• Korean Journal of Metals and Materials
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• v.46 no.12
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• pp.780-787
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• 2008

For the advanced high strength steels (AHSS), high-manganese TWIP (twinning induced plasticity) steels exhibit high tensile strength (800-1000 MPa) and high elongation (50-60%). However, the TWIP steels need to be understood of delayed fracture following the cup drawing test. Among the factors to cause delayed fracture, i.e, martensite transformation, hydrogen embrittlement and residual stress, the effects of martensite transformation (${\gamma}{\rightarrow}{\varepsilon}$ or ${\gamma}{\rightarrow}{\alpha}^{\prime}$) were investigated on the delayed fracture phenomenon. Microstructural phase analysis was conducted for cold rolled (20, 60, 80% reduction ratio) steels and tensile deformed (20, 40, 60% strain) steels. For the Al-added TWIP steels, no martensite phase was found in the cold rolled and tensile deformed specimen. But, the TWIP steels with no Al addition indicated the martensite transformation. The cup drawing specimens showed the martensite transformation irrespective of the Al-addition to the TWIP steel. However, the TWIP steel with no Al exhibited the larger amount of martensite than the case of the TWIP steel with Al addition. For the reason, it was possible to conclude that the Al addition suppressed the martensite transformation in TWIP steels, therefore preventing the delayed fracture effectively. However, it was interesting to note that the mechanism of delayed fracture should be incorporated with hydrogen embrittlement and/or residual stress as well as the martensite transformation.

• Korean Journal of Materials Research
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• v.16 no.3
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• pp.173-177
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• 2006

Porous surfaced Ti implant compacts were fabricated by electro-discharging-sintering (EDS) of atomized spherical Ti powders. Powders of $50-100{\mu}m$ in diameter were vibratarily settled into a quarts tube and subject to a high voltage and high density current pulse in Ar atmosphere. Single pulse of 0.7 to 2.0 kJ/0.7 gpowder, from 150, 300, and $450{\mu}F$ capacitors was applied in less than $400{\mu}sec$ to produce twelve different porous-surfaced Ti implant compacts. The solid core formed in the center of the compact shows similar microstructure of cp Ti which was annealed and quenched in water. Hardness value at the solid core was much higher than that at the particle interface and particles in the porous layer, which can be attributed to both heat treatment and work hardening effects induced by EDS. Compression tests were made to evaluate the mechanical properties of the EDS compacts. The compressive yield strength was in a range of 12 to 304MPa which significantly depends on input energy. Selected porous-surfaced Ti-6Al-4V dental implant compacts with a solid core have much higher compressive strengths compared to the human teeth and sintered Ti dental implants fabricated by conventional sintering process.

• Proceedings of the KSME Conference
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• 2007.05a
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• pp.114-119
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• 2007

Sulfide stress cracking (SSC) of materials exposed to oilfield environment containing hydrogen sulfide $(H_{2}S)$ has been recognized as a materials failure problem. Laboratory data and field experience have demonstrated that extremely low concentration of $H_{2}S$ may be sufficient to lead to SSC failure of susceptible materials. In some cases, $(H_{2}S)$ can act synergistically with chlorides to produce corrosion and cracking failures. SSC is a form of hydrogen embrittlement that occurs in high strength steels and in localized hard zones in weldment of susceptible materials. In the heat-affected zones adjacent to welds, there are often very narrow hard zones combined with regions of high residual stress that may become embrittled to such an extent by dissolved atomic hydrogen. On the base of understanding on sulfide stress cracking and its mechanism, SSC resistance for the several materials, those are ASTM A106 Gr B using in the oil industries, are evaluated.

• Korean Journal of Materials Research
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• v.26 no.10
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• pp.535-541
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• 2016

The present study deals with the effects of micro-alloying elements such as Ni, V, and Ti on the recrystallization behavior of carbon steels at different strain rates. Eight steel specimens were fabricated by varying the chemical composition and reheating temperature; then, a high-temperature compressive deformation test was conducted in order to investigate the relationship of the microstructure and the recrystallization behavior. The specimens containing micro-alloying elements had smaller prior austenite grain sizes than those of the other specimens, presumably due to the pinning effect of the formation of carbonitrides and AlN precipitates at the austenite grain boundaries. The high-temperature compressive deformation test results indicate that dynamic recrystallization behavior was suppressed in the specimens with micro-alloying elements, particularly at increased strain rate, because of the pinning effect of precipitates, grain boundary dragging and lattice misfit effects of solute atoms, although the strength increased with increasing strain rate.

• Carbon letters
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• v.16 no.3
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• pp.203-210
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• 2015

This study fabricated low thermal conductive polyacrylonitrile (PAN)-based carbon fibers containing cellulose particles while maintaining their mechanical properties. The high thermal conductivity of carbon fibers limits their application as a high temperature insulator in various systems such as an insulator for propulsion parts in aerospace or missile systems. By controlling process parameters such as the heat treatment temperature of the cellulose particles and the amount of cellulose added, the thermal and mechanical properties of the PAN-based carbon fibers were investigated. The results show that it is possible to manufacture composite carbon fibers with low thermal conductivity. That is, thermal conductivities were reduced by the cellulose particles in the PAN based carbon fibers while at the same time, the tensile strength loss was minimized, and the tensile modulus increased.

