발간논문

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Recrystaltization at the surface of specimen was observed during heat treatment due to lathe machining or shot peening. In order to investigate the effect of surface recrystallization on creep properties, creep test was conducted at 1050℃ under a stress of 123MPa. Creep rupture lives were compared among DS specimen without surface recrystallized layer, specimens with surface recrystallized layer of 80∼100㎛ and 160-180㎛. Creep rupture time of DS specimen without surface recrystallized layer was longer than DS specimen with surface recrystallized layer (160-180㎛), which was reduced more than the specimen with surface recrystallized layer(80-100㎛). Microcracks were formed around γ´ denuded zone.

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DC-arc furnaces are used as new smelting units for the production of Fe and ferro-alloys. In order to obtain a better understanding of electrophoretic phenomena occurring in the slag phase during the smelting process, the electrophoresis data for the TiO₂ rich slag are examined. The discussion is extended to the determination of metal drop velocity in the slag phase. Results including reduction results obtained in the titanium stags and Cu-secondary stags are presented. The effect of polarity change on the reduction degree of stags in the arc furnace is discussed in theoretical and experimental viewpoints.

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Raipidly solidified Al-Ti-Si and Al-Ti-Si-Cr base alloys were prepared by melt spinning process. The effects of solute on the microstructural change and age hardening after heat treatment have been studied to improve the high temperature properties in the Al alloys. In the supersaturated Al-Ti-Si alloy, age hardening occurred after lhr aging in the range of 400℃∼500℃, which seems to be attributed to the precipitation of metastable L1₂-(Al, Si)₃Ti phase. And the addition of Cr to Al-Ti-Si alloy system affected the precipitate morphology by forming (Al, Si)₃Ti containing Cr to stabilize the microstrucure at elevated temperature. As a result, it is found that there is a strong correlation between the microstructure and the precipitation hardening for the aged alloys.

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Energetistic consideration of shape memory alloys(SMAs) is carried out in this work. ΔW_(cri), a criterion of energy, is developed to characterize and evaluate intrinsic property of a SMA. Through an experiment with Mn-Cu SMA, ΔW_(cir) is measured for a spiral and a spring and their efficiency is discussed by calculating their geometry factors.

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Poly-silicon thin films were crystallized by ZMR(Zone Melting Recrystallization) using a halogen lamp as a heat source and the effects of intensity of the radiative heat source and the substrate heating temperature on the solidification behavior were investigated through computer simulation and experimental observation. It has been found that the solidifying front becomes stable with increase of intensity of the radiative heat source and substrate heating temperature, which is consistent with computer simulation results. According to computer simulations, degree of supercooling is mainly responsible for the solidifying front instability and dendritic and cellular growth which occured when the solidifying front was unstable disappeared with increase of intensity of the radiative heat source and substrate heating temperature and random defects in recrystallized thin film was observed. SOI/MOSFET fabricated in this work showed the electron mobility of 1000㎠/Vsec.

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Tensile property variation due co the modification of the volume fraction and average size of primary a in the bimodal structured Ti-6Al-4V has been studied. For the modification of volume fraction and size of the primary α, various solution heat treatments were conducted in the (α+β) field. Strength as well as ductility was maximized at a critical distribution of the primary α in the bimodal microstructure, where volume fraction and average size were 9.0% and 74.1㎛², respectively. Both of the strength and the ductility improvement were dominantly affected by volume fraction rather than average size of the primary α. The strength improvement is due to the combination of dominantly existing strengthened lamellar matrix and of finely distributing hardened primary α. Ductility improvement is obtained as a result of combined resistance to the microvoid generation and microcrack proipagation at a critical amount of the primary α. Microvoid generation and microcrack propagation are dominantly appeared at the conditions of less and more than the critical amounts of the primary α, respectively. Resistance to the microcrack propagation gives better ductility rather than microvoid generation in the bimodal structured Ti-6Al-4V alloy.

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The purpose of the current research work is to develop a technology for preparing pure AlNi using SHS process. The preparation of AlNi intermetallic compounds was affected by the mole ratio, compaction pressure, and particle size. Combustion temperature is decreased with increasing the compaction pressure and the particle size of aluminum. In the course of combustion reaction, high densification of products was directly attained by inducing pressure and forming simultaneously.

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The effect of an in-mold electro-magnetic stirring(MEMS) on the subsurface defects and macrosegregation in the CC billets has been examined by a plant test and a new evaluation method. The reduction behavior of macrosegregation by a combination stirring has also been discussed through analyzing the thicknesswise cast structures at the centre region and mushy zone depending on the various EMS conditions. It is found that the subsurface defects and macrosegregation can effectively be reduced under an optimum stirring condition. The reduction of macrosegregation in CC billets by (M+S+F) EMS appears to be caused by 1) increasing the flow resistance of solute enriched interdendritic liquid in the mushy zone due to highly close-packed fine equiaxed crystals, 2) decreasing the flow distance of solute enriched interdendritic liquid in the mushy zone due to increased solidification rate at the centre region, and 3) decreasing ferrostatic pressure at the time of macrosegregation formation caused by the shortened distance of crater end.

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This study makes use of the transmission electron microscopy to examine the crystal structure of eutectic flake graphite extracted from commercial gray cast iron. The result shows that the flake graphite contains rotational stacking faults of various rotation angles. The fault is observed to occur more frequently in fine graphite flakes than in coarse flakes. The fault formation seems to be responsible for the characteristic growth mode of flake type graphite, especially in the fine flakes precipitated at high rates.

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