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Now showing items 1 - 16 of 34

  • Mechanism for Z-phase formation in 11CrMoVNbN martensitic heat-resistant steel

    Myung-Yeon Kim   Sung-Min Hong   Kyu-Ho Lee   Woo-Sang Jung   Young-Su Lee   Young-Kook Lee   Jae-Hyeok Shim  

    Abstract The mechanism for the formation of Z-phase was investigated for samples of an 11CrMoVNbN steel aged at 593 °C for up to 50,000 h. X-ray diffraction indicates that Z-phase appears after 5000 h of aging, and its amount gradually increases up to 50,000 h aging at the expense of mainly Cr 2 N. Transmission electron microscopy shows that Z-phase nucleates in the V-enriched rim region of Cr 2 N precipitates, which forms due to the diffusion of V into the precipitates from the matrix. Nucleated Z-phase tends to grow relatively rapidly compared with the preexisting precipitates such as Nb(C,N) and M 23 C 6 . Z-phase seems to consume Cr 2 N by the nucleation and growth mechanism rather than the transformation mechanism. The main difference in the aged samples is that the Nb content of Z-phase is lower and that Z-phase does not replace Cr 2 N completely, compared to the results of the crept samples. Graphical abstract Image 1 Highlights • Precipitation is studied for a martensitic steel aged at 593 °C for up to 50,000 h. • Z-phase nucleates in the V-enriched rim region of Cr 2 N precipitates after 5000 h. • Z-phase seems to consume Cr 2 N by the nucleation and growth mechanism.
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  • Atomistic simulation of hydrogen diffusion at tilt grain boundaries in vanadium

    Jae-Hyeok Shim   Won-Seok Ko   Jin-Yoo Suh…  

    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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  • Hydrogen embrittlement in high interstitial alloyed 18Cr10Mn austenitic stainless steels

    M.P. Phaniraj   Han-Jin Kim   Jin-Yoo Suh   Jae-Hyeok Shim   Seong-Jun Park   Tae-Ho Lee  

    Abstract The effect of hydrogen on the tensile behavior of two 18Cr10Mn austenitic steels having the similar stacking fault energy but different nitrogen and carbon contents was investigated. The steels were thermally charged with hydrogen at 15 MPa and 300 °C. The hydrogen content was determined using thermal desorption spectroscopy. The microstructure and fracture surface of the specimens were characterized using X-ray diffraction, optical and scanning electron microscopy. The ductility loss after hydrogen charging was greater in the steel with higher nitrogen content. The fracture behavior is discussed in terms of austenite phase stability and the localization of slip in the presence of nitrogen and hydrogen. Highlights • Cr–Mn–N–C steels have similar stacking fault energy but different ductility loss. • Hydrogen in solution increases the flow stress. • Deformation induced martensite forms in steel with greater ductility loss. • Fracture surface micrographs indicate hydrogen enhanced dislocation processes.
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  • Effect of preexisting plastic deformation on the creep behavior of TP347 austenitic steel

    Sung-Min Hong   Dong-Joon Min   Yong-keun Chung   Ju-Heon Kim   Dong-Ik Kim   Jin-Yoo Suh   Jae-Hyeok Shim   Woo-Sang Jung   In-Suk Choi  

    Abstract We investigated the effect of preexisting plastic deformation with the strain of 30% on the creep behavior of TP347 austenitic steel at 650 °C with different applied stresses. The results showed that the creep rupture life of the prestrained sample increased more than that of the unstrained sample tested at 300, 250 and 200 MPa due to the strengthening of the mechanical twins induced by preexisting plastic deformation. Conversely, the prestrained samples were ruptured earlier than that of the unstrained sample tested when the lower creep stresses (150 and 125 MPa) were applied. The shorter creep rupture life of the prestrained sample was due to the reduced amount of time to a minimum creep rate and the shorter tertiary-stage creep time which can be related to the accelerated precipitation of sigma phase at the grain boundaries in the strained sample.
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  • Method for the preparation of titanium nitride powder

    Nano-sized titanium nitride powder can be prepared by a simple process comprising subjecting mixed powder of titanium trichloride and lithium nitride to high-energy ball milling using a plurality of balls in an airtight reactor vessel under an inert gas atmosphere to form composite powder, and recovering the titanium nitride powder therefrom.
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  • Method for manufacturing transition metal boride power

