발간논문

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The ultrafine particles of relatively pure SiO₂ were prepared by means of a water/isooctane/ AOT microemulsion system. The reactant added to the microemulsion system was tetraethoxysilane and the aqueous solution of ammonia was also added to adjust the PH of the micoremulsion system. The SiO₂ particles precipitated in a microemulsion reactor was dried and then calcined properly in order to obtain ultrafine SiO₂ powders. The condition of drying and calcination process was found to influece the purity of final product. The SiO₂ powders prepared in this way(especially water-in-oil microemulsion) had substantially increased number of micropores in them compared to the powders prepared by other methods. The combination of materials forming the microemulsion system was also found to influence the size and porosity of SiO₂ powders as well as the stability of microemulsion systems.

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The variation of SME by the cyclic deformation process that, Fe-30Mn-5Si-1Co is annealed at 600℃ after 4.5% deformation has been studied. The cyclic deformation effect was investigated by measuring SME using the method of tensile cyclic deformation and bending cyclic deformation. In the result of measurement of SME by the tensile cyclic deformation, SME reached at constant value with small deviation after gradually increased until 7 cycle. And the bending cyclic deformation showed similar result. Investigation of the variation of the microstructure by cyclic deformation using X-ray and optical microscope showed that by the cyclic deformation the amount of ε martensite increased and the variants were gradually oriented. We could know that the increase of ε martensite and the re-orientation of the variants is concerned with the increase of SME. And after the saturation, the SME was no more increased. In the result of the measurement of Neel temperature, Neel temperature gradually increased and reached at constant value. This means that the gradual stabilization of matrix hinders the increase of ε martensite. A_s showed the same inclination, too. The addition of Co to improve the shape memory effect of FeMnSi alloys did not affect the enhancement of corrosion property nearly.

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In order to determine the optimum condition for batch annealing of STS 420J2 martensitic stainless steel, several annealing treatments were done above and below the Ac₁ transformation temperature. At higher annealing temperature, the amount of dissolved carbides increased and more homogeneous and finer carbides precipitated after cooling. Deformability in the hot rolling direction of STS 420J2 martensitic stainless steel was improved because of remaining Cr-C laminates after hot rolling.

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Preparation of double-layered grain structure was stuided in molybdenum (Mo) sheet. Mo powder doped with CaO and MgO (D-layer) and pure Mo powder (P-layer) were compacted at 300MPa in sequence of P-D-P and D-P-D. The compacts were sintered at 1850℃ and straight rolled by 70% reduction in thickness. Microstructures in thickness direction were observed by optical, scanning electron and transmission electron microscopies. Pole intensities were also calculated for a specific [hkl] orientation. To understand the role of dopants, i.e, CaO and MgO during annealing, P-D-P and D-P-D specimens were annealed at 1200℃ for 10hrs, at 1776℃ for 2hrs and at 1950℃ for 2hr under hydrogen atmosphere. D-layers at the surface of D-P-D specimen revealed smaller grain size than D-layer at the center of P-D-P specimen in the range of primary recrystallization (l200℃), At 1776℃, both P-D-P and D-P-D specimens showed secondary recrystallization phenomenon from the surface of specimen, At 1950℃, double-layered grain structure developed in D-P-D specimen while coarse grains formed in P-D-P specimen.

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The effects of atomizing gas pressure and geometry of melt delivery nozzle on the aspiration pressure, water flow rate and gas flow rate were studied in two different types of gas nozzles(converging gas nozzle, and converging and diverging gas nozzle). In case of converging gas nozzle, adjustment of protrusion length of the melt dilivery nozzle below the gas nozzle is required to get stable high aspiration pressure. The optimum extrusion distance varies with outer diameter of melt delivery nozzle. In case of converging and diverging gas nozzle, high aspiration pressure is obtained without any extrusion of melt dilivery nozzle. the effect of inner diameter of melt delivery nozzle on the aspiration pressure is insignificant in both types of gas nozzles. Variation of water flow race with aspiration pressure can not be interpreted with Bernoluui`s equation because the aspiration pressure was measured without water flow, which affects the has field during atomization. Gas flow rate increases linearly with atomizing gas pressure. With increasing atomizing gas pressure the ratio of gas flow rate to melt flow rate increases.

