Papers by Author: Kae Myung Kang

Abstract: Acoustic emission (AE) technique was applied to evaluate both tensile deformation of 316L stainless steel and bending-fatigue damage of socket-welded pipe. AE activity was monitored during tensile deformation of plate-type specimens subjected to various heat treatments. Variation in RMS voltage of AE signal was correlated with the amount of strain-induced martensite phase. Secondly, actual size of socket-welded pipe specimen was bending-fatigued under various stresses. Crack initiation was determined by observing an abrupt increase in AE count, and confirmed by radiographic examination before and after the crack initiation cycle. Potentials of AE technique for monitoring fatigue crack initiation were discussed.

Abstract: We have studied on the wear behavior of Cu-TiB2 composites, produced by thermal extraction, as a function of dispersed particle sizes and volume fractions of TiB2. The depth of subsurface zone was increased with applied loads from 25 μm with 20 N to 25 μm with 80 N. From the analyses results of the formation of wear debris generated by cracks between work hardened layer and matrix, wear mechanism is changed from abrasive wear to adhesive wear with increasing loads from 20 N to 80 N. The formation of sub-grains in the region of subsurface was clearly observed from the sample tested over 60 N loads. We attribute the formation of sub-grains to the plastic lubricant effect of TiB2 which gives arising from the contact stress and local heat generated during wear test.

Abstract: An optimum route to synthesize Al2O3-based composite powders with a homogeneous dispersion of carbon nanotubes (CNTs) was investigated. CNT/Metal/Al2O3 nanocomposite powders were fabricated by thermal chemical vapor deposition (CVD) over a metal catalyst homogeneously dispersed into an Al2O3 matrix by the means of chemical and selective reduction processes. The nanocomposite powders were densified by Pulse Electric Current Sintering (PECS). The experimental results show that the CNT/Metal/Al2O3 nanocomposites have unique electrical properties.

Abstract: The microstructure and electrical conductivity of CNTs dispersed Al2O3 nancomposites depending on the powder processing and CNTs content were demonstrated. The composite powders with homogeneous dispersion of CNTs could be synthesized by a catalytic route for direct formation of CNTs on nano-sized Fe dispersed Al2O3 powders. The sintered nanocomposite using the composite powder with directly synthesized CNTs showed homogeneous microstructure and enhanced elelctrical conductivity. The influence of powder processing on the properties of sintered nanocomposites was discussed by the observed microstructural features.

Abstract: In this study, mechanical degradations in 2.25Cr-1Mo steel were evaluated by quantitative morphological analysis of carbides. Based on the morphology, carbides were classified as globular, fine acicular, rod, and grain boundary one. Mean size of carbides were determined as a function of morphology and thermal degradation time at 630°C. Area fraction of grain boundary carbides and fraction of grain boundary M6C carbides were observed to increase rapidly in the initial stage of degradation and then gradually afterwards. Both mean size of globular carbide and fraction of grain boundary M6C carbides were linearly correlated with strength. Potentials of carbide morphology analysis as a health monitoring technique were discussed, in term of correlation coefficient with strength.

Abstract: Plasma electrolytic oxide coatings were produced on both pure magnesium and binary Mg-Al alloys containing 3, 6, 7 wt% Al, and the effect of Al on the characteristics of coatings was investigated under the processing conditions of various coating times and current. The coatings on Mg-Al alloys showed porous microstructure and some volcano top-like pores on the surface, and they consist of MgO and Al2O3. The surface roughness became larger with increasing coating time, while it was unchanged with increasing Al content. The thickness increased with increasing coating time and showed slight dependence on Al content. The hardness was almost unchanged with increasing coating time and Al content in the coatings grown with the ratio of current, C1:C2=100:75.

Abstract: Both the Al-5wt%Mg powder mixture and Al-5wt%Mg/SiCp composite mixture were separately ball-milled for 50h, followed by compaction under a pressure of 350MPa. To obtain the bi-materials, the compacted Al-5wt%Mg and Al-5wt%Mg/SiCp composites were bonded by sintering at 873K-1173K for 1-3h. At a relatively low temperature, 873K, the sound bi-materials could not be obtained. However, the bi-materials with the macroscopically bonded interface between Al-Mg and its composite were obtained at temperatures higher than 873K. The length of a well-bonded interface became longer with increasing temperature and time, indicating the improved contact in the bonded interface. The relative density in the bi-materials increased as the sintering temperature and time increased, and the bi-materials sintered at 1173K for 5h showed the highest density.

Abstract: Microstructure of hot-pressed Al2O3/Cu composites with a different temperature for atmosphere switching from H2 to Ar has been studied. When the composite atmosphere was switched at 1100°C it lead to a more homogeneous microstructure and the highest fracture strength with smaller deviation than when the atmosphere was switched at 1450°C. The strong sensitivity to atmosphere, especially the oxygen content in the atmosphere, was found to be responsible for observed changes, based upon the interfacial phenomena related to the formation of CuAlO2

Abstract: CNTs/metal/Al₂O₃ nanocomposite powders were fabricated by thermal chemical vapor deposition (CVD) of C₂H₂ gas over metal/Al₂O₃ nanocomposite catalysts prepared by the selective reduction of oxide/Al₂O₃ powders. The yield and diameter of CNTs significantly depended on the kind of metal catalyst and catalyst size. X-ray diffraction (XRD) was used for phase identification. The morphology of CNTs was determined using field emission scanning electron microscopy (FESEM). FT-Raman spectroscopy revealed that the CNTs have single- and multi-walled carbon nanotubes structure. The relationship between the CNT yield/diameter and the characteristics of the composite catalyst was systematically investigated.