    The present invention relates to a method for manufacturing a transition metal boride powder. The method for manufacturing a transition metal boride powder includes: i) manufacturing a mixed powder by mixing a transition metal halogenide powder and an alkali metal borohydride powder; ii) charging the mixed powder and a plurality of balls into a reaction vessel; iii) charging an inert gas into the reaction vessel and sealing the reaction vessel; iv) high energy ball milling the mixed powder and manufacturing a composite powder containing a transition metal boride and an alkali metal halogenide; v) washing the composite powder in water, dissolving the alkali metal halogenide in the water and filtering the transition metal borides; and vi) drying the filtered transition metal boride and collecting the transition metal boride powder.
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  • Effects of alloyed aluminum and titanium on the oxidation behavior of INCONEL 740 superalloy

    Majid Abbasi   Dong-Ik Kim   Jae-Hyeok Shim   Woo-Sang Jung  

    Abstract An attempt has been made to improve the oxidation resistance of the INCONEL 740 nickel-base superalloy by altering its composition. The effects of aluminum and titanium were studied in particular. Electron microscopy was utilized to characterize and quantify the oxides. The addition of aluminum up to 2.4 wt.% at low titanium contents provides the lowest mass gain. Aluminum internal and grain boundary oxidation, especially at nickel high-angle boundaries, was observed. The surface oxide consists of predominantly Cr 2 O 3 . The oxide thickness and defect density increase with the increasing titanium content. Graphical abstract Highlights • Compositional effect on water vapor oxidation of INCONEL 740 has been investigated. • Addition of Al up to 2.4 wt.% at low Ti contents exhibited the lowest mass gain. • Aluminum oxidation was observed, especially at high angle Ni boundaries. • High Ti contents lead to internal titanium nitride and oxide formation. • Dominant surface oxide was Cr 2 O 3 , varying in grain size, thickness, and cavity size.
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  • Method for fabricating magnesium-based hydrogen storage material

    A method for fabricating a magnesium-based hydrogen storage material according to the present invention comprises a) forming a mixture of a magnesium hydride powder and a transition metal halide powder, b) adding the mixture and balls into a vessel, c) filling the vessel with an inert gas or hydrogen, and d) subjecting the mixture to high energy ball milling.
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  • Thermal decomposition of Li3AlH6 with TiAl3 catalyst

    Jae-Hyeok Shim   Gil-Jae Lee   Byeong-Joo Lee   Young-Joo Oh   Young Whan Cho  

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  • Microstructure evolution in Alloy 617 B after a long-term creep and thermal aging at 700 °C

    Magdalena Speicher   Florian Kauffmann   Jae-Hyeok Shim   Mahesh Chandran  

    Abstract Changes in the microstructure of heat-resistant materials may influence their long-term behaviour. For this reason, materials chosen for high temperature-based applications, e.g. advanced ultra-supercritical (A-USC) steam power plants, must exhibit long-term microstructure stability. Therefore, changes in the microstructure of frequently used materials must be determined and correlated with their creep behaviour to assure a reliable operation of components. In this work, a long-term study investigated the microstructure of a creep rupture specimen made of a nickel-based alloy 617 B. The creep tests were carried out at 700 °C for up to 45,148 h. By using optical microscopy (OM), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) the virgin, thermal-loaded and creep states were characterized. The precipitate location, number and size have been determined. The experiment results were compared, discussed and correlated to the creep and failure behaviour. Furthermore, the long-term precipitation kinetics of Alloy 617 B was simulated, which considered its thermal history, using the MatCalc software. The simulated precipitates fraction and size were compared with the experimental data obtained in this study. The comparison between the experimental and simulation results demonstrated comparable gamma prime (γ') phases. The simulation results of the carbides identified at the grain boundaries were satisfactory. However, the size of the intragranular fine carbides M 23 C 6 was not reproduced correctly.
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  • Reaction products between TiCl3 catalyst and Li3AlH6 during mechanical mixing

