발간논문

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In this study, three alloys with nominal compositions(at%) of Ti48Al, TiAl-1V, TiAl-2V were prepared by plasma are melting and quenched from α₂＋γ, α＋γ and α phase regions, respectively. V additions to TiAl enhanced the plasticity of the alloy, and maximum plasticity in a compressive test occurred in the TiAl-2V alloy quenched from α＋γ phase regions. This is the reason that V additions to TiAl tend to refine grain size and increase twinning deformation. The peak stress of the alloys shows the same tendency as the strain. Vickers hardness of the alloys heat treated in the α phase region is, however, higher than the one in the α₂＋γ, α＋γ phase regions, and the reason is that α₂phase of the alloys heat treated in the α phase region is more than the one in the other regions. Therefore maximum hardness occurs at the TiAl-1V alloy heat treated in the α phase region.

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Dynamic features of dephosphorization of Cr containing steels by the oxidizing fluxes consisting of BaO, CaO and halides, are examined to determine possibility of efficient dephosphorization employing a laboratory-scale induction furnace. These oxidizing fluxes can dephosphorize steels more than 50% within refining times less than 10 minutes. In addition, these fluxes simultaneously show a very fast rate of desulfurization with a degree of desulfurization higher than 95%. The refining rate of dephosphorization was found to be controlled by liquid phase mass transfer. The results also show that a degree of dephosphorization increases as decreasing both initial Cr contents in steels and refining temperatures. The initial presence of Cr₂O₃in fluxes diminishes the amount of Cr loss in steels. The higher Cr₂O₃content within it`s solubility in a slag, the higher degree of dephosphorization. The highest degree of dephosphorization was obtained at slag compositions with high basicity and good fluidity.

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We have investigated the effect of heat treatment on tensile properties of SiC_p/Al composites. Heat treatment were carried out at aging of 200℃ upto 30hrs after solid solution treated at 500℃. After heat-treatment, the specific modulus of the composites was observed to be increased as the volume fraction of SiC particle increases. The specific modulus values of composites with 30 vol% SiC particle exhibited the increase portion of 15%, in comparison with the as-fabricated composites, increasing to 127 ㎬ at 30 vol% SiC_p/2024Al composites. Therefore, it is noted that the elastic modulus is a function of the amount of SiC particle. On the other hand, the specific strength value of 10 vol% SiC composites demonstrated the great increase portion of 50% compared with composites with 20 vol% and 30 vol% SiC particle. The specific strength of SiC_p/2024 Al composites was found to be greatly influenced by 2024 Al matrix.

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A Pure and doped Mo wire were fabricated by direct current sintering and swaging. The specimens were annealed at various temperature and investigated mechanical properties, recrystallization temperature and bubble formation mechanism by EDAX. The bubble rows which were existed in doped Mo wire enhanced the mechanical properties and recrystallization temperature. It was considered that complex oxide of Al, K and Si components was contributed to the bubble formation.

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Laboratory experiments were performed to investigate the effect of alloying elements on the reverse transformation behaviour from martensite(α`) to austenite(γ). The amount of deformation induced martensite was increased with decreasing the carbon content and increasing carbide precipitates. The mechanism of the austenite reversion was changed from the shear transformation to the diffusional transformation with increasing the carbon content in Fe-16% Cr-10% Ni alloys. The age hardening was observed during reversion heat treatment in the Ti-added Fe-Cr-Ni steel. Abnormally coarse grain structure was observed in the steels with high austenite stability which contained both strain induced martensite and retained austenite. The grain structure was formed due to the strain induced grain boundary migration.

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The microstructural alteration of α phase by various solution treatment and cooling conditions have been studied for Ti-6Al-4V alloy. Edge shape α phases were formed when it was the first solution treated at β region and the second solution treated at 960℃ or 930℃, followed by water quenching. Floral pattern structures were formed when it was the first solution treated at β region and the second solution treated at 960℃ or 930℃, followed by air cooling. When the specimen was the second solu-tion treated at 960℃ or 930℃, and furnace cooled, edge shape α phase changed its morphology to equiaxed structure. The second solution treatment at 900℃, followed by water quenching, had fine and homogene-ous mixed structures of equiaxed and widmanstatten structure with retained β phase as martensite. This is considered to be optimum microstructure to improve mechanical properties.

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An on-line computer system was developed for prediction of tensile properties of C-Mn steels manufactured in the hot strip mill. To predict the final microstructure and properties in plain carbon steel hot coils, a mathematical model based on physical metallurgy was developed. Computer modelling was carried out to describe the microstrucal evolution during hot rolling and cooling and the interrelationships existing between microstructure and mechanical properties. The mathematical model consists of three sub-models : austenite conditioning model, transformation model and mechanical property model. The austenite conditioning model deals with recovery, recrystallization and grain growth which occur during hot rolling. The allotropic phase transformtaiton from austenite to ferrite, pearilte and bainite are described by the transformation model. The final mechanical properties are determined from the structure-property relationships established in the mechanical property model. An Avrami-type kinetic equation was used for mathematical representation of recrystallization and austenite/ferrite transformation. The structure-property relationship was formulated by combining the Hall-Petch equation with the mixture rule. In order to apply the model to the practical hot rolling process, an on-line computer system was installed at a hot strip mill. The on-line system consists of a high speed process computer and software packages. The predicted values of tensile strength from the system were in reasonable agreement with the measured ones when hot strips were processed under various rolling conditions.

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Al-8.5Pb-1.5Sn-4Si-(1∼5)Cu powders were manufactured using centrifugal atomizer. The powders were cylindrical shape and showed bimodal size distribution. These bearing powders with pure Al powders at both outer side were cold rolled using rolling machine. The strip porduced showed enough strength for further handling when the reduction ratio exceeds 70%. The strips were sintered at 500℃ under Ar atmosphere to reduce the porosity. Mild steel strip, which is used as a back-up plate in sheetbearing, was surface treated in HNO₃- Ethylene alcohol solution to produce proper surface condition for cold bonding. The bonding between mild steel strip and powder-rolled strip were performed using a rolling machine. In this case, 20∼25% of reduction ratio was sufficient to form a sound bonding.

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The microstruture of the solution-treated and subsequently quenched Fe-10Cr-10Ni-2W maraging steel consists of fully lath martensite with high dislocation density. The orientation relationship between adjacent laths in maraging steel has been studied using transmission electron microscopy. A twin relationship between some adjacent martensitic laths is observed. It is identified that the twinned laths are transformed from the austenite of two specific variants with a Kurdjumov-Sachs orientation relationship.

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The mechanical properties of the Al/Al₂O₃composite materials prepared by oxide dispersion strengthening were investigated. The tensile strength and hardness were increased with increasing the weight percent of Cu/Al₂O₃composite powder of the Al melt up to a constant concentration. In the case of Vol. ratio of Al₂O₃Sol./Cu(NO₃)₂, 1:1, the tensile strength exhibited a maximum at 0.39 Vol.% of Cu/ Al₂O₃addition. However the tensile strength was decreased above 0.39 Vol.% of Cu/Al₂O₃addition, On the other hand, the elongation was decreased with increasing the weight percent of Cu/Al₂O₃addition to the Al melt. It was also revealed that the tensile strength of the materials was not influenced by the melting temperature but was influenced mainly by the volume ratio of Al₂O₃Sol./Cu(NO₃)₂, while the effect of melting temperature was quite small.

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