발간논문

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Texture and deep drawability have been investigated in Al-Mn-Mg alloy by means of orientation distribution functions(ODFs) analysis. The effect of heat treatment and cold rolling on texture varied with the process variables. Intermediate annealing before final cold rolling led to the formation of cube texture and 0°/90°earing. While, the variation of cold rolling ratio can balance the proportion of the recrystallization texture to the deformation texture so that the overall anisotropy is minimized. In the specimens cold rolled directly after hot rolling, cold rolling had more influence on earing rate than intermediate annealing. On the contrary, intermediate annealing exerted more effect on earing rate than cold rolling in the specimens intermediate annealed directly after hot rolling.

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Two and three layer composite models were developed using a beam vibration theory and the models were applied for measuring the Young`s moduli of thin metallic films. Two-layer composite of Cr(or Ni, Co/Si(111)) and three-layer composite of (Cr/Ti/Si(111)) were prepared by RF magnetron sputtering and used to test the developed models. The Young`s moduli of (Cr)films obtained by three-layer composite model agree well with those of (Cr)films obtained by two-layer composite model considering (Ti/Si) as one layer and (Cr) as the other layer. This suggests that moduli of multilayer films may be obtained by using a two-layer composite model repeatedly.

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In this study, the influence of impurity elements Ca, P on the Sr modification treatment in Al-7wt%Si-0.3wt%Mg alloys was examined by the observation of microstructure, DSC analysis, tension test and fractographs. In the case of modification treatment by Sr 50ppm, the increase of Ca deteriorated eutectic modification rate and increased the primary and eutectic temperature. With the increase of Ca, ultimate tensile strength(UTS) and elongation were decreased and the fraction of intergranular fracture increased. The coexistence of Ca and P increased eutectic modification rate and depressed the eutectic temperature.

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Ni22Cr10Al1.0Y/ZrO₂-25CeO₂-2.5Y₂O₃ and Ni22Cr10Al1.0Y/ZrO₂-8Y₂O₃ duplex(bond+top) thermal barrier coatings were prepared using the air plasma spray process. Properties such as phase transformation in the ceramic top coating, bond coat oxidation and thermal barrier effect were investigated to compare ZrO₂-CeO₂-Y₂O₃ with ZrO₂-Y₂O₃ under the 1300℃ high temperature thermal cycles. In condition of as-sprayed, both coatings showed 7∼11% porosity fraction and typical lamellar structures formed by continuous wetting of liquid droplets. In the ZrO₂- CeO₂- Y₂O₃ coating, the phase ratio of tetragonal to cubic was 75:25, and ZrO₂-Y₂O₃ coating had 100% untransformable tetragonal phase. There was no monoclinic phase in both coatings. However, the phase ratio of coatings was changed after 1300℃ thermal cycles. In the ZrO₂-CeO₂-Y₂O₃ coating, the ratio of tetragonal to cubic was changed to 88:12 and monoclinic phase was not detected, but in the ZrO₂-Y₂O₃ coating 10∼19% monoclinic phase had formed. The life of coatings was found to be strongly dependent upon the temperature of the bond coat experienced during exposure to peak temperature of 1300℃. When the bond coat experienced a temperature higher than 1100℃, the campaign life of bath TBC decreased drastically and this was related to the oxidation behaviour. Al₂O₃ was formed preferentially along the bond and top coat interface and the other oxides such as NiO and Ni(Cr,Al)₂O₄ spinel, which were believed to decrease the coating life by oxide growth stress, were formed rapidly at the top coat side of interface at temperature higher than 1100℃. The ZrO₂-CeO₂-Y₂O₃ coating was superior to ZrO₂-Y₂O₃ coating because there was no phase transformation from tetragonal to monoclinic(which induces volume expansion) and the effect of oxide growth stress was relatively small by better thermal insulation.(i.e, bond coat experienced a temperature lower than 1100℃ for a given TBC thickness)

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Under deposition conditions limited by gas transport process through a gas boundary layer to the substrate, the etching effect of Cl/H input ratio on the chemical vapor deposition rate of silicon on molybdenum substrate was investigated in the temperature range of 1173K and 1473K using hot-wall horizontal reactor and a gas mixture of SiCl₄ and H₂. The deposition amount of silicon increased with increasing Cl/H input ratio but decreased at higher Cl/H input ratio and finally attained a constant value. At 1173K, the deposition rate of silicon obeyed linear law at low Cl/H input ratio in which the outer layer of molybdenum substrate was pure silicon but parabolic law at high Cl/H input ratio in which that was MoSi₂. At 1473K, however, the deposition rate of silicon obeyed parabolic law irrespective of Cl/H input ratio in which the outer layer of molybdenum substrate was MoSi₂. The etching effect of Cl/H input ratio on the deposition rate of silicon were explained by thermodynamic calculations in which the variation of silicon activity on substrate surface induced from the difference of silicon flux between gas transport process and solid diffusion process was considered.