    Jae-Hyeok Shim   Gil-Jae Lee   Young Whan Cho  

    The reaction between TiCl 3 and Li 3AlH 6 was investigated to understand reaction products in Ti-catalyzed Li 3AlH 6 using mechanical milling and thermodynamic calculation. As reaction products, LiCl, TiAl 3 and TiH 2 are observed by X-ray powder diffraction, which is in agreement with the results of thermodynamic calculation. TiAl 3 forms in an L1 2 structure rather than an equilibrium D0 22 structure during milling. It is predicted that the mole fraction of TiAl 3 increases and that of TiH 2 decreases as temperature increases during milling. Differential scanning calorimetry confirms that TiAl 3 acts as a dehydrogenation catalyst in Li 3AlH 6. [All rights reserved Elsevier]
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  • Atomistic modeling of nanosized Cr precipitate contribution to hardening in an Fe–Cr alloy

    Jae-Hyeok Shim   Dong-Ik Kim   Woo-Sang Jung   Young Whan Cho   Brian D. Wirth  

    Molecular dynamics simulations of the interaction between an edge dislocation and nanosized Cr precipitates in bcc Fe have been performed to investigate the hardening effect of alpha' phases in high Cr ferritic/martensitic steels. The critical resolved shear stress needed for an edge dislocation to overcome Cr precipitates of diameter between 3 and 6 nm is larger than for dislocation glide in the bcc Fe lattice containing 10% Cr solute atoms. This indicates that the precipitation of alpha' phases leads to hardening in high Cr ferritic/martensitic steels. The MD simulations reveal that the interspacing of Cr precipitates plays a more crucial role in the hardening of Fe-Cr alloys than the precipitate size. An attractive interaction exists between an edge dislocation and nanosized Cr precipitates, which is evident as a decrease in total energy when an edge dislocation is placed within in a Cr precipitate. [All rights reserved Elsevier].
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  • Molecular dynamics simulation of primary irradiation defect formation in Fe–10%Cr alloy

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

    Molecular dynamics simulations of displacement cascades in Fe and Fe-10%Cr have been performed for primary knock-on energies from 1 to 20keV using two different Finnis-Sinclair style interatomic potentials. The different potentials were fit to describe the extremes of positive (attractive) and negative (repulsive) binding between substitutional Cr atoms and Fe self-interstitial atoms. As expected, the effect of Cr on the collisional stage of cascade evolution and on the number of point defects and point defect clusters produced is quite minimal. However, the quantity of mixed Fe-Cr dumbbells produced is sensitive to the choice of potential. [All rights reserved Elsevier]
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  • Thermodynamics of the dehydrogenation of the LiBH4–YH3 composite: Experimental and theoretical studies

    Jae-Hyeok Shim   Young-Su Lee   Jin-Yoo Suh   Wonchul Cho   Sang Sup Han   Young Whan Cho  

    Pressure-composition-temperature curves were constructed for the dehydrogenation of the LiBH 4-YH 3 composite. The Van't Hoff plot of the plateau pressure values showed that the reaction enthalpy and entropy were 51 kJ/mol E 2 and 101 J/Kmol H 2, respectively.Thermodynamic calculations we re performed for the reaction in combination with first principles calculation. The calculated reaction enthalpy and entropy are in good agreement with the measured values. The equilibrium temperature for the reaction at 1 bar of hydrogen was estimated to be 232degC. [All rights reserved Elsevier].
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  • Molecular dynamics simulation of the crystallization of a liquid gold nanoparticle

    Jae-Hyeok Shim   Seung-Cheol Lee   Byeong-Joo Lee   Jin-Yoo Suh   Young Whan Cho  

    Molecular dynamics simulation of the crystallization behavior of a liquid gold (Au) nanoparticle, about 4 nm in diameter, on cooling has been carried out based on the modified embedded-atom-method potential. With decreasing cooling rate, the final structure of the particle changes from amorphous to crystalline via icosahedron-like structure. While the outer shell of the icosahedron-like particle shows crystalline feature with {111}-like facets, the inner core remains amorphous. It is found that the structure of the fully crystallized particle is polycrystalline face-centered cubic (fcc). The fcc structure of the gold nanoparticle is proved energetically the most stable form
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  • Mechanochemical synthesis of nanocrystalline TiN/TiB2 composite powder

    Jae-Hyeok Shim   Jung-Soo Byun   Young Whan Cho  

    TiN/TiB 2 composite powder has been synthesized from a mixture of Ti and BN powders by high-energy ball milling. An abrupt displacement reaction between Ti and BN occurs in about 2 h. After the reaction, the crystallite sizes of TiN and TiB 2 decrease continuously and become about 10 nm, after 16 h of milling
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