발간논문

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The effect of inert fillers and Si contents on pack siliconizing kinetics of pure molybdenum was investigated in the range of 1273K-1473K. Si contents were varied from 1wt%Si to 40wt.%Si. Al₂O₃, and SiC powders were used as inert fillers. Similar coating microstructures were obtained and the growth rate of MoSi₂ coatings was observed to be proportional to the square root of time regardless of whether inert filler was Al₂O₃, or SiC. The dependence of parabolic growth rate constant of MoSi₂ coatings on the square root of Si contents was changed at the standard of 10wt.%Si. Apparent activation energy for the growth of MoSi₂ coatings was obtained to be 193 KJ/mole. Under the quasi steady state assumption, if the transport rate of Si containing gases in the pack is varied with Si contents and inert fillers, the surface composition of coatings tends to shift to another value which will re-satisfy the requirement of zero Si transport rate difference between gas diffusion and solid diffusion on the coating surface Therefore, pack siliconizing kinetics of pure molybdenum was controlled by a self-adjusted mechanism which combined the Si transport capability of gas diffusion in the pack and solid diffusion in the Mo substrate.

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A method for "Al+Y codeposition" was developed to improve the resistance of high temperatrue oxidation of TiAl using a single process of EB-PVD. Isothermal and cyclic oxidation tests at 950℃ showed that the Al+Y codeposition improved the oxidation resistance of TiAl significantly. During oxidation, the oxides formed in the Al+y codeposition layer are composed of two parts ; the outer (Y,Al)O layer and the inner Al₂O₃, layers containing small amount of Y-rich oxide. Both the outer (Y,Al)O and inner Al₂O₃ layers have a fine-grained structure which prevents effectively inward diffusion of oxygen and consequently prohibits formation of porous TiO₂. During cyclic oxidation, these (Y,Al)O and Al₂O₃, layers relieve easily both the growth and thermal stresses by permitting easy plastic deformation. However, when exposed to high temperature for an extended period of time, an Al₂O₃, layer having a columnar structure is developed underneath the existing fine-grained Al₂O₃, layer by reaction with Al from the substrate TiAl. This columnar Al₂O₃, is responsible for degradation of the oxide layer.

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The possibility to obtain high density ultrafine W/Cu composite alloy with ultrafine microstructure without addition of sintering activator by chemical processing method using metallic salts as starting material is investigated in this study. Nanostructure W/Cu composite powder which is loosely agglomerated homogeneous clusters of nanoscale size W and Cu particles can be obtained by thermochemical process. Thermochemically synthesized W-/Cu-metal composite powder passed through milling process shows higher sinterability than un-milled powder. It knows that combination of thermochemical process and mechanical process was useful method to obtain high density W/Cu composite alloy with ultrafine microstructure. W-/Cu-metal composite powder reduced by the multi-step reduction process of mechanochemically synthesized W-/Cu-oxide composite powder shows higher sinterability than reduced powder by the one-step reduction process. W-20wt.%Cu composite alloy has average W particle size 1㎛ and relative density 98% can be obtained without addition of sintering activator by combination of mechanochemical process and liquid phase sintering method.

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Milling media of steel and partially stabilized zirconia(PSZ) were used to produce Mo_5Si₃ by mechanical alloying(MA) of Mo-37.5 at%Si powder mixture. The effect of milling medium materials on MA of the powder mixture has been investigated by SEM, XRD, DTA and in-situ thermal analysis. The homogeneity of the powder mixture, the reaction rate and the end-product markedly depended upon the milling medium materials. MA of the powder mixture by steel ball-milling produced the homogeneous structure faster than by PSZ ball-milling. The formation of Mo_5Si₃ by PSZ ball-milling occurred after 15hr of MA and was characterized by a slow reaction rate as Mo, Si and Mo_5Si₃, coexisted for a long period of milling time. The formation of any new phase by steel ball-milling, however, did not occur even after 96hr of MA. DTA and annealing results demonstrated that Mo and Si were transformed into Mo_5Si₃, phase after heating the powder specimen steel ball-milled for 24hr to 1040℃ Mo and Si were transformed into Mo5Si₃ and a little Mo_5Si₃ after heating the powder specimen PSZ ball-milled for 12hr to 1040℃. The formation of Mo_5Si₃ by mechanical alloying of the powder mixture and the reaction rate seemed to depend upon the milling medium materials (steel and PSZ) as well as the thermodynamic properties of Mo_5Si₃, compound.

