발간논문

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The behavior of black oxide layer formation on the high speed steel(HSS) roll materials for hot strip mill has been studied. High temperature oxidation experiments were performed at both dry and wet atmospheres. Surface oxide scale was preferentially formed on the matrix and grew laterally to the top of carbides with oxidation time. Spalling of the oxide scale initiated at the interface between external oxide layer and carbide. Ti-treated specimen showed an increased weight gain during oxidation. This can be attributed to the fact that Ti treatment results in finer crystallized carbides, leading to the increment of interface area between carbides and matrix, thereby promoting break away oxidation. Under the dynamic wet oxidation atmosphere, growth of oxide scale at scale/air interface was restricted physically by the applied force, whereas the growth at scale/metal interface progressed independently.

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The photoluminescence characteristics of lithium and trivalent rare-earth ions(Ce^(3+), Dy^(3+), Tm^(3+)) doped ZnGa₂O₄ with solid state reaction methods were studied. Under 254 ㎚ excitation, the absorption band at 245 ㎚ and the emission band at 380 ㎚ in reduced ZnGa₂O₄ phosphor were observed, which were originated from ⁴T₂→⁴A₂ transition of the octahedral Ga^(3+) ions in the spinel lattice. The incorporation of lithium and trivalent rare-earth ions into the ZnGa₂O₄ lattices results in the emission peak shift of nearly 50 ㎚ towards long wavelength region and improving the emission intensity and color purity.

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Cementation reaction was performed in 1∼1.5 wt% HCl solution using pure magnesium and aluminum plate as the reductant. Sponge lead prepared by cementation was oxidized at 250, 300 and 350℃ and then oxidized sponge lead was pulverized by ball milling for 1, 3 and 5 hrs. Therefore, The aim of this work is to establish the optimum condition of oxidation and ball milling of sponge lead which cementation leady oxide has best physicochemical characteristics. Utilization of active material prepared by cementation oxide was about 72% which this is higher than ball mill oxide.

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The heat affected zone of a welded STS304 steel has been irradiated in an 1300 keV high voltage electron microscope at 673K and up to 5.4 dpa (displacements per atom) to study the effect of electron-beam irradiation on irradiation damage microstructure. The dislocation loop density of initial irradiation state was increased with electron-beam irradiation. Void size, void number density and void swelling percentage were increased and then saturated gradually with irradiation dose. The depletion of Cr and the enrichment of Ni were also recognized in HAZ part of welded STS304 steel at the grain boundary by EDS analysis, as reported already the same tendency for matrix material.

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This study was carried out to investigate the effects of alloying elements and vertical centrifugal casting(V.C.C.) conditions on microstructural behavior and mechanical properties, including hardness and wear characteristics in high speed steel. The hardness and wear test were used to evaluate the mechanical property of high speed steel manufactured by both gravity casting and V.C.C. With the increase in V element addition, the hardness increased. It was found that the amount of MC carbide was increased and M_7C₃ carbide were decreased with the increase in V and Nb content, The cell and carbides became finer and the amount of M_7C₃ carbide was decreased, according to the increase in revolution speed due to increase cooling rate.

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Deformation behaviors of Al-MgB₄C and Al-Mg/TiC composites fabricated by pressureless infiltration technique were studied in the temperature range between 298K and 773K. The strength decreased similarly for Al-MgB₄C and Al-Mg/TiC composites, but the ductility of Al-Mg/B₄C was higher and reached to 150% at 773K. The stress exponents and the activation energy for high temperature deformation were observed to be 7∼8 and 180∼220 KJ/mole respectively. The relatively high stress exponent and high activation energy can be explained in terms of dynamic solution-dislocation interaction in the matrix of Al-Mg composites. The activation volume for high temperature deformation of Al-Mg matrix composites increased from 50-80b³ at 573K to 200-800b³ at 773K. The rate controlling mechanism of the deformation for Al-Mg matrix composites was well explained by the dislocation trough model in which the segregation of alloying elements on dislocation determines the activation length.

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The effect of surface active elements an reaction mechanism of nitrogen absorption of iron melts were investigated in the iron melting process using Ar plasma arc. Because change of rate constant according to increasing Ar gas flowrate is very small, mass transfer in gas and liquid phase is not affect to kinetics of nitrogen absorption of the melt. Cr in melt increases reaction rate of nitrogen absorption by decreasing activity of nitrogen in melt. With increasing the content of surface active elements in the melts, the solubility and reaction rate of nitrogen absorption for both melts have increased and this phenomenon is also shown similarly in alloy. Therefore, the rate controlling step for nitrogen absorption is the dissolution reaction step into the iron melts of adsorbed nitrogen atom in chemical reaction of the interface between gas and melts phase.

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