발간논문

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Microstructure of direct bonded copper-alumina substrate has been investigated using SEM, EDX, and EPMA. Its interfacial strength was also evaluated by peel test. When the alumina bonded at 1337 K for 0.3 ks under 4.4 Pa in contact with the copper which preoxidized at 1273 K for 0.3 ks under the oxygen partial pressure of 10 Pa, the substrates had the highest interfacial (peel) strength of 8.2㎏/㎝. Microstructural investigation demonstrated that its bonding resulted from the formation of the layer consisting of copper and Cu₂O at the interface between copper and alumina. When the density of Cu₂O particles was higher along the grain boundaries than in the grain of the copper, the substrates had the higher peel strength. In addition, the substrate had the higher strength when the shape of Cu₂O particles was spherical, not nodular.

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The Fe-Cr-C alloys having high hardness and good wear resistance have been used as the wear resistance materials, valve seat, tappet, etc. mainly made by sintering of powder. For the examination of the role of P that is added with gas atomized Fe-Cr-C alloy powder which has a finer microstructure than chemical reduction powder in view of liquid phase and phase sta ilization, this study was carried out. In this study, Fe-14wt%Cr-1.77wt%C alloy powder compact was sintered for 1 hr at 1170℃ in H2 atmosphere. The effects of P were examined by OM, SEM, EDS, DSC and micro Vickers hardness of quenched specimen. During sintering, P was diffused throughout Fe(γ) matrix, and changed it from FCC to BCC structure and the sintering rate was increased by rapid self-diffusivity, but the hardness of quenched specimen decreased because the amount of FCC phase which carried out martensitic transformation decreased. And liquid phase (Fe3P + Fe matrix) asisted that P diffused to matrix, but the excess liquid phase hampered densification for increasing swelling effect.

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A new process for the metal fiber production by the powder extrusion has been developed. Being compared with the conventional processes, such as the bundle drawing method, the shatter machining method and the in-rotating water melt spinning method, the process has two distinctive advantages. One is that the fibers having a diameter below l0㎛ can be easily manufactured, the other is that the diameter, the length and the cross-sectional shape of the fibers can be controlled easily by the selection of the diameter of initial metal powders, extrusion ratios and extrusion die shapes. The silver fibers of 2㎛ wide and 30㎛ long and aluminum fibers of 5-10㎛ wide 1-3㎜ long were demonstrated.

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The present study aims at investigating the effect of SiC particulates size and heat-treatment on the tensile properties of SiC_P/AZ91 Mg composites. The AZ91 Mg alloy powders were made by wet attrition milling from AZ91 Mg machined chips and the powders size was below 240㎛. The SiC particulates size used as a reinforcement was 8, 30 and 50㎛, respectively. The Mg powders and SiC particulates (10 vol%) were mixed using a V-blender for 2 hours. Both AZ91 Mg alloy powders and mixtures of AZ91 Mg alloy powders and SiC particulates were hot pressed at 400℃ to make round billets. The billets were hot extruded at extrusion temperature of 300℃ under extrusion ratio of 40:1. The extruded Mg matrix and SiC_P/AZ91 Mg composites were solution treated at 410℃ for 17 hours, quenched into ice water, and then aged at 170℃ for 40 hours. It was found that the ultimate tensile strength of as-extruded composites is much higher than that of the as-extruded Mg matrix. As SiC particulates size decreased, the U.T.S. of as-extruded composites tended to increase. This could be attributed to the matrix grain refinment caused by the present of SiC particulates and the reduction of crack formation by the fracture of SiC particulates during hot extrusion. After T_6 heat treatment, the U.T.S. of heat-treated composites was found to be superior to that of as-extruded composites. Also, the U.T.S. of the composites was considerably increased with decreasing particulates size after T_6 heat-treatment. The reason for this is the restriction of the matrix grain-coarsening by fine SiC particulates during solution treatment.

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Solidification of an initially superheated aluminum filled within a rectangular cavity is numerically studied. Emphasized is the influence of both volume contraction caused by solid-liquid density change and natural convection in the melt. Difficulties associated with the complex time-dependent solid and liquid domains, whose shapes are also a part of the solutions, are overcome by employing the boundary-fitted coordinate system. Computed results are shown in forms of transient position of the interface, temperature distribution, flow pattern, Nusselt number, and solidification fraction with respect to time.

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A kind of hybrid numerical method between a finite volume method (FVM) and a finite element method (FEM) is applied for thermal stress analysis in a two-dimensional solidification of a pure metal allowing natural convection in the melt and volume contraction due to solid-liquid density change. A hypoelastic-viscoplastic constitutive model and a rate form of the principle of virtual work are involved to model the stress and deformation. Stable numerical solutions are obtained for the solidification process with solid-liquid density change and natural convection in the melt. Thermal stress distributions in a solidifying body are presented and discussed.

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By the Nb(OC₂H_5)_5 source, Growth process of Nb₂O_5 films formed by AP-_MOCVD has been examined by in-situ ellipsometry as a function of the substrate temperature, carrier gas flow rate, source temperature and reactant O₂ gas flow rate. Δ-Ψ curve obtained in experimental results corresponded to the theorical Δ-ψ curve of transparant homogenious film, and showed a rectilineal growth behaviour of the one stage at a low temperature range of 473∼673K and indicated a rectilineal growth behaviour of the two stage at the high temperature of 673∼753K. A refractive index meaning density of the deposited film increased until 673K of the substrate temperature but decreased above 673K. The thin film deposited at 723K showed the most excellent corrosion resistance, because the deposited film was crystallized into the δ-Nb₂O_5 above 723K. Dielectric constant of Nb₂O_5 thin films deposited at 675K show the maximum vlue. Dielectric constant of Nb₂O_5 thin films deposited above 675K was decreased.

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In order to develope a high capacity hydrogen-absorbing alloys for Ni-MH secondary battery, charge and discharge characteristic, of Ti-based and Zr-based Laves phase alloys were investigated. Ti-based Laves phase alloys showed low discharge capacity because of their poor electrode catalysis and lower hydrogen diffusivity in the alloys. However, some Ti-based alloys revealed relatively high discharge capacities up to 200mAh/g indicating possibility to use as electrode materials by a modification of alloys. Zr-based alloys showed higher discharge capacities and better electrode characteristics than Ti-based alloys. In the Zr-based alloys. Zr_(0.4)Ti_(0.6)Mn_(0.4)V_(0.6)Ni_(0.8)Fe)(0.2) electrode exhibited remarkably improved discharge capacity of 385mAh/g, in the case of slow charge and discharge. However in order to use this alloy for MH electrode, it is necessary further improvement of charge and discharge rate capability and cycle life.

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The effects of thermal cycling treatment on mechanical properties and transformation temperatures of Fe-20%Mn alloy were studied. With increasing number of the thermal cycling between room temperature and 260℃, the amount of ε martensite sharply increased up to 3 cycles and showed parabolic increase beyond 3 cycles, but the amount of γ austenite showed exactly the reverse. When the alloy was deformed to fracture at room temperature, the amount of ε martensite decreased but α′ martensite increased gradually with thermal cycling. On the other hand, γ could not be found except ε and α′ martensites in the fractured samples. As the number of thermal cycling increased, the hardness, UTS and YS of the alloy increased, the ε martensite plate became finer, a large number of dislocations were introduced in γ parent phase, and M_s was lowered.

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