system of Fe-0.30C-0.23Si-1.37Mn-0.14V-0.22Cu (in wt%) was selected, and thermodynamic and kinetic calculations were carried out using MatCalc and JMatPro software. Second, in order to reduce V content in the steel for economic reasons, a cooling process designed using finite element (FE) simulation based on the thermodynamic database was performed. For this application, Fe-0.34C-0.22Si-1.34Mn-0.04 V (in wt%) alloy was chosen, and the FE software ABAQUS was applied for modeling the TempCore process. The mechanical properties of the steel products with a diameter of 32 mm produced based on the simulated results satisfy the required properties for grade 600 MPa seismic reinforced steel bars.</P>
논문 : 상변태 ; Fe-P 합금계의 열역학 성질과 상태도 계산
심재혁 ( Jae Hyeok Shim ),오창석 ( Chang Seok Oh ),이동녕 ( Dong Nyung Lee ) 대한금속재료학회 ( 구 대한금속학회 ) 1996 대한금속·재료학회지 Vol.34 No.11
Thermodynamic and phase equilibrium data of the Fe-P system have been critically assessed by employing the CALPHAD method. The regular type solution model was applied to α(bcc), γ(fcc) and liquid Fe-P phases. The stoichiometric compound model was applied to the intermediate phases such as Fe3P, Fe2P and FeP. The parameters of the thermodynamic models have been evaluated from the thermodynamic and phase equilibrium data in the literature. Stable and metastable phase equilibria, To curves and thermodynamic properties have been calculated using the evaluated parameters. The calculated results are in fairly good agreement with the experimentally measured data.