발간논문

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An analysis on the morphological development in the planar solid-liquid interface of dilute Sn-Sb alloys has been investigated by means of the directional solidification process with the help of a novel “Pettier Interface Demarcation” method for the recording of the instantaneous interracial morphology during solidification cycle. The transition from planar to cellular interface under the condition of corresponding change of G_L and V was agreed generally with constitutional supercooling criterion but it was deviated slightly from the CS criterion as the growth velocity was increased. The perturbed solid-liquid interface in its initiation stage was analyzed to preserve the planar interface in accordance with the CS and Mullins Sekerka`s criterion for the stability of the planar solid-liquid interface. Although the solid-liquid inter face in polycrystalline growth was statisfied macroscopically with a planar shape, the microscopic difference in the growth velocity of each grain resulted from some effects due to the slight difference in cry stallographic orientation and the grain boundary energies of each grain.

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To study the effect s of heat treatment on the magnetic properties of ductile Fe-Cr-Co permanent magnets, a composition of Fe-25Cr-l4Co-2Si-1.5Ti alloy was prepared by induction melting, and the optimum heat treatment procedure and the microstructures of the alloy were investigated. After step aging treatments, it was concluded that aging at high temperature effects largely on the squareness of the demagnetization currve and aging at low temperature effects largely on the coercive force. The sample which was thermomagnetically treated for 10 minutes at 675℃ showed clearly the evidence of spinodal decomposition of the FeCo-rich phases from the Cr-rich matrix.

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Effects of compressive stress generated in the oxide layer and its subsequent relief on oxidation rate and post-oxidation creep characteristics of Zircaloy-4 were investigated by oxidation studies in steam with and without applied tensile stress and by creep testing at 700℃ in high purity argon. The thickness of oxide layer increased with the magnitude of tensile stress applied during oxidation at 650℃ in steam whereas similar phenomenon was not observed during oxidation at 800℃. Zircaloy-4 specimens oxidized at 600℃ in steam without applied stress exhibited higher creep strain than that shown by unoxidized specimens when creep-tested in argon. Zircaloy-4 specimens oxidized at 600℃ steam under the applied stress of 8.53MPa and oxidized at 800℃ under the applied stress of 0 and 8.53MPa exhibited lower strain than that shown by unoxidized specimen. The above experimental results were accounted for on the basis of interactions among applied stress during oxidiation, compressive stress generated in the oxide layer and elasticity of Zircaloy-4 matrix.

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Through the application of mathematical modelling and water model experiments, a reasonable understanding has been obtained of flow pattern and mixing characteristics in ladle injection metallurgy. It is seen that the flow pattern in gas-stirred cylindrical vessel produces a recirculating vortex located high in the vessel and displaced toward the side wall. Stagnant zone is existing especially at the bottom of the vessel and increased as the H/D ratio is decreased. Mixing time of gas-stirred system is measured by means of a conductivity method. It was found that the time required for the homogenization of the tracer could be readily correlated with the rate of disspiation energy or Froude number. Mixing time also depends on geometry of a vessel. As the H/D ratio of a vessel is decreased, the mixing time is increased. The increase of mixing time in case of low H/D ratio is considered to be the increase of stagnant zone in bottom region.

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A mathematical model of a fluid flow in a bottom blown gas-stirred vessel which is widely used in refining process of steel has been developed with the use of Homogeneous Flow Theory corrected by Drift Flux Model. Time-averaged continuity equation, Navier-Stokes equation and K-εturbulence model are applied to the two-dimensional axisymmetric turbulent recirculating flow in a cylindrical vessel and mean velocity distributions, turbulent kinetic energy distributions are computed by Finite Difference Method. The computed results of the velocity distributions are reasonably well agreed with the experimental data of investigators and those of turbulent kinetic energy show semi-quantitative agreements. This numerical experiments show that the fluid flow field in gas-stirred vessel is highly non-uniform and that velocity and turbulent kinetic energy show relatively higher values near free surface and plume zone than the bottom regions of vessel which has very lower values.

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High temperature corrosion behaviors of type 304 and 410 steels in dry oxidizing gas-argon mixtures have been studied in detail over a temperaturerange of 800-1100℃ using thermogravimetry, metallography, x-ray analyses and EPMA. From the results obtained, the oxidation rate as a function of the ratio of oxidizing gas to argon and the temperature has been determined. The overall apparent rate was close to parabolic law for all experimental specimens. It was confirmed that the apparent activation energies calculated for the oxidation in all cases were ranged to be 15-27 and 10-21 ㎉/㏖ respectively for type 304 and 410 steel. The spinel layer in the product scale containing (Cr.Fe)₂O₃, NiFe₂O₄ and NiCr₂O₄ in type 304 steel, and that of(Cr.Fe)₂O₃ in type 410 steel were also observed.

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Effects of neodymium content, particle size, sintering and heat treatment on magnetic properties of Nd-Fe-B system permanent magnets were investigated. In the study, the best magnetic properties were achieved for the magnets containing 33 wt% Nd in the Nd-Fe-1.3B system. An energy product of up to 37 MGOe has been resulted. The highest coercivity was observed when the average particle size was about 3㎛. When the content of Nd was less than 31 wt%, the coercivity was not exhibited. It was also observed that very little sintering has taken place due to lack of Nd-rich phase with in the Nd-Fe-1.3B system. The absence of coercivity was due to easy nucleation of reverse domains with in the irregular shape of grains in the sintered body.

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