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The microstructural evolution and sheet resistance of copper-metallized alumina substrate have been investigated in terms of calcination temperature and hydrogen reduction temperature. The paste was made by mixing Cu₂O powder with binder. The alumina substrates were screen-printed with Cu₂O paste to 50㎛ thickness and heated at the temperature range from 1050℃ to 1200℃ for 30min in air atmosphere. The hydrogen reduction was performed at the temperature range from 200℃ to 800℃ for 30min. The calcined surface was densified at the temperature range from 1150℃ to 1200℃ due to the full melting of copper oxide powders. It was observed that the morphology of hydrogen reduced surface was more porous than that of calcined surface. Also, it was found that the morphology of hydrogen reduced surface depended upon calcination temperature. The sheet resistance value of copper-metatlized alumina substrate was varied with calcination temperature and hydrogen reduction temperature

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We have studied the hardfacing valve seat to improve the resistance against failure, using an austenitic heat resisting steel STR35 precipitation-hardened by (Cr·Fe)_(23)C_6 carbides. Failure of the hardfacing valve seat was due to stress concentration at the vicinity of the interface between STR35 and stellite layer. It was thus necessary to avoid weakening the matrix through the valve forming process. In the conventional forming process of the valve head, the grain size of STR35 was about 20-50㎛ but increased remarkably up to 60-80㎛ at the vicinity of the interface region overheated for overlaying stellite. The post welding hardness of the reaction interface depended on the heat treatment conditions prior to welding. The hardness value of the solution treated specimen was very low, HRc 3-5 (matrix HRc 35), and this was associated with the solute-dilution during welding and makes them more susceptible to failure. The danger of wear and failure of the valve seat face could be minimized by reducing the solutedilution rate to a low value with plasma welding method and by producing very stable carbides with annealing below 700℃ prior to welding to prevent the intergranular corrosion and by tempering at 760℃ for 60 minutes after welding to improve: the hardness of substrate STR35.

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A numerical analysis of heat transfer and solidification phenomena in vertical-type continuous casting for cladding process(CCC process), which can produce a composite hot working roll of Fe-Cr-V-C and Fe-C alloy, was carried out. A commercial program, FLUENT, was used to analyze the heat transfer coefficients between molds, and then used to optimize the pouring temperature and casting speed on solidification phenimena. By increasing the heat transfer coefficient between molds, the size of mushy zone reduced and the starting position of solidification moved upward. Increase of the pouring temperature moved the starting position of solidification downward, while mushy zone remains constant. With increasing average casting speed, the size of mushy zone increased as well and the starting position of solidification moved downward. The decrease of ratio of the holding to withdrawal time decreased the temperature variation during casting.

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Corrosion behavior of austenitic stainless steel containing Ti has been studied by using electrochemical techniques. The samples containing Ti from 0.1 to 1.0 wt% were solutionized at 1050℃ for lhr and then sensitized at 650℃ for 5hr under argon atmosphere. Microstructure and phase analysis of the samples after heat treatment and corrosion tests were carried out by using XRD, TEM, SEM and optical microscope. The amount of a-ferrite and TiC precipitates in matrix increased as the Ti content increased. In the sensitized samples, Cr_(23),C_6 precipitates were observed at γ/δ interface. Degree of sensitization(DOS) was lower than 1.0 in all of the solutionized samples and the sensitized samples of Ti content above 0.4 wt% whereas the sensitized samples of Ti content lower than 0.4 wt% showed DOS higher than 1.0. Intergranular attack appeared mainly at grain boundaries in the sensitized sample containing 0.1 wt% Ti and at the γ/δ interface of the higher Ti content. In the latter, however, the attack was not so severe. Pitting potential(E_(put)) and repassivation potential(E_(tep)) of the solutionized and sensitized samples were increased with increasing Ti content. The number and size of the pits decreased with increasing Ti content in the sensitized samples. The pits nucleated at Cr_(23)C_6 site and the γ/δ interface.

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