발간논문

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To obtain fundamental informations for fine tungsten carbide powders, the preparation of fine tungsten carbide by varpor phase reaction of WCl_6-C₃H_8-H₂ system has been carried out in the temperature range of 700-1200℃ The effects of reaction conditions on particle size and morphology of tungsten carbide powders have been investigated by utilizing X-ray diffraction, SEM and TEM techniques. The synthesis temperature of only WC powders is above 1100℃ in the condition of the flow rate of argon and H₂ gas 200㎖/min respectively and C₃H_8 gas 30㎖/min. The particle size of the reaction products obtained at 1200℃ is about 0.02 ㎛ and the shape of WC is spherical. The particle size and the crystallite size decreases gradually with increasing reaction temperature.

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To obtain fundamental informations for fine tungsten carbide powders, the preparation of fine tungsten carbide by vapor phase reaction of WCl_6-C₂H₄-H₂ system has been carried out in the temperature range of 800-1300℃ The effects of reaction conditions on particle size and morphology of tungsten carbide powders have been investigated by utilizing X-ray diffraction, SEM and TEM techniques. The content of WC increased remarkably with increasing the reaction temperature and the maximum content was about 95% at 1100℃ in the condition of the flow rate of argon and H₂ gas 200㎖/min respectively and C₂H₄, gas 15㎖/min. The particle size of the reaction products obtained at 1100℃ is about 0.007㎛ and the shape of WC is spherical. The particle size decrease linearly with increasing reaction temperature. On the other hand, the agglomerate particle size increase linearly with increasing temperature. The lattice parameters a and c increase gradually with increasing reaction temperature.

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In this study we were examined the effects of austempering time on the fracture properties of Mo-Cu alloyed ADI(austempered ductile iron). All the specimens were austenitized at 900℃for 1.5hr and austempered at 320∼380℃ for various holding time of 0.25∼4hrs. In the microstructure of this specimens austempered for 0.25hr and 0.5hr at all austempering temperatures was shown untransformed austenite volume partially and it was embrittled by martensite transformation. The strenth of these ADI decreases with the increases of austempering temperature, but the fracture toughness increases. When austempering time at various austempering temperatures is 2∼3hr, the strength and fracture toughness was had best value. Also, it`s ductility and fracture tohghness have been proportion with retained austenite volume and SEM fractography of high toughness ADI was showed the dimple and fibrous pattern.

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The pressure dependence of the nucleation temperature for the pure metals, (e.g. Bi and Sn) was considered in terms of the classical nucleation theory and the structural model for a liquid-solid interfacial energy. With a suitable selection of kinetic parameter values, the calculated trend of nucleation temperature on pressure can be shown to reproduce the experimental observations. Form the results of the comparison, the pressure dependence on the liquid-solid interfacial energy, which has been usually ignored, was discussed. However, for a complete understanding of the uncleation kinetices, the structural model may require modification to interpret the liquid-solid interfacial energy. Also, other uncertainties such as a pressuer dependence of the catalytic potency factor, f(θ), should be studied further.

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Spray forming process has been known to the only method by which bulk materials solidified with a fairly high cooling rate of l0²-10³K/sec. This process has been reported to be applicable to production of conventional alloys as well as production of materials which need a rapid solidification processing technique because of difficulties in manufacturing. Thus, the spray forming process has attracted a considerable attention as one of the new important solidification process techniques. Among the operating conditions in spray forming process, melt flow rate and gas flow rate are the most important factors. Aspiration pressure, however, is also a very important factor determining the success of atomizing process as will as influencing the melt flow rate. In this study, the aspiration pressure was measured according to not only the gas nozzle geometry; spraying gas angle, the width of annulus gas exit and the diameter of the annulus gas exit but also operating conditions; outer and inner diameter of orifice, the geometry of orifice tip and spraying gas pressure. Thus the variation of the aspiration pressure is able to be known. On the basis of the data, we can introduce the model to estimate the possible operating region according to the relation of the gas nozzle, orifice and spraying gas condition.

