Papers by Author: Hyoung Seop Kim

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Authors: Hyoung Seop Kim, Sun Ig Hong, Min-Seok Sohn
Authors: Seung Chae Yoon, Do Minh Nghiep, Sun Ig Hong, Z. Horita, Hyoung Seop Kim
Abstract: Manufacturing bulk nanostructured materials with least grain growth from initial powders is challenging because of the bottle neck of bottom-up methods using the conventional powder metallurgy of compaction and sintering. In this study, bottom-up type powder metallurgy processing and top-down type SPD (Severe Plastic Deformation) approaches were combined in order to achieve both full density and grain refinement of metallic powders. ECAP (Equal-Channel Angular Pressing), one of the most promising processes in SPD, was used for the powder consolidation method. For understanding the ECAP process, investigating the powder density as well as internal stress, strain and strain rate distribution is crucial. We investigated the consolidation and plastic deformation of the metallic powders during ECAP using the finite element simulations. Almost independent behavior of powder densification in the entry channel and shear deformation in the main deformation zone was found by the finite element method in conjunction with a pressure dependent material yield model. Effects of processing parameters on densification and density distributions were investigated.
Authors: Hyoung Seop Kim
Abstract: Equal channel angular pressing (ECAP) is a convenient forming procedure among various severe plastic deformation processes. It is based on extruding material through specially designed entry and exit channel dies to produce an ultrafine grained microstructure. The properties of the materials obtained depend on the plastic deformation behaviour during ECAP, which is governed mainly by the die geometry, the material itself and the processing conditions. As the mechanical properties of the severely deformed material are directly related to the deformation history, understanding the phenomena associated with strain and strain rate development in the ECAP process is very important. In this study, the results of continuum modelling of ECAP are described in order to understand strain and strain developments. For this purpose, the results of modelling ECAP using the finite element method and analytical solution are presented for various geometric conditions. It was concluded that although deformation is nonuniform due to geometric effects, the strain and strain rate values obtained by the analytical solutions are not much different from the average results of the finite element method.
Authors: J. Kim, Y.C. Choi, Hyoung Seop Kim, Sun Ig Hong
Abstract: Biomimetic apatite deposition behaviors and mechanical performance for as-rolled and annealed Ni-Ti plates were investigated. Apatite nucleation and growth on Ni-Ti in SBF (simulated body fluid) was not appreciably influenced by heat treatment. But, the apatite deposition rate increased slightly by NaOH surface treatment. The nodular apatite on the deposited layer is favored on a macro-scale since the surface energy of polycrystalline apatite particles can be reduced by forming nodules. The weight gain after apatite deposition for Ni-Ti (0.004 g/cm2) after 10 days were found to be smaller that that of NaOH treated Ti-6Al-4V, but it was comparable to that of non- NaOH-treated Ti-6Al-4V (0.004 g/cm2). The stress-strain responses of annealed Ni-Ti displayed the pseudoelastic behavior associated with stress-induced martensite formation with the transition stress for the martensite formation equal to 320 MPa. On the other hand the cold worked Ni-Ti displayed no appreciable pseudoelastic region and the yield stress was ~500MPa. A good biomimetic apatite formation and excellent mechanical performance of Ni-Ti suggests that Ni-Ti can be an excellent candidate material for orthopedic implants.
Authors: J. Kim, Y.C. Choi, Hyoung Seop Kim, Sun Ig Hong
Abstract: Biomimetic apatite deposition behaviors on Zr-1Nb and Ti-6Al-4V plate with various surface conditions were examined. Both alloys were polished with abrasive papers to have different roughness and some of them were treated in NaOH before deposition of apatites in the simulated body fluid. After, 10 days immersion in a SBF, NaOH treated Zr-1Nb and Ti-6Al-4V were completely coated with apatite. The deposition rate of apatite was higher on NaOH-treated Ti-6Al- 4V than on NaOH-treated Zr-1Nb initially, but the deposition rate on Zr-1Nb accelerated after 2 days and the total weight gain due to the deposition on Zr-1Nb approached to that of Ti-6Al-4V. NaOH treatment was found to enhance the deposition rate of apatite on Ti-6Al-4V significantly. On the other hand, the deposition rate of Zr-1Nb was not influenced by NaOH treatment. Without NaOH treatment, the polished Zr-1Nb with abrasive paper was found to induce more apatite nucleation than the polished Zr-6Al-4V. The presence of apatite was confirmed by XRD analysis. SEM observation revealed a conglomerated granular structure with elongate plates.
Authors: Hyoung Seop Kim, H.H. Yang, Ik Hyun Oh, Seock Sam Kim
Authors: Quang Pham, Young Gi Jeong, Seung Chae Yoon, Sun Ig Hong, Soon Hyung Hong, Hyoung Seop Kim
Abstract: Carbon nanotubes (CNTs) have been the subject of intensive study for applications in the fields of nanotechnologies in recent years due to their superior mechanical, electric, optical and electronic properties. Because of their exceptionally small diameters (≈ several nm) as well as their high Young’s modulus (≈ 1 TPa), tensile strength (≈ 200 GPa) and high elongation (10-30%) in addition to a high chemical stability, CNTs are attractive reinforcement materials for light weight and high strength metal matrix composites. In this study, bottom-up type powder processing and top-down type SPD (severe plastic deformation) approaches were combined in order to achieve full density of CNT/metal matrix composites with superior mechanical properties by improved particle bonding and least grain growth, which were considered as a bottle neck of the bottom-up method using the conventional powder metallurgy of compaction and sintering. ECAP (equal channel angular pressing), the most promising method in SPD, was used for the CNT/Cu powder consolidation. The powder ECAP processing with 1, 2, 4 and 8 route C passes was conducted at room temperature. It was found by mechanical testing of the consolidated CNT/Cu that high mechanical strength could be achieved effectively as a result of the Cu matrix strengthening and improved particle bonding during ECAP. The ECAP processing of powders is a viable method to achieve fully density CNT-Cu nanocomposites.
Authors: Y. Choi, Nam Ihn Cho, Hyoung Seop Kim, Sun Ig Hong
Abstract: Effect of equal-channel angular pressing (ECAP) on the corrosion and mechanical properties of Cu-35%Zn alloy were studied. Two types of feed direction were selected. One is parallel pass and the other is 180°degree rotated ECAP pass after each pass. Both ECAP passes made texture in each specimen in which shear band with 45 degree on transverse direction and twins exist. The specimen prepared by parallel ECAP pass has finer shear band. Relative amount of twins to shear band on the microstructure becomes decrease with number of ECAP pass. Microhardness increased from 75 Hv to 210 Hv by ECAP. The corrosion potential and rate of the ECAPed Cu-35%Zn alloys in aerated aqueous 1 M-H2SO4 solution were –92.3 mVSHE and 3.72x10-2 A/cm2 for route- A and –38.6 mVSHE and 5.08x10-2 A/cm2 for route-C, respectively. The corrosion potential and rate of depended on the feed direction and number of pass.
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