발간논문

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The atomic structures of RF magnetron sputtered amorphous Fe_(80)Tb_(20), Co_(75)Tb_(25) and Fe_(58)Co_(15)Tb_(27) films with 1㎛ thickness have been investigated by X-ray diffraction measurements in a low-angle geometry. Atomic number density functions of Fe-Tb and Co-Tb alloy systems show that the inter-atomic distance for the Co-Tb pair is 0.25Å shorter than that for the Fe-Tb pair but the structural difference in the topology between tho two alloy systems has not been observed. The local structures of amorphous Fe-Tb and Co-Tb systems are identified similar to those of the crystalline Fe_(23)Tb_6 and Co₃Tb, respectively. The local structure of amorphous Fe-Co-Tb system appears to be the weighted average structure of amorphous Fe-Tb and Co-Tb alloy systems according to the compositional ratio of Fe and Co atoms.

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A numerical methodology enabling thermal stress analysis in a two-dimensional solidification process has been developed and successfully applied to a vertical unidirectional solidification of pure aluminum in a confined rectangular mold. Although the boundary conditions of the vertical surface have a significant influence on the thermal stresses due to the formation of air-gaps at the solid-mold interface, stable numerical solutions are obtained by this numerical method. The maximum compressive stress was found to develop along the symmetry (center) surface, while the maximum tensile stress was generated at the outer surface close to the cold surface. Both of these stresses became smaller as the distance from the cold surface increased.

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The precipitation behavior in Al-2.28wt%Li-1.11wt%Cu-0.57wt%Mg-0.18wt%Zr alloy has been studied by using a high resolution electron microscope. In the alloy quenched from 530℃, the Ll₂-type microdomains with a few unit cells and regions having lattice modulation parallel to {100} planes are observed in an atomic resolution image. After an aging at 190℃ for 10min, the discrete δ´-ordered precipitates with 4 to 8㎚ in size are recognized in fcc disordered matrix. In the θ´/δ´complex precipitates, interface boundary on both faces of θ´ plate is atomically flat and coherent with surrounding δ´ phase and an antiphase relation is formed between the δ´ phases on both faces of θ´ plate. Nucleation sites of S´ phase are observed at not only δ´/matrix interface and subgrain boundary, but Cu and Mg clusters. In the β´/δ´ complex precipitates, β´ and δ´ phases are slightly misoriented with respect to each other and a lattice misfit of about 10㎚ width is partially observed in the interface boundary.

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The RF sputtered MoS₂/PTFE multilayer lubricant film has been studied to reduce the rapid oxidation tendency of sputtered MoS₂ film. The AES depth profile of MoS₂ film shows evenly distributed oxygen throughout the layer. However, by sputtering the PTFE as a top layer, the penetration of oxygen has been drastically suppressed. When the thickness of PTFE is about 500Å the friction coefficient of multilayer reduces from 0.34 to 0.15 in air. The optimum working pressure of argon and temperature of substrate are about 10m torr and 150℃, respectively, which show the largest grain size of MoS₂.

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The effect of dissolved carbon on the recrystallization behavior was studied in a low carbon steel by texture measurement and TEM observation. Two initial specimens having different amount of dissolved carbon atom in the ferrite matrix were cold rolled and recrystallized to compare the formation of recrystallization texture. The γ-fiber orientations in the recrystallization texture was explained by the preferred recrystallization nucleation and growth of {111}//ND oriented grains. The deformation was more inhomogeneous in the specimen with higher dissolved carbon atoms. The inhomogeneously deformed microstructure caused the random oriented recrystallization. Thus the orientation density along γ -fiber was Lower in the specimen with higher dissolved carbon.

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The effects of Al addition (0-3wt%) to Fe-30Mn-12Cr-xAl cryogenic alloys on tensile properties and microstructures were investigated within temperature ranged from -196℃ to 25℃. The addition of Al increased the stability of δ-ferrite and Fe-30Mn-12Cr-3Al alloy showed the phase transition from single γ phase to γ plus δ-ferrite. The Fe-30Mn-12Cr-0Al alloy showed the deformation induced ε-martensite in deformed structure at -196℃. The deformation twins were observed in deformed cryogenic alloys except Fe-30Mn-12Cr-2Al alloy, in which the stacking fault energy of γ phase exceed about 50mJ/㎡. The formation of deformation twins during the tensile test was found to be beneficial to enhance the elongation through multiple necking. Fe-30Mn-12Cr-(0, 1, 3)wt% alloys showing deformation twins exhibited the elongation peaks between -196℃ and 25℃. The elongation peak shifted to lower temperature with increased Al content, since the stacking fault energy increases with increasing Al content. The stacking fault energies of the cryogenic alloys at temperatures showing the elongation peaks were calculated to be about 35-40mJ/㎡. The gradual formation of deformation twin during the test was found to be important to maximize the elongation.

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The abnormally high terminal solubility of hydrogen (TSH) in Nb-Ta alloys at 250 K was previously explained by Oates and Flanagan using the concept of para- (or pseudo-) equilibrium. In this paper, in order to provide a more accurate explanation for the low temperature TSH and furthermore the phase diagram of Nb-Ta-H system, we recalculate the TSH of Nb-Ta alloys by applying the same method used previously but considering more experimental thermodynamic results available. The present calculation is found to give a better agreement between the experimental and calculated TSH results. Also the phase diagram obtained from the present calculation is well in accord with the detailed structural information, whilst that from the previous calculation is not. The present work emphasizes the need to use accurate thermodynamic data in model calculations.

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