발간논문

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Segregation of Nb during solidification results in the formation of Laves phase in the interdendritic regions of cast Alloy 718. The Laves phase can be dissolved in the matrix by suitable heat treatments. In the present study, the effect of Nb content and homogenization treatment on tensile properties and hardness was studied at room temperature. Nb content was varied within 5.2∼6.2wt.% Nb. Homogenization treatments were conducted on as-cast alloys at 1100℃/1hr, 1100℃/10hr, 1200℃/1hr and 1200℃/10hr. The hardness increases with increasing Nb content to a different extent depending on homogenization treatment. Tensile properties vary with Nb content and homogenization treatment in a complex way. The effects of Nb content and homogenization on mechanical properties are attributed to the combined effects of Laves dissolution and γ´/γ˝ precipitation.

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Homogenization treatments were performed at 1200℃ on cast Alloy 718 and its Nb modified version in order to investigate the effects of alloy chemistry and homogenization treatment on age-hardening behavior. Regardless of alloy chemistry, age hardening occurs faster at 1200℃/1hr than at 1200℃/10hrs condition. The remaining Laves phase affects the age-hardening behavior through the control of γ˝ precipitation. The larger volume fraction and even distribution of Laves phases, containing high Nb content, enhance the formation of γ˝ near the Laves phases. A peak hardness was accordingly obtained only after 8hrs of aging at 720℃ in the sample homogenized at 1200℃/1hr. This peak time is in contrast to 28hrs of aging in the sample homogenized ac 1200℃/10hrs. The Laves phase and γ˝ precipitates in the aged alloys were identified with electron microscopy.

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Bending of the steel/polymer/steel layer laminate steel under plane strain condition is simulated by an elastic-plastic finite element program MARC. It is confirmed that the forming defects such as double-bent and slipping-off which are often found in bending of steel-plastic laminate sheets, can be simulated for different kinds of core resins exactly by using FEM. The calculated sheet geometry, load and folding angles agree well with those of the experiment. The stress state during loading and the residual stress after unloading are studied.

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Cube texture after cold rolling has been studied to clarify effects of annealing time and temperature for Al foils with varying purifies from 99% to 99.99%. The foils of 0.1㎜ thickness were annealed finally at temperatures from 300℃ to 600℃ for 1, 3, 5 and 7 hours in 10^(-3)torr vacuum. It was found that the highest cube texture was obtained by repeating hot and cold rolling, and final annealing at 600℃ for 3 hours, while the formation of cube texture was reduced as the annealing time augmented from 3 to 7 at 600℃, because of the coarse recrystallization grain growth during annealing process. It could be known that forming time and leakage current more increased in low purity Al foil than high purity.

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The effect of grain refinement on dynamic recrystallization controlled rolling(DRCR) of V-microallyed structural steel was performed in the temperature range of 750℃∼1000℃ and strain rate (ε˙) range of 0.1∼3.0/sec with decreasing temperature and increasing ε˙ pass strain of 0.3 and/or 0.5 by hot torsion. From the analysis of the values of the critical strain of ε_c for the initiation of dynamic recrystallization and the peak strain of ε_p, the relationship ε_c≒0.7ε_p was obtained. By increasing the pass strain from 0.3 to 0.5 at the penultimate pass in the DRCR schedule, dynamic recrystallization could be induced and accelerated the grain refinement.

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The water-based magnetic fluids were prepared with the synthesized ultrafine magnetite using sodium oleate and lauric acid as surfactants. The dispersion ratio of water-based magnetic fluids was maintained constantly when the added amounts of sodium oleate and lauric acid for magnetite(20g) were more them 1.6×10^(-2)㏖, 3.0×10^(-2)㏖ respectively. The stable pH region for the magnetic fluids prepared with sodium oleate and lauric acid was range of pH 8.0 to pH 10.0. As the solid content increased from 0.05 g/cc to 0.5g/cc, the viscosity of water-based magnetic fluids increased from 0.97 cp to 12.08 cp, and its saturation magnetization at 5kOe increased from 3.1 emu/g to 20.6 emu/g. Water-based magnetic fluids were able to redisperse to kerosene-based or water-based magnetic fluids by adding kerosene and NH₄OH solution to dried water-based magnetic fluids. However in the case of redispersed water-based magnetic fluids, the saturation magnetization decreased with increasing iteration.

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The corrosion and the microstructural properties of evaporated Al-Cr films on the steel strip by two-source evaporator have been investigated. The corrosion resistance of Al-Cr coated steel was evaluated by the salt spray test as well as the anodic polarization test in 0.1N NaCl. Porosity of coated Layer was also compared with the corrosion resistance property. Corrosion resistance was improved with increasing the Cr content in Al-Cr films, and it was the maximum in the Cr content of 10 to 15wt%. However, corrosion resistance was decreased over 15wt% of Cr content films, it was thought to be due to porous microstrucute resulting from the formation of intermetallic compound phases.

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In order to refine the grain size and to improve their mechanical properties in Cu base shape memory alloys, the methods such as the addition of alloying element, powder metallurgy and rapid solidification technology are being studied. This work was carried out to establish the basic concept for attaining the above purpose by studying on the changes in the grain size and studying on the forming behavior of γ₂ precipitates by aging of the melt spun ribbons manufactured by single roll melt spinning. Melt spun ribbons, as it is, exhibited shape memory behavior. Grain size in the melt spun ribbons was on the whole below about 3㎛, it was finer than that of castings. When they were aged, the phenomena of the increase in hardness and of the disappearance in the shape memory ability were started generally after about 10³ seconds at 400℃, 10^5 seconds at 300℃ and l0^6 seconds at 200℃ in ribbons. But in case of the alloy being added compoundly Ti, Zr and B, the phenomenon of the increase in hardness appeared after about 10⁴ seconds and the shape memory ability was maintained for the longest time at 400℃. The cause of the phenomena of the increase in hardness and of the disappearance in the shape memory ability is due to the formation of the γ₂ precipitates. In this work it is considered that the alloy being aged compoundly Ti, Zr and B is more superior than any other alloys from the viewpoint of the grain size, aging characteristics and shape memory ability.

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In case of cryogenic structures such as superconducting magnets and pressure vessels, the structural material experiences unrestricted force. It is, therefore, desirable to characterize the load-controlled tensile properties at cryogenic temperatures to simulate operating condition. At 4 K, however, discontinuous yielding occurs, which arises mainly from low specific heat and thermal conductivity of the materials at low temperatures, and it affects the load-displacement or stress-strain curves. In this study, load-controlled tensile tests were conducted at 77 ad 4K on AISI 300 series stainless steels. The load rate was varied from 5 to 5000 N/s. The effect of load rate and testing temperature on tensile properties of 300 series stainless steels were observed. A model which correlate discontinuous yielding and ultimate tensile strength was proposed.

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Processing techniques to synthesize Ti_5Si₃ from a mixture of elemental powders were studied. Two routes of synthesis, self-propagating high-temperature synthesis(SHS) and reactive sintering, as well as the effect of Cu as an activator for the sintering reaction were studied. For the final densification treatment, elemental powders were mixed and refined either by mechanical milling using an attritor or by ball milling. The optimum milling conditions for the maximum density, such as revolution speed and milling time, as well as the compaction pressure were identified. Due to the difficulties involving reaction rate control and gas removal, the SHS method did not yield high density. However, the reactive sintering resulted in a significant reduction of porosity. Cu addition proved to be effective in densification, and a sintered density over 90% of the theoretical density was obtained in a sample containing 1wt.% of Cu. The effect was attributed to the promotion of diffusion, which was interpreted in terms of the `figure of merit` concept.

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