발간논문

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The magnetic field dependence of the magnetic domain and domain wall in the transparent monocrystalline YFeO₃(60㎛) have been studied using the magneto-optic Faraday effect. The domain pattern consists of stripe domain and bubble domain. The width of the domain and domain wall are measured as dependent on the applied magnetic field parallel to the C-axis. Domain patterns have been photographed by means of Faraday effect utilizing the xenon lamp. The width of the domain wall is calculated by measuring the change of the Faraday rotation in the near domain wall. The magnetic field dependence of the stripe domain and bubble domain are good explained by the equation of the minimizing the total energy. The width of the domain wall of the stripe domain and bubble domain are the same and independent on the applied magnetic field.

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This study was attempted to improve the mechanical properties such as strength and elongation in martensitic high carbon steel by increasing Ms temperature. In order to raise the Ms temperature cobalt was used, and high carbon steel was selected to be one containing 1% C (wt.). For the purpose of investigating the effect of cobalt addition, three alloys were prepared, such as Fe-1% C alloy, Fe-1% C-14% Co alloy, and Fe-1% C-23% Co alloy, by means of a high frequency induction furnace in air. The optimum austenitizing temperature for obtaining martensitic structure was 900℃ in Fe-1% C alloy and 1250℃ in the other alloys having cobalt. As-quenched strength of the alloy having no cobalt was lower than the strength in the two alloys having cobalt. The ultimate tensile strength of Fe-1%C steel was increased with increasing tempering temperature showing a maximum strength around 300℃, and then decreased with further tempering temperature. In the case of Fe-1%C-14%Co alloy, the strength was gradually increased until tempering of 400℃, and then decreased with tempering temperature above the 400℃. However, in the case of Fe-1%C-23%Co alloy, as-quenched strength was maintained nearly constant up to 450℃ and then decreased gradually with further tempering. The different change in strength for these three alloys during tempering can well be explained by the variation of Ms temperature due to cobalt addition. It was noted that the cobalt contained alloys showed little improvement in elongation during tempering. Thus, although the combination of strength and elongation at tempering temperatures below 200℃ or 300℃ was better in Fe-1%C steel than in the cobalt contained alloys, it was reverse on tempering at temperatures above the 300℃.

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Using the characteristic phenomenon of the strain-induced phase transformation (ε→α-martensite) in Fe-13% Mn steel, the mechanism of hydrogen embrittlement was studied. The differences in mechanical property and in the amount of the phase transformed with hydrogen-charged and uncharged specimens were investigated. When tensile stress is applied to the hydrogen charged specimens, cracks are formed on their surface layers as the stress reaches the yield point and result in brittle fracture. It is also investigated by the X-ray diffraction analysis that the charged hydrogen could make a phase transformation (ε→α-martensite) on the surface layers of the thin specimen without any applied stress. These experimental results may satisfactorily explain the hydrogen embrittlement phenomenon in terms of the planer pressure theory, which demonstrates that molecular hydrogen with high pressure accumulated in the lattice defects is the cause of hydrogen embrittlement.

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A rigorous method for obtaining isothermal phase boundaries in ternary systems has been presented when free energy of mixing values are known over the ternary field. The method is equivalent to the common tangent method to obtain phase boundaries in binary systems and is therefore rather conceptual than for the accurate determination of the phase boundaries. For demonstration the method has been used to obtain the ternary phase diagram of the Au-Ni-Cu system whose free energy values were calculated by making use of regular solution equations.

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