발간논문

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Dry sliding wear behavior was investigated in SiC, TiC and B_4C particle reinforced 6061 Al matrix composites that were manufactured by a pressureless infiltration technique. Pin-on-disk wear tests were performed on the composite disk specimen against a hardened AISI 52100 steel ball at various applied loads. Wear resistance of the composites was always higher than that of the monolithic alloy under all test conditions. The resistance increased with the increase of the size and volume fraction of the reinforcing particles. Subsurface deformation and delamination of the deformed (comminuted) layer was a major wear mechanism of the composites at high wear rates, while the composites displayed abrasive-wear-like behavior at low wear rates. Among the composites tested, the SiC reinforced composite showed the lowest wear resistance. Wear resistance of all the composites increased remarkably after T6 heat treatment. Variation of matrix hardness immediately below worn surfaces and the role of surface-iron-oxide layers formed during wear of the composites were also discussed.

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Fracture toughness of a SA 533 B-1 steel was characterized in ductile-brittle transition temperature region by means of a RKR-type model. The original RKR model has been used to predict the plane strain fracture toughness(K_IC) behaviors in lower shelf region by assuming two material parameters, ie, the critical fracture stress and the characteristic distance. In this study, the fracture surface of every specimen was thoroughly investigated using scanning electron microscope to locate the actual cleavage initiation and to measure the cleavage initiation distance(CID) from the initial crack. The local fracture stress(σ^*_f) of material was determined from the elastic-plastic stress field at the measured cleavage initiation location in the notched and precracked specimen. The local fracture stress of the precracked specimens was much higher than that of the notched specimen. The measured CIDs were strongly dependent on the test temperature and also on the fracture toughness. Based on the observations, it is found that, in the RKR-type cleavage fracture models, the characteristic distance should not be treated as a constant material parameter in the ductile-brittle transition region where the cleavage initiation controls the overall fracture process.

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The present study is concerned with the microstructural evolution and property improvement of VC/ carbon steel surface-alloyed materials fabricated by high-energy electron-beam irradiation. The mixtures of VC powders and flux (50%MgO-50%CaO or CaF_2) were placed on a plain carbon steel substrate, and then electron beam was irradiated on these mixtures using an electron beam accelerator. The surface-alloyed layers of 1.2∼3 ㎜ in thickness were homogeneously formed without defects, and contained a large amount (up to 10 vol.%) of VC precipitates in the bainitic or martensitic matrix. This microstructural modification including the formation of hard precipitates and hardened matrix in the surface-alloyed layers improved hardness and wear resistance. Particularly in the surface-alloyed material fabricated with the lower input energy density, the wear resistance was greatly enhanced over the steel substrate because of the increased size and volume fraction of VC particles, although the thickness of the surface-alloyed layer decreased. Microstructural modifications including melting, solidification, precipitation, and phase transformation processes of the surface-alloyed layer could also be explained from a thermal transfer modeling and a Fe-V-C ternary phase diagram.

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We have investigated the etching possibility of ITO(Indium Tin Oxide) thin films by using the plasma that was developed to generate a low-temperature plasma torch at atmospheric pressure. Hydrogen and methane radicals generated from the plasma were observed and their intensity was found to be dependent on the methane flow rate due to an analysis with optical emission spectroscopy. The etching ability of this plasma was evaluated by the evaluation of methane radicals emission intensity.

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Fe_21Ni_79 nanowire arrays have been fabricated by the electroforming method using AAO(anodic aluminum oxide) as a template, which was prepared by anodizing the pure aluminum foil. According to the magnetic property of Fe_21Ni_79 nanowire prepared, it was found to have the coercivity more than 1 kOe due to the shape anisotropy and squareness(Mr/Ms) very close to 1. Especially, it could be noted that Fe_21Ni_79 nanowire showed the preferred crystallographic orientation of (220). Annealing treatment of Fe_21Ni_79 nanowire at 500℃ resulted in the enhancement of coercivity by 18% while the squareness was not varied by annealing treatment. However, the random orientation of Fe_21Ni_79 disk and the preferred orientation of nanowire arrays were maintained without respect to the annealing treatment up to 500℃.

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The equilibrium study has been made on the vanadium distribution between CaO-SiO_2-MgO_ast.-Fe_tO slag and liquid iron over the temperature range from 1540℃ to 1640℃. The vanadium distribution ratios(L_v) were approximately larger than phosphorous distribution ratios(L_p) by a factor of 10∼100 times. The L_v dependence on slag composition and temperature was found to be similar to the case of L_p. The vanadium distribution ratio increases with the slag basicity and Fe_tO content in slag, but decreases with the increase of temperature. A linear relationship was observed between logarithm of (V)/{[V][O]^n} (n=2 or 2.5) and (CaO+0.3MgO)/SiO_2. It is assumed that the vanadium ion is in the form of V^4+ or V^5+ in slag. The mutual function relating the slag components, F_s=0.58(CaO/ SiO_2)+0.025Fe_tO, was introduced to explain quantitatively the influence of slag composition on the vanadium distribution ratio. The log values of vanadium distribution ratio as a function of Fs were derived at the fixed temerature of 1590℃ and 1640℃.

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