발간논문

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Effect of Ti content on the bake hardening behavior was investigated in cold-rolled and continuously-annealed ultra-low carbon steels containing 0.075% P. TiN and TiS precipitates were observed in steel with Ti content lower than (48/14)N+(48/32)S while in the steel with Ti content higher than (48/14)N+(48/32)S, in addition to TiN and TiS, (Fe, Ti)P and Ti(C, N) were observed. A bake hardening of about 3kgf/㎟ was obtained in a steel with carbon content of about 25 weight ppm and Ti content controlled in N(48/14) ≤ Ti ≤ N(48/14)+S(48/32). Substantial amount of solute carbon might remain without affecting bake hardening. In the 0.0029-0.021% range, sulfur had little effect on the bake hardening. On the other hand, the solute carbon levels that resulted in yield point elongation of about 4.0 percent or less in as-annealed steel appeared to be too small to contribute to bake hardening and aging index.

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Effects of cooling rate, quenching temperature and holding time on the morphology and the composition of the precipitates at the prior austenite grain boundary were investigated in boron steel. Morphology of the precipitate was plate type and independent of quenching temperature and holding time. But the increase in the cooling rate decreases the thickness and length of the precipitates. The precipitate at the prior austenite grain boundary was Fe₃(CB) and independent of cooling rate, quenching temperature and holding time. Boron content in Fe₃(CB) was 7.5at% at 20℃/sec and 11.0 at% at 1℃/ sec.

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Microstructure and micro-indentation fracture of pressureless-sintered Al₂O₃/ZrO₂(0%Y₂O₃) composite containing different Y₂O₃, content were investigated by transmission and high-resolution electron microscopy. In the Al₂O₃/ZrO₂(0%Y₂O₃) composite, strong strain field contrasts were observed in Al₂O₃ near m-ZrO₂ grains. ZrO₂ with a lamella-shaped twin structure produces compressive and expansive strain field in adjoining Al₂O₃ matrix and microcracks at the interfaces between Al₂O₃ and ZrO₂ with no twins are joined without any impurity phases. However, amorphous phases appear at the interfaces and triple points with increasing Y₂O₃ contents. The main fracture toughening effect in Al₂O₃-ZrO₂(0%Y₂O₃) is due to crack deflection and plastic deformation in m-ZrO₂ grains. In the Al₂O₃/ZrO₂(3%Y₂O₃), main fracture mode is a mixed type with intergranular, and transgranular, and stress-induced phase transformation in tetragonal-ZrO₂ grains partially occurs by crack propagation. In Al₂O₃/ZrO₂(8%Y₂O₃), fracture was mainly transgranular, but serious plastic deformation is not observed in transgranularly fractured cubic-ZrO₂ grains.

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The composite coating of aluminide-yttrium has shown excellent corrosion resistance under cyclic high temperature hot corrosion environment. To understand the effect of the yttrium on the stability of the composite coating, the specimens were prepared with various coating parameters such as thickness of Y layer, sequence of post heat treatment, and surface condition before Y ion plating. Performance of the composite coating was evaluted by the isothermal oxidation and cyclic high temperature hot corrosion. Test results of isothermal oxidation show that the Y in the composite coating helps to form a thick and dense Al₂O₃ scale which is ductile and resistant to thermal stress. The Y in Al₂O₃ may act as a donor which leads to increase in the concentration of interstitial oxygen and thus increase in oxidation rate. The presence of Y₂O₃ and (Y, Al)O type compounds in the grain boundaries of Al₂O₃ itself and the boundaries between the Al₂O₃ and NiAl effectively prohibits the fast diffusion of oxidants, and Al along the grain boundaries. Consequently, it may induce slow diffusion through the matrix, and thus the corrosion resistance of the conposite coating under cyclic hot corrosion increases substantially.

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In this study, aluminum powders were sintered to porous aluminum plate, and the sound absorbing characteristics and mechanical properties was investigated. The difference of thermal expansion coefficient between aluminum and alumina was used in sintering aluminum powder and in order to increase sintering property of aluminum powder, Al-8%Si powder was mixed as a binding material. The cross section of sintered plate was obserbed with optical microscope and EDAX. The result showed that silicon was mainly distributed near the neck between sintered aluminum powders. and Al-8%Si alloy powder worked as a binding material. The result of measurement of sound absorbing coefficient showed resonator type characteristics by increasing the thickness and density of sintered plate and maximum sound absorbing coefficient was transfered to lower frequency by increasing air space of the back side of sound absorbing plate. Tensile strength of the sound absorbing plate was increased to 31∼37㎏/㎠ by increasing density.

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The B₂⇔ B_(19) transformation behaviours in a ternary Ti_(50)Ni_(35)Pd_(15)(at.%) alloy were investigated by means of electrical resistivity measurements, differential scanning calorimetry and constant load thermal cycling tests and they were compared with the B₂⇔ R and B₂⇔ B_(19) transformations in a thermo-mechanically treated binary Ti-50.2Ni alloy. The transformation start temperature of B₂→ B_(19) transformation and transformation finish temperature of B_(19) → B₂ transformation increased lineary with increasing applied stress. The stress dependence of the transformation start temperature, transformation elongation and hysteresis of the B₂⇔ B_(19) tranformations in the ternary alloy were 10.0MPa/K in the reciprocal form(dσ/dT) where σ in the critical stress for inducing the transformation, 2.1% and 25K, respectively. The B₂→ B_(19) transformation in the ternary Ti-Ni-Pd alloy lies between the B₂→ R and B₂B_(19)` transformation in the binary Ti-Ni alloy. And the transformation elongation associated with the B₂⇔ B_(19) transformation increased rapidly for stresses less than 80MPa and slowly above 80MPa with increasing applied stress.

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