발간논문

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The effect of the initial grain size on grain growth and texture evolution was studied by the Monte-Carlo simulation with a condition of isotropic grain boundary energy and mobility. The initial microstructure was composed of six different texture components which were classified by the grain size. It was shown that the texture component of smaller grains and/or grains with less number of sides than those of the average grains disappeared at the early stage of growth regardless of its initial areal fraction. Shrinking of the small grains reduced drastically the total energy of the system but it did not affect the grain growth kinetics. The texture components possessed by the larger grains sharpened continuously during growth with their intensity proportional to the areal fraction.

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In an attempt to control the defect morphology and density of the ZMR-SOI thin film, the temperature distribution across the melting zone was modified by a dual heat source consisting of the two tungsten-halogen lamps focused by a dual elliptical mirror. The intensity distribution of the focused beam was calculated to estimate the asymmetry of the temperature distribution. The intensity gradient at the solidifying interface became less steep when the right lamp power increased and left lamp power decreased keeping the sum of the each lamp power constant. On this condition, the defect spacing increased to the critical point and decreased dramatically, while the spacing decreased when the left lamp power exceeded the right. A model was proposed to explain the change of defect morphology and spacing.

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Halogen lamp ZMR(Zone-Melting-Recrystallization) system was examined to evaluate the radial temperature distribution on the surface of a Si wafer. To measure the temperature change on the surface, PR thermocouples were directly contacted at the various points on a wafer surface. It was proved experimentally that the thermal distribution could be approximated as a Gaussian or a cosine function. Uniform heating of the entire wafer surface was difficult due to the heat conduction from the wafer circumference to ambient without any heat absorber beneath the wafer. Heat transfer mode was changed abruptly at about 400℃ and the hot zone was formed on the surface.

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Experiments for the combustion of coke pellet mixed with lime and limestone had been performed to investigate the combustion kinetics and NO and SO₂ emission behavior between 700℃ and 1300℃ under pure oxygen atmosphere. Intrinsic rate constants for the combustion of C, N and S were obtained from thermogravimetry and gas analysis data. The amounts of SO₂ reduced by lime were nearly 99% below 1100℃ and decreased to about 75% at 1300℃. Limestone as a sorbent for SO₂ was found to be superior to lime because the calcined lime was more active in reaction with SO₂ than lime. Pelletizing coke with lime/limestone had negligible effect on reducing the amounts of NO during combustion of coke pellet in the experimental ranges. The combustion kinetics of coke pellet mixed with lime/limestone was nearly same as that of coke in pellet made from pure coke.

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Solvent extraction and stripping experiments had been performed to separate indium and gallium in sulfuric acid solutions. In extraction, nearly 99% of indium in the mixed solutions could be extracted to the organic phase with the stoichiometric concentration of D₂EHPA in the experimental ranges. It was found that kerosene as a diluent was superior to benzene in respect of loading capacity. In stripping experiments of the loaded organic phase with acid solutions, the stripping percentage of indium decreased from 95% to 5% with pH, while separation factors increased with the pH of the stripping solutions. From simulation experiments for the multistage counter-current extraction, the possibility of nearly perfect separation of indium and gallium was obtained by a two-stage extraction of aqueous feed with a solvent in which the concentration of D₂EHPA was maintained at the stoichiometric ratio to the concentration of indium.

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Microstructural modelling for prediction of the grain size of steels were reviewed and the approaches to obtain a fine grain structure were discussed based on the underlying physical metallurgy and the availability of various process control parameters. The control parameters which determine the ferrite grain size are the initial austenite grain diameter, the amount of retained deformation, the type of the nucleation site, the cooling rate and the transformation temperature. The ferrite grain size can be expressed in terms of the nucleation and the growth rates, but an exact evaluation of it seems to be difficult due to uncertainties involved in determining various physical factors, such as surface energy, diffusivity etc. Therefore, various regression equations are used for the grain size prediction. Large deformation and rapid cooling appears to be effective for grain refining. Attempts are made to estimate the obtainable grain size under a given processing condition. In addition, the effectiveness of the magnetic force are also discussed.

