Papers by Author: Kyu Hwan Oh

Abstract: Deformation behavior of high nitrogen austenitic Fe-18Cr-18Mn-2Mo-0.9N stainless steel was investigated utilizing electron back-scattered diffraction (EBSD) and transmission electron microscopy (TEM). During deformation, the <110> grains rotated and the trace of these grains moved towards into the dodecahedral plane (the line connecting (001) and (111) planes). Misorientation mapping in EBSD showed that the special boundaries (almost Σ3) gradually diminished whereas the low-angle boundaries were developed. TEM observation showed that (i) the low-angle boundaries developed corresponded to the deformation twinning with {111}<112> component, (ii) the deformation twinning showed the strong orientation dependence relative to tensile axis, and (iii) the deformed microstructure was characterized by extended stacking faults, planar dislocation array in low strain regime, and by well-developed deformation twinning in high strain region, respectively.

Abstract: This paper studies the microstructure of drawn gold wires to equivalent strain of 10 and to equivalent strain of 8.5 then heat-treated. The texture of gold wire drawn to strain of 10 is mainly composed of <100> and <111> fibers. Tensile strength of the gold wire increases with <111> fiber fraction, while the grain size does not appear to affect the tensile property. With an exception at heat treatment at 600oC, the texture of gold wire drawn the strain of 8.5 is replaced with <100> fiber component by heat treatment process at 400~700oC. Heat treatment at 600oC produces <110> fiber or <112> fiber, depending upon annealing time.

Abstract: Microstructural characteristics of friction-stir-welded Al 6061-T651 with varying rotating and advancing speed were examined by the electron backscattering diffraction (EBSD) installed in field emission-scanning electron microscopy (FE-SEM). It was found that FSW produced an equiaxed fine-grained microstructure in weld zone and the grain size in weld zone decreased up to about 4~6 µm with decreasing rotating speed. The primary textures developed in weld zone were {100}<001>, {110}<001> and {111}<110>. In thermo-mechanical affected zone, the change in grain size was not significant, however, large number of low angle grain boundaries were observed, which seems to be concerned with the formation of subgrains due to the development of dislocation cells.

Abstract: Tensile specimens cut from the surface layer to the center layer of a 12.7 mm thick 2090 Al-Li alloy plate were solution treated at 550°C for 30 min and subsequently peak-aged at 190°C for 18 h. They were tensile tested along the rolling direction at 25°C at various strain rates. The solution-treated specimens gave rise to serrated flows at a strain rate of 2×10-4 s-1. On the other hand, for the peak-aged alloy, the surface-layer and subsurface-layer specimens underwent complex, serrated flows (fine and coarse types superimposed each other), whereas the center-layer and near-center-layer specimens were devoid of serrated flows. The textures of the surface-layer and subsurface-layer specimens were approximated by the {001}<110> orientation, while those of the center-layer and near-center-layer specimens were approximated by the {011}<211> orientation. The different flow behaviors were discussed based on the crystallographic textures, microstructures and the strain rates.

Abstract: Under a high-pressure condition, the wear characteristics of alloyed and unalloyed ductile cast iron were investigated. Mn, Cu, Si and Mo were chosen as alloying elements, and their effects on mechanical properties of hardness and toughness were correlated with wear properties. In most cases, high-pressure wear of alloyed and unalloyed DCI were sequentially developed by mild wear region followed by severe wear. The transition of wear mode was indicated by the drastic increase of friction coefficient and wear rate. The observation of worn surface at mild-wear region suggested that wear had been concentrated at graphite phases, whereas severe wear mode damaged both of graphite and matrix. It was found that DCI alloyed with Si-Mo exhibited the most prominent resistance to the beginning of severe wear while the toughness of DCI was also a crucial factor.

Abstract: The evaluation of elastic property for thin diamond-like carbon (DLC) films has been presented with buckle analysis of compressive stressed film on patterned substrate. When substrate has been patterned with adhesion release layer, the morphologies of buckle configuration on patterned layer has been controlled from straight sided (Euler) buckle to nonlinear telephone cord type buckle with respect to the pattern width. By using the simple equation for Euler buckle, the elastic modulus has been easily calculated, shown well consistent with the results by nano-indentation test.

Abstract: The failure assessment for the defects in multiple corroded region has been considered with the real-size pipeline burst tests and the associated Finite Element Method. The corroded gas pipeline made of X65 steel with various types of artificially machined pits has been used for a number of series of burst tests on gas pipeline. The failure pressures for the designed multiple corroded defect on gas pipelines have been measured by using in-situ burst testing and classified with respect to each designed defect sizes - length, width and depth - and distance between defects. The result from the experiments has been modeled and compared with Finite Element Method. As the result of FEM simulation, the failure pressure in multiple defects is lower than that in single defect, and closer to failure pressure in single defect of total length of defects in multiple defects as the distance between each defect is closer.

Abstract: An analysis for non steady state heat transfer of a hot pressing roller was suggested in 1-dimensional model. The surface temperature on hot pressing roller was predicted by using surface contact heat transfer coefficient calculated with induced analytic solution. We calculated the size of iron powder, influencing on surface contact heat transfer coefficient. Since coarse iron powder has reduced heat transfer coefficient during contacting on roll surface with smaller contact area, temperature on roller surface has been expected to decrease. This predicted temperature by the analytic model was fairly reasonable in comparison with experimental data and finite element model.