발간논문

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The effects of Mn addition on the magnetic properties were investigated for Co_(76-x) Mn_xCr₄B_(16)Si₄(X=0, 2, 4, 6, 8) soft magnetic amorphous alloys which have high saturation magnetization, low coercive force and high permeability. The magnetic properties such as saturation magnetization, coercive force and maximum permeability of these alloys were measured by VSM and B-H loop tracer. With increasing Mn content, the crystallization temperature increased whereas Curie temperature decreased. The saturation magnetization had a maximum value at X=4, but the coercive force and the maximum permeability had best values at X=6. The improvement of the soft magnetic properties of the amorphous alloys were attributed to formation of a ferromagnetic Co-Mn coupling and decrease of magnetostriction induced by Mn addition.

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The magnetic properties and microstructures of Misch Metal(MM) - Fe - B and MM-(Fe, Co)-B melt-spun ribbons were investigated. The magnetic properties depended upon the wheel velocity of the single roll, the optimum velocity for good magnetic properties being varied with the composition. MM_(16.25)Fe_(70.06)B_(6.69)Co_7 ribbon which was prepared with a wheel velocity of 20.84m/sec showed a maximum energy product of 4.45MGOe and intrinsic coercive of 8.71KOe in the external field of 20KOe. The microstructure of ribbons melt-spun with the optimum velocities consisted of fine crystalline grains. Magnetic hardening was thought due to these fine grains. The coercive force of MM- Fe - B melt -spun ribbons was decreased by the addition of Co. The velocity showing the highest (BH)_(max) value was rather larger for the Co containing alloy, compared to the Co-free alloy.

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The chemical rate phenomena associated with the flash reaction of chalcopyrite have been examined by the in-flight flash reactor system. In the gas-solid reaction stage, dissociation and subsequent oxidation of labile sulfur near the reacting particle enhance the heat ing of particles which eventually leads to the rapid preferential oxidation of iron sulfide in dissociated particles. The rate determining step of this stage is found to be mass transfer through the boundary layer. Due to the exothermicity of reaction, particles are melted during the reaction. It is found that because of the rapid increase of particle temperature and gas evolution within the molten core, a particle is fragmented to spherical droplets any oxide crust. The extend and shape of fragmented particle varies with the initial size and surroundings. Typical shapes under different experimental conditions are presented and discussed in this study.

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The growth characterisitics of complex carbides precipitated in Fe-5. 6Cr-C steel and Fe-8. 9Cr-C steel during spheroidizing treatment have been investigated. In order to examine changes in the the crystal structure and composition of complex carbides, they were electrolytically extracted from spheroidized steels. The coarsening of complex carbides has been found to take place by the concentration of chromium in the carbides which have been determined to be (Fe, Cr)_7C₃. The activation energies for the coarsening of complex carbides have been calculated to be 45.9Kcal/mole and 55.7Kcal/mole, respectively. In case of Fe-5. 6Cr-C steel, when spheroidizing for 400 hours at 700℃ the compositional change of the complex carbide has reached to an equilibrium state and consquently the chemical formula for this carbide at that stage has been analysed to be(Fe,_(1.5)Cr_(5.5))C₃.

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Systematic studies were carried out to investigate the extraction kinetics of cobalt by di-2-ethyl-hexyl phosphoric acid in kerosene using a single drop cell. The extraction rates were increased with increasing pH and cobalt concentration in the aqueous phase. The concentration of D₂EHPA had a little effect on the extraction rate within the experimental range. Extraction kinetics correlated well with the Handlos-Baron model coupled with an intermediate complex formation at the organic-aqueous interface

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When the rapidly solidified standard high Mn steel strips were reheated, the precipitation curve of carbide was likely a C-curve which had the nose at about 600-700℃. The precipitation curve of pearlitic constituents(PC) was also likely a C-curve having the nose at about 600℃. By addition of alloying elements(Cr, V), the temperature ranges of the carbide and PC precipitation were enlarged and the time of their precipitation was earlier. The quantities of the carbide and PC precipitation were increased and were observed much more in the V-added specimen. Regardless of the alloying elements(Cr, V), carbide was formed in the shape of film at the grain boundaries at 500-700℃. With the increase of heating time, the shape of carbide film was grown like plate and subsequently PC nucleus was formed at the plate-like precipitations and grown. The hardness of the (Cr, V) added specimen was higher than that of the standard specimen. The increased hardness was mainly affected by PC rather than carbide. V has more accelerated the formation of carbide and PC than Cr.