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The purpose of this study is to investigate the effect of carbon content on the formation of nitride layer onto the plasma nitrided Cr-Mo Steels. Plasma ion-nitriding was performed for SCM415, SCM420 and SCM440 at temperature ranges from 525℃ to 625℃, for 1,4,9 and 16hours, in the atmosphere of 80% N₂ and 20% H₂ gases in vol.%. In case of plasma ion-nitrocarburising, CH₄ gas of 1 and 3 vol.% was mixed with N₂ gas. Optical microscope, scanning electron microscope, X-ray diffractometer, Auger electron spectroscope and micro-vickers hardness tester were used to examine the characteristics of nitrided layer. It was found that grain boundary diffusion process played an important role in the rapid growth of compound layer at the early stage of ion-nitriding reaction. In plasma ion-nitriding, the compound layer of SCM440 was thicker than those of SCM415 and SCM420. However, the thickness of compound layer was decreased for all specimens in plasma ion-nitrocarburising. The amount of micropores in the compound layer of the nitrocarburised specimen was found to be quite diminished and the case depth was decreased with increasing carbon content in both the matrix and the carrier gas.

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TiO₂ films deposited on glass were prepared by plasma sputtering method. The dependence of structural properties on the deposition conditions and the photocatalytic degradation of methylene blue were investigated. When the oxygen partial pressure was higher than 5%, as-deposited TiO₂ films exhibited an almost amorphous structure under DC plasma condition. A transition at which metallic mode deposition(Ti) changed to dielectric mode deposition(TiO₂), was observed at the 5∼10% oxygen partial pressure under RF plasma condition. The amorphous structure changed to anatase structure by heat treatment at 400℃ for 4 hours. Crystallized anatase structure had a preferred orientation along 〔101〕 direction. For 150 min of photocatalytic degradation, the concentration of methylene blue was reduced by half.

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The effect of Si addition on the scale adhesion of 19Cr-Ti ferritic stainless steel, which is used for stud pin as a sealing material for cathode ray tube, has been studied by using press compression test, visual inspection, optical micrograph and electronic microanalysis. A spalling behavior of scale formed on the surface by oxidation treatment at 1100℃ for 30∼40 minutes in 75%H₂+25%N₂ gas mixtures was mainly governed by the morphology of SiO₂ layer formed at scale/metal interface. As Si content exceeded more than 0.5%, a continuous silica interlayer was produced and then the adhesion of scale which is mainly 2∼4㎛ thickness of Cr-oxide layer was abruptly decreased. It is due to a low fracture toughness of SiO₂ formed at scale/metal interface and a generation of residual stress at scale/metal interface during cooling after oxidation treatment. The residual stress might originate from a great discrepancy in thermal expansion coefficient between matrix and silica.

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Zr-Cu binary alloys for nuclear fuel cladding were prepared by changing Cu content from 0.1wt.% to 0.8wt.% to investigate the effect of Cu on the corrosion behavior in water at 360℃ under a pressure of 2660 psi. Specimens containing 0.1∼0.5wt.%Cu had the black and uniform oxide film and the transition of corrosion rate didn`t occur during 50 day corrosion test. However, 0.8wt.%Cu specimen had the white oxide film and the transition of corrosion rate occurred at the 10 day corrosion test. The weight gain in pre-transition regime generally decreased with the decreasing Cu content from 0.8 to 0.2wt.% though 0.1wt.%Cu specimen showed higher weight gain than 0.2wt.%Cu specimen. Cu content had no effect on changing the oxide structure formed in pre-transition regime but decreased electrical resistivity of the oxide due to short-circuit effect of precipitate. The fraction of precipitates in the Zr-Cu binary alloys, identified as tetragonal Zr₂Cu increased with increasing Cu content. Increase in the fraction of precipitates is thought to be responsible for the increase in weight gain.

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Fe₂O₃-TiO₂artificial passive films were formed at formation temperatures of 200∼300℃ by low pressure MOCVD technique. The relationship between formation temperature and corrosion of the Fe₂O₃-TiO₂films was examined in acid solutions. Fe₂O₃-TiO₂ films deposited above 300℃ were crystalline and the films deposited below 250℃ were amorphous. The dissolution rate of the Fe₂O₃-TiO₂films deposited at 200 and 250℃ decreased with increasing the formation temperature in 5 M-HCl, but the films deposited at 300 and 350℃ hardly dissolved in the solution. The films deposited at 200 and 250℃ easily dissolved at cathodic potentials in 1 M H₂SO₄ and HCl owing to the selective reduction of iron oxide components in the films. But, the selective reduction in the films deposited at 300 and 350℃ was suppressed. The Fe₂O₃-TiO₂ films deposited above 300℃ have higher corrosion resistance than that of the films deposited below 250℃, because the former have high crystallinity and contain high chemical resistant oxides.

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