Papers by Author: Hyoung Seop Kim

Abstract: In most of the simulation studies of equal channel angular pressing (ECAP) it has been assumed that materials obey isotropic hardening law. However, in the case of precipitation hardenable alloys, an accurate prediction of the deformation behavior requires incorporation of kinematic hardening model. In this study, the influences of kinematic, isotropic and combined hardening laws on deformation behavior have been investigated. For this purpose, an ECAP die consisting of two 120° channels has been selected, and the effect of hardening law on the strain profile and ram pressure at the final exit channel has been studied. The simulation results showed that the hardening mechanism does not affect the strain profiles extensively; but, when kinematic hardening mechanism was considered the ram pressure decreases significantly due to less hardening of the material during reverse loading in the final exit channel.

Abstract: Carbon nanotubes (CNTs) have been the subject of intensive studies for applications in the fields of nanotechnologies in recent years due to their superior mechanical, electric, optical and electronic properties. Because of their high Young’s modulus (≈ 1 TPa), tensile strength (≈ 200 GPa) and high elongation (10-30%) as well as high chemical stability, CNTs are considered to be attractive reinforcement materials for light weight and high strength metal matrix composites. In this paper, we described a scheme for multi-scale modeling for the elastic and plastic properties of CNT/metal nanocomposites using the numerical analyses of the three-dimensional finite element method based on the continuum mechanics of a unit cell. In particular, the quantitative effects of the distribution and the array of the CNT reinforcement (viz. cross-over, vertical and horizontal distributions) on the elasticity and plasticity of the nanocomposites were investigated and the anisotropic characteristics of elasticity and plasticity of the nanocomposites were linked with the extremely high aspect ratio of CNTs.

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.

Abstract: Mg and Mg alloys are promising materials for light weight high strength applications. In this paper, grain refinement of pure Mg using severe plastic deformation was tried to enhance mechanical properties of the hard-to-deform metallic material. The microstructure and the mechanical properties of Mg processed by equal channel angular pressing (ECAP) at various processing temperatures were investigated experimentally. ECAP of channel angle of 90o and corner angle of 0o was successful without fracture of the samples at 300 oC. The hardness of the ECAP processed Mg decreased with increasing ECAP processing temperature. The effect of temperature on the hardness and microstructure of the ECAP processed Mg were explained by the dislocation glide in the basal plane and non-basal slip systems and the dynamic recrystallization and recovery.

Abstract: In this study, a deformation mechanism map of metallic nanocrystalline materials (NCMs) using the phase mixture model is proposed. It is based on recent modelling that appears to provide a conclusive description of the phenomenology and the mechanisms underlying the mechanical properties of NCMs. The proposed models adopted the concept of a ‘phase mixture’ in which the grain interior and the grain boundaries are treated as separate phases. The volume fraction of this grain boundary ‘phase’ may be quite appreciable in a NCM. Based on the theoretical model that provides an adequate description of the grain size dependence of plasticity covering all grain size range from coarse down to the nanoscale, the tensile deformation response of NCMs, especially focusing on the deformation mechanisms was investigated. The deformation mechanism map is newly proposed with axes of strain rate, grain size and temperature.

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.

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.

Abstract: In this study, bottom-up type powder 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 with least grain growth, which is considered as a bottle neck of the bottom-up method that uses the conventional powder metallurgy of compaction and sintering. ECAP (Equal channel angular pressing), one of the most promising method in SPD, was used for the powder consolidation. In the ECAP process of not only solid but also powder metals, it is important to get a good understanding of the density as well as internal stress, strain and strain rate distribution. We investigated the consolidation, plastic deformation and microstructure evolution behavior of the metallic powders during ECAP using an experimental method. It was found that high mechanical strength could be achieved effectively due to the well bonded powder contact surface during ECAP process of gas atomized Al-Si powders. The experimental results show that SPD processing of powders is a viable method to achieve both fully density and nanostructured materials.

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.

Abstract: In this study, powder processing and severe plastic deformation (SPD) approaches were combined in order to achieve both full density and good particle-matrix bonding in CNT and Cu powder mixtures without grain growth, which was considered as a bottle neck of the bottom-up method in the conventional powder metallurgy of compaction and sintering. Equal channel angular pressing (ECAP), one of the most promising methods in SPD, was used for the powder consolidation. The powder ECAP processing with 1, 2, 4 and 8 passes was conducted at room temperature. It was found by microhardness tests and microstructure characterization that relatively high mechanical strength could be effectively achieved as a result of the well bonded powder contact surface during powder ECAP. The SPD processing of powders is a viable method to achieve both fully density and good particle bonding in CNT-metal matrix nanocomposites.