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High temperature oxidation of Fe-13%Cr and Fe-17%Cr stainless steels at the temperatures between 1200℃ and 1300℃ in dry oxygen environment has been studied. The oxidation rate was more influenced by the partial pressure of oxygen than the temperature. The oxidation was proceeded by forming nodular oxides at the scale/metal interface. The decrease in oxidation rate with increasing temperature was due to the prevention of nodular oxide growth at the scale/metal interface by lowering oxygen potential which was caused by the evaporation of CrO₃(g). The scale breakaway of the Cr₂O₃ layer formed at the scale/metal interface was restrained by the formation of internal oxides constituting mainly of SiO₂

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Statistical evaluation of strength has been performed for SiC particle reinforced Al base composites made by two fabrication processes in order to predict their reliability. In the case of compocast SiC_P/A356 Al composites, it was found that both tensile strength and elongation of hot extruded composites are much higher than those of as-cast composites. This might be associated with the reduction of defects in the matrix and the homogeneous distribution of the SiC particles due to reorientation of the particles along the extrusion direction. Statistical evaluation of strength for these composites indicated that the scatter in tensile strength of extruded composites were remarkable decreased in comparison with the as-compocast composites. In particular. the weibull modulus was highly increased due to the extrusion process. In the case of SiC_P/2024Al composites by P/M process, the tensile strength of the composites was increased with volume fraction of SiC particle. However, it was found that. the scatter of tensile strength is increased as volume fraction of SiC particles increases. The Weibull modulus of the composites tents to gradually decrease by adding SiC particles into the matrix, exhibiting ductile to brittle fracture mode. The weibull modulus, m, was 25 in the composites with 20 Vol.% SiC particle, 43 in the 2024 matrix, respectively.

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Quasicrystalline and quasicrystal-related crystalline phases in rapidly solidified Al-Fe alloys have been investigated by a detailed TEM(transmission electron microscopy). Rapidly solidified Al-10at %Fe mainly consists of α-Al and microquasicrystalline phase. The decagonal phase particles with a dendritic shape are also present as a primary solidification product in the relatively slowly cooled region of the ribbon. Rapidly solidified Al-20at%Fe consists of α-Al and Al_(13)Fe₄ phase. The Al_(13)Fe₄ phase particles have a dendritic shape with ten branches due to (201) and (100) ten fold twinning of Al_(13)Fe₄ monoclinic structure. Detailed SADP(selected area electron diffraction pattern) analysis has shown that there exists a close similarity between the diffraction characteristics of the decagonal phase and Al_(13)Fe₄. The present study also shows that the quasicrystalline structure of the decagonal phase decomposed into ten fold twinned Al_(13)Fe₄ structure with heat treatment.

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Recently the plane strain stretching test called RIST-PSST has been developed as a formability test designed for the sheet material industry. To evaluate the stability of the RIST-PSST against several noise factors such as specimen size, specimen surface condition, test speed and blank holding force, Taguchi`s orthogonalarray experiments for the RIST-PSST were carried out and the result was compared to the case of hemispherical punch stretching test known as the limiting dome height test (LDH test). Results showed that the RIST-PSST was more stable than the LDH test for each noise factor. Moreover specimen surface preparation was very important factor on the LDH but the RIST-PSST was not affected largely by surface state of specimen. Therefore due to its simplicity of the test procedure the RIST-PSST can be used easily as a robust testing method to evaluate the press formability of sheet materials in sheet making plant and also to monitor the incoming material in stamping plant.

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The effect of final stage electro-magnetic stirring on the macrosegregation behavior in CC high carbon steel blooms(C contents:0.71, 0.82 and 1.01wt.%) was examined by using the continuous bloom caster, equipped with an in-strand stirrer(SEMS) and a final stirrer(FEMS). The properties of mushy zone were also examined through white band measurement and calulation of iso-solid fraction line and permeability together with a complimentary permeability measurement using a Tamman furnace. An optimum stirring pool thickness(d_o) at the position of FEMS is then determined for each high carbon steel tested, at which the best centre soundness is obtained. The optimum value of d_o is found to decrease almost linearly with increasing the carbon content, due to a longer solidification time interval(t_c) for higher carbon content. The specific permeability is also found to decrease with the carbon content suggesting that for a higher carbon content, the centre defects are formed at a little later stage of final solidification with the reduced optimum stirring pool thickness at FEMS. The cast structure at the centre region, for the most effective reduction of macrosegregation, is then determined as less developed dendritic equiaxed structure, with finely distributed solute segregation spots through thickness, and having small secondary dendrite arm spacing.

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