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Recent studies show intermetallics with 5∼20 ㎚ grains show improved physical, chemical and mechanical properties. For example, intermetallics can have some amount of ductility due to ㎚ scale grains and increased portion of grainboundaries. The nanocrystalline intermetallics can be fabricated by using various techniques such as gas condensation, rapid solidification and mechanical attrition. The nanocrystalline intermetallics also have good thermal stability which is one of the important factors for successful consolidation, thus providing high performance bulk materials. Nanoscale crystallization and decomposition behavior of intermetallics have been reviewed with special emphasis on nanocrystalline FeAl, NiTi and NiAl intermetallics.

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The effect of compact structures on the phase transition and subsequent microstructure evolution during sintering of ultrafine powder compacts has been investigated. The ultrafine ceramic powders having a capillary-induced high pressure phase, n-TiO₂ powder of anatase phase and the n-Al₂O₃ powder of gamma phase, were tested. The compact density was varied by applying different compaction pressures either 65_(㎫) or 4.5_(㎬). The compacts fabricated at the low pressure, called MC, had about 60% of theoretical density (TD) and the compacts consolidated at the high pressure, called GC, had about 80% TD, irrespective of powder systems. The shrinkage behavior of compacts was affected by compaction density. The low density MC compacts exhibit two inflection temperatures, whereas the relatively high density GC compacts exhibit one inflection temperature, in the shrinkage curve obtained by uniform heating. The shrinkage curve of the MC is separated into two parts: one comes from the capillarity effect and the other comes from the effect of phase transition on the microstructure evolution. The volume contraction due. to anatase→rutile TiO₂ and γ→α-Al₂O₃ phase transition was proved not to contribute to the whole compact shrinkage. The microstructures resulting from phase transition in the loose MC compacts retarded significantly the subsequent densification from the onset of phase transition. The effect of phase transition on the subsequent densification in the sintering of n-Al₂O₃ compacts seems to be of great detriment more than that in the sintering of n-TiO₂. In spite of detrimental effect of phase transition on the subsequent densification, high density compaction is proved to be beneficial to complete densification and fine sintered microstructure evolution.

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Nitrided Fe-3%Si alloys were secondary recrystallized by intermittent annealing without MgO coating (Model anneal)and by annealing with MgO coating(Box anneal). the optimum grain size and nitrogen content for the maximum B8 in both annealing methods were quite different. However, the maximum B8 obtained was the same(B8=1.94) in both annealing methods. The B8 after Model anneal was closely associated with the onset temperature of secondary recrystallization (T_(cr)). The maximum B8 was obtained when T_(cr) was about 1075℃ and T_(cr) increased with increasing initial grain size or initial nitrogen content. These findings were discussed utilizing recent information on the migration characteristics of Σ9 boundaries derived from an Fe-3% Si bicrystal. It was concluded that Σ9 boundaries determine the sharpness of Goss texture evolution and the future possibility of improving production technology of grain-oriented silicon steel was demonstrated based on this finding.

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The main objective of this study is to investigate the microstructural evolution during mechanical milling of prealloyed (Ti_(52)Al_(48))_(100-x)B_x powders and also to investigate the effects of B contents and heat treatment on the microstructure of mechanically milled TiAl alloys. Microstructure of binary Ti_(52)Al_(48) Powders consists of grains of hexagonal phase whose structure is very close to Ti₂Al. (Ti_(52)Al_(48))_(95)B_5 powders contain TiB₂ in addition to matrix grains of hexagonal phase. The grain sizes in the as-milled powders of both alloys are nanocrystalline. The mechanically alloyed powders were consolidated by vacuum hot pressing (VHP) at 1000℃ for 2 hours, resulting in a material which is fully dense. Microstructure of consolidated binary alloy consists of γ-TiAl phase with dispersions of Ti₂AlN and A1₂O₃ phases located along the grain boundaries. Binary alloy shows a significant coarsening in grain and dispersoid sizes. On the other hand, microstructure of B containing alloy consists of γ-TiAl grains with fine dispersions of TiB₂ within the grains and shows the minimal coarsening during annealing.

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