• Proceedings of the KSME Conference
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• 2008.11a
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• pp.20-25
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• 2008

Sulfide stress corrosion cracking (SSCC) of materials exposed to oilfield environment containing hydrogen sulfide ($H_2S$) has been recognized as a materials failure problem. Laboratory data and field experience have demonstrated that extremely low concentration of $H_2S$ may be sufficient to lead to SSC failure of susceptible materials. In some cases, $H_2S$ can act synergistically with chlorides to produce corrosion and cracking failures. SSC is a form of hydrogen embrittlement that occurs in high strength steels and in localized hard zones in weldment of susceptible materials. In the heat-affected zones adjacent to welds, there are often very narrow hard zones combined with regions of high residual stress that may become embrittled to such an extent by dissolved atomic hydrogen. On the base of understanding on sulfide stress cracking and its mechanism, SSC resistance for the several materials, those are ASTM A106 Gr B using in the oil industries, are evaluated.

• Journal of Korea Foundry Society
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• v.33 no.3
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• pp.122-126
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• 2013

Recently, heat-resistant aluminum alloy has been re-focused as a downsizing materials for the internal combustion engines. Heat-resistant Al alloy development and many researches are still ongoing for the purpose of improving thermal stability, high-temperature mechanical strength and fatigue properties. The conventional principle of heat-resistant Al alloy is the precipitation of intermetallic compounds by adding a variety of elements is generally used to improve the mechanical properties of Al alloys. Heat resistant aluminum alloys have been produced by CrW homogeneous solid solution to overcome the limit of conventional heat resistant aluminum alloy. From EPMA, it is found that CrW homogeneous soild solution phases with the size of $50-100{\mu}m$ have been dispersed uniformly, and there is no reaction between aluminum and CrW alloy. In addition, after maintaining at high temperature of 573 K, there is no growth of hardening phase, nor desolved, but CrW still exists as a homogeneous solid solution.

• Korean Journal of Metals and Materials
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• v.49 no.3
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• pp.243-249
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• 2011

This study describes the feasibility of producing high-strength Al alloy sheet with a high solute content using a combined technique of twin-roll strip casting and asymmetric rolling. The Al sheet produced in this study exhibited excellent formability ($\overline{r}$ =1.0, $\Delta$r=0.16) and mechanical properties ($\sigma_{TS}$~305 MPa, $\epsilon$~33%), that, cannot be feasibly obtained via the conventional technique based on ingot casting and rolling. The structural origin of the observed properties, especially enhanced formability, was clarified by examining the evolution of textures associated with strip casting and subsequent thermo-mechanical treatments. Our evaluation of the mechanical properties and formability leads us to conclude that the combination of strip casting and asymmetric rolling is a feasible process for enhancing the formability of Al alloy sheets to the level beyond what the conventional techniques can reach.

• Composites Research
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• v.35 no.3
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• pp.121-126
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• 2022

Owing to advantages of polymeric materials for hydrogen tank liner like light-weight property and high specific strength, polymer based composites have gained much attention. Despite of many benefits, polymeric materials for fuel cell tank cause problems which is critical to applications as low gas barrier property, and poor processability when adding fillers. For these reasons, improving gas barrier property of polymer composites is required to study for expanding application fields. This work presents impermeable polymer nanocomposites by introducing thin barrier coating using layer by layer (LBL) deposition method. Also, bi-layered and quad-layered nanocomposites were fabricated and compared for identifying relationship between deposition step and gas barrier property. Reduction in gas permeability was observed without interrupting mechanical property and processability. It is discussed that proper coating conditions were suggested when different coating materials and deposition steps were applied. We investigated morphology, gas barrier property and mechanical properties of fabricated nanocomposites by FE-SEM, Oxygen permeation analyzer, UTM, respectively. In addition, we revealed the mechanism of barrier performance of LBL coating using materials which have high aspect ratio.

• The Korean Journal of Nuclear Medicine Technology
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• v.27 no.1
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• pp.23-28
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• 2023

Carbon materials are widely used in many areas of our lives. A fiber having a carbon content of 90% or more obtained by heating an organic fiber precursor is referred to as a "carbon fiber". Carbon fibers are currently used in the medical market to manufacture radiation transmission device parts, artificial joints, and medical aids, as many developments have been made to utilize carbon fibers' characteristics such as light weight, radiation permeability, biocompatibility, high strength, high heat resistance, thermal conductivity, and electrical conductivity. In order to maintain body temperature and increase immunity in long-lasting nuclear medical examination and treatment through the idea of convergence of carbon materials and radiation technology, the quality of medical services can be improved by utilizing carbon materials. We should be aware of the domestic carbon-based medical device industry and make efforts to contribute to the development of medical devices. As a radiation expert, we should try to use our skills and experience to find items that can be fused with medical devices to develop various nuclear medical examination fields and radiographic examination fields that can be widely applied. We should actively engage in future technology development and carbon material research to strengthen the global competitiveness of the domestic medical device industry and improve the quality of medical services.