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Microstructure and magnetic property correlations have been sought for unidirectionally solidified Bi/MnBi eutectic alloys. The growth rate was varied from 0.5 to 60 ㎝/h and the temperature gradient was varied from 56 to 256 K/㎝. Annealing for 60 hours at 248℃ was necessary for enhancing the magnetic property. The MnBi phase morphology under various growth conditions was characterized by four types;rod, trinangular, V letter and broken lamellar type. The slope of a logarithmic plot of MnBi phase spacing with respect to growth rate was 0.5148, which was nearly in agreement with the theoretical slope of a nf/nf eutectic alloy. This means the MnBi phase to be independent of temperature gradient. The coercivity of the Bi/MnBi eutectic alloy solidified unidirectionally were greatly imporved by annealing. This seems to be caused by the transformation of the MnBi phase from HTP to LTP. The more increased the growth rate, the more the intrinsic coercivity improved. As a result, the maximum intrinsic coercivity of 27.8 kOe was obtained at the growth rate of 60 ㎝/h.

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Bi-Pb-Sr-Ca-Cu metallic precursor for high Tc superconductor was manufactured in a vaccuum arc melting furnace and the oxidation mechanism of this precursor was studied by differential scanning calorimetry and X-ray diffractometry. The optimal melting process was to melt the whole alloying elements except Sr in one charge. The master alloys such as Cu-Ca, Bi-Sr-Pb, Cu-Sr, and Pb-Bi do not give the sound melting stock retaining volatile Sr and Ca. The melting of Cu-Bi-Ca-Pb in one charge and then the alloying of this melting stock with Sr gives a sound ingot. The separation of liquid phases during melting was reduced in extent by the addition of Pb in the Bi-Sr-Ca-Cu system. The mechanism for the oxidation of the precursor was revealed as follows. 1. Formation of SrPbO₃ from the matrix in the low temperature range. 2. Formation of the low Tc 2212 phase in the SrPbO₃ with diffusion of Bi and Ca from the matrix and Cu from the separated phase. 3. Formation of high Tc 2223 phase in the 2212 phase with further diffusion of Ca and Cu from the separated phase.

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The elasto-plastic finite element analysis for deformation of sintered porous metal specimen has been made. This analysis takes into account yield surface expansion due to work hardening and density increase and contraction due to tensile stress as well. Density is calculated based on mass conservation. Below a critical density, the yield surface is assumed to have a very small finite value with penalty of 10^(-10). The method has been applied to upsetting of porous metal cylinder. The calculated results are in very good agreements with Shima`s experimental results. Folding and crcaking due to barreling are predicted.

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Hot-pressing of cast Nd-Fe-B alloy was tried out for the shortening of manufacturing process for anisotropic R-Fe-B magnet. The effects of composition and working variables on the magentic properties for Nd-Dy-Fe-B-Cu magnets were studied. The results are as follows: 1. Dendritic growth to the direction of mold wall was observed in as-cast Nd-Dy-Fe-B-Cu alloys. Magnetization measured to the normal direction to the solidification showed higher values than the values measured to the parallel direction. 2. Hot-pressing of Nd-Dy-Fe-Cu alloy invoked alignement of grains to press direction 3. In Nd_xDy₁Fe_(92.5-x)B_5Cu_(1.5)(x=13∼16) and Nd_(14) Dy₁Fe_(83.5-y)B_yCu_(1.5)(Y=4∼8) alloys, the optimal magnetic properties of hot-pressed magnets were attained at x=14 and y=5. 4. It was confirmed that added Cu existed in Nd rich phase.

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The anisotropy of R(R=Pr, Nd)-Fe-B-Cu magnets prepared by hot-pressing was investigated. For that purpose, anisotropy factor(f_a)was introduced. Also, the effects of thickness reduction ratio, working temperatures and heat treatment on the magnetic properties for R-Fe-B-Cu magnets were studied. Induced anisotropy parallel to press direction by hot-pressing was confimed through the variation of magnetic properties and domain patterns of R-Fe-B-Cu magnets. Anisotropy factors were increased according to the increment of thickness reduction and working temperature. But heat treatment of the R-Fe-B-Cu magnets had no relation with anisotropy factors. Irregularities and swelling of the specimen during the pressing were alluded.

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