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Molybdenum was deposited onto nickel substrate by chemical vapor deposition from MoCl_5 and H₂ gas mixture. Effects of temperature, flow rate of the gas, and MoCl_5 partial pressure on the deposition rate, microhardness, and x-ray pattern were thoroughly investigated. The deposition rate of Mo was found to be zero order reaction and was increased with increasing temperature, flow rate of the gas, and MoCl_5 partial pressure It was apparent that the reaction mechanism changes at 900℃, at temperatures lower than 900℃ the activation energy is 35.8Kcal/mole, whilst at temperatures higher than that is 3.6Kcal/mole. In the region of diffusion control, the deposition rate of Mo was proportional to the square of total flow rate. The microhardness of deposited layer was increased and the preferred orientation(220) distinctively appeared with increaing above three factors, (reaction temperature, flow rate of the gas, and Mocl_5 partial pressure) however, the former was highest and the latter was lowest at 800℃.

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Transparent alumina was prepared by a sol-gel process. The fabrication process comprises precipitation of aluminum monohydroxide, formation of sol and gel, and calcination of alumina. Thin Al₂O₃coatings were prepared on stainless steel by slow dehydration of an aqueous sol and subsequent heating in air. The alumina coating was transparent by calcination at temperatures up to 1000℃ above which it became opaque due to the formation of α-alumina phase. The total volume of micropores and the volume of pores larger than 80A in the synthesised alumina powder were decreased with increasing calcination temperature upto 1200℃ above which the latter was abruptly increased. The grain size was affected by PH of solution, mole ratio of H₂O/ASB, and reaction temperature. The lattice parameter was not affected by the above three parameters. The alumina powder synthesised by this method has both ultra-fine size and high purity. The high reactivity owing to high surface energy is thought to have contributed to obtaining this alumina powder at a relatively low sintering temperature of 1600℃.

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The effect of SiC whisker reinforcement on the high temperature tensile properties and deformation behavior of Al-Cu-Mg alloys was investigated using high temperature tensile test and scanning electron microscopy. The tensile strength of the SiC whisker reinforced composites was not greater than that of the matrix alloys without whisker and decreased more rapidly with increasing temperature. Whereas the tensile stength of matrix alloys decreased abruptly above the incipient melting temperature, that of the composites decreased continuously at that temperature and above without abrupt change. The elongation-to-failure of the extruded Al-Cu-Mg-SiC_w composites increased with increasing temperature and reached the maximum value near the incipient melting temperature of matrix alloys. Addition of Mg to Al-Cu-SiC_w composites increased the elongation-to-failure at the incipient melting temperature of matrix alloys and above. The elongation-to-failure of the rolled Al 2024-20SiC_w composite near the incipient melting temperature of matrix alloy increased with increasing strain rate and reached the maximum strain of 100% at a strain rate 0.2s^(-1) Fractographs for the composites revealed that the final fracture at low strain rates was mainly due to the elongation of matrix and that at high strain rate was mainly due to the whisker-matrix sliding. An theoretical approach was suggested to explain the high temperature deformation behavior of the composite. Predictions by the proposed model for the strain rate sensitivity and fracture mode transition with stain rates were well consistent with the experimental results.

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The carbothermic smelting reduction of Nb₂O_5 and successive refining of Nb crude metal were studied by Ar/Ar-H₂plasma arc. The Ar plasma reduction of Nb₂O_5 proceeded rapidly to form 2 N grade Nb metal in 15minutes and the Ar-H₂plasma was very effective to the simultaneous decarburization/deoxidation of Nb crude metal. The mixing ratio of carbon and plasma gas composition were main factors to attain the high purity Nb metal. The solubility product, [wt%C] [wt%0], was shown the minimum value of 0.0017 under the carbon ratio(C/Nb₂O_5) of 5.2 and Ar-20%H₂plasma gas. This results confirmed that the high Nb metal over 3 N grade can be refined from Nb₂O_5, by the one step process using the Ar and Ar-H₂plasma.

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