발간논문

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As the demand for high thrust gas turbine engines increases, critical components such as high pressure turbine discs are normally exposed to ever extreme conditions of high temperature and stress. Consequently a thorough understanding of material degradation mechanisms during service is indispensable. The precipitation kinetics of the detrimental sigma phase in the disc alloy Udimet 720Li have been characterized and quantified. The present investigation includes the exposure of the alloy to temperatures between 700℃ and 775℃ and times up to 5000 hours. Stress effect on the sigma precipitation has also been examined by applying stresses up to 250㎫. Minor phases such as sigma, carbide and boride were extracted from the exposed samples by the electro-chemical extraction method, and subsequently the extracted residues were quantitatively analysed using the X-ray diffraction technique combined with the Rietveld method. Variations of minor phases contents have been investigated with exposure time, temperature and applied stress. It is shown that applied stress considerably accelerates the precipitation rate of sigma phase. For example, after 2000 hours exposure at 725℃. 0.4wt.% sigma phase is found. If a stress of 250㎫ is applied. the amount of sigma phase rises up to 2.5wt.%. From these quantitative examinations, time-temperature-transformation diagram is presented far sigma phase formation in Udimet 720Li. Such observations have been correlated with microstructural characterization carried out using optical microscopy.

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In this study, the internal variable theory of structural superplasticity has been applied to Pb-Sn alloys. The basic concept of the theory is that superplastic deformation of crystalline materials consists mainly of interface sliding and a dislocation glide process accommodating incompatibilities caused by the former process. A special attention has been focused on the effects of grain size and the volume fraction of constituent phases on deformation characteristics of the typical microduplex superplastic material, generally known to reveal a pronounced phase boundary sliding. For this purpose, a series of load relaxation tests has been conducted to obtain the flow curves at room temperature. A new quantitative method to determine the optimum strain rate for superplastic forming process has also been suggested. The phase boundary sliding in the two phase alloy was characterized as a viscous flow process with the power index value of M_g=0.5 which is much less than the previously reported value of M_g=1.0 for grain boundary sliding. The optimum strain rate was found to shift into a faster region as the grain size decreased.

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In this study, the high temperature superplastic deformation behavior of Al-Mg-X(X:Cr,Zr,Mn) alloys were investigated. The effect of the transition elements and thermomechanical processing on the microstructure and superplastic behavior of Al-Mg-X alloys were also examined. Various intermetallic particles were distributed randomly throughout the matrix and large particles were observed frequently at grain boundaries, suggesting they are effective in refining grain size. In Al-Mg-Cr alloy which was thermomechanically processed with an intermediate annealing, the maximum elongation (close to 730%) was observed at 530℃. The strain rate at which the maximum elongation occurred was faster in Al-Mg-Cr(1x10^(-3)/sec) alloy than in Al-Mg-Mn(1x10^(-4)/sec) alloy. In Al-Mg-Zr alloy, the maximum elongation was observed at 500oC when it was thermomechanically processed without an intermediate annealing and annealed at 530℃ before testing. The observation of the maximum elongation at a lower temperature (500℃) in Al-Mg-Zr alloy may be associated with the finer size of dispersoids in this alloy. The superplastic behavior of Al-Mg-X alloys was found to be greatly influenced by the nature of intermetallic particles.

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The hot deformation process of Al 6061 alloy was simulated by hot torsion test. The torsion tests were performed in the ranges of 400-550℃ and 5.0×10^(-2)∼5.0×10^0/sec to predict the deformation resistance. The flow curves showed the characteristic of dynamic recrystallization(DRX) and the dependence of the flow stress on temperature(T) and strain rate (E) was expressed by hyperbolic sine law. (ε) = 2.9x10^(-3) (sinh 0.0295σ)^(6.2) exp(-207080/RT). The ZenerHollomon parameter obtained to predict the flow curves was Z= (ε) exp(207080/RT) and peak stress was expressed as σ_p=4.1ln∼(Z/A)+29.5. The modelling σ-ε curves of Al 6061 alloy were completely agreed with experimental results over the whole strain. since the hardening caused by deformation and the softening caused by dynamic recrystallization were taken into consideration in this modelling equation.

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The cause of the surface defect in the cold-rolled ultra-low carbon steel was investigated through the analyses of the chemical composition and the size of the defect itself and of the correlation between the defect and the surface deterioration of the finishing work roll in the hot strip mill. It was found that the surface defect was originated from the surface peeling of the fourth finishing work roll, which was related to the severe thermal fatigue damage due to higher hotrolling temperature. The amount of the defective strips was considerably decreased by intermediate change of the fourth work roll and by increasing the roll coolant and the lubricant. The defect was also decreased by reducing the number of the ultra-low carbon steel coils in one rolling campaign. Complete prevention was achieved by changing the roll material from nickel-grain cast iron to high-chromium cast iron (Received December 8, 1997)

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The effect of friction on the development of the cold rolling texture in extra low carbon steel sheets was investigated using X-ray texture measurement. The through-thickness texture inhomogeneity was found to be dependent on the friction acting between rolls and the rolled sheet. The degree of the through-thickness inhomogeneity was higher in the specimen rolled without lubrication than that rolled with lubrication. Cold rolling without lubrication led to the formation of Goss-Orientation at the surface layer by shear deformation, whereas the typical cold rolling texture was obtained at the surface of the specimen rolled with lubrication. The evolution of the through-thickness texture inhomogeneity was successfully explained by variation of the friction induced shear component e13 in the roll gap.

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The addition of RE elements in the form of misch metal is considerably effective in the modification of Al-7.5wt.%Si alloy. The morphology of eutectic Si was drastically changed from coarse flake to fine fibrous when RE was added. 1wt.% addition of RE appeared to be favorable to the modification of eutectic Si in terms of the size reduction of eutectic Si and the improvement of mechanical properties. Like Sr and Na, a decrease in the eutectic temperature was also found in RE-added alloy. Most of the RE participate in the formation of coarse intermetallic compounds and only a small quantity of RE solute still remaines in retained liquid phase. RE solute dissolved in liquid phase may affect the modification of eutectic phase by reducing the anisotropy and preventing the TPRE(twin plane re-entrant) growth of eutectic Si phase.

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Powder processed(P/M) and spray formed(S/F) Al-20Si-5Fe-2Ni billet were reheated into mushy zone by high frequency induction heating. Evolution of microstructures in the semi-solid stste was investigated according to various heating, cooling and post heat treatment conditions. The results showed the feasibility of net-shape manufacturing of Al-high Si part by thixoforging of spray formed billet. Application of P/M Al-20Si-5Fe-2Ni for thixoforming revealed to be problematic because of the formation of pores in the semi-solid state. It was possible to control the average size of Si intermetallic phases within 10㎛ after thixoforging of S/F Al-20Si-5Fe-2Ni. Cooling rates depending on die temperatures strongly affected the homogenity of microsructures and hardness distribution of the thixoforged part. Microstructural study implied that optimal die temperatures for thixoforging of S/F Al-high Si billet should be in the range of 200-350℃. Inhomogenious macro-& microstructure of thixoforged part could be homogenized by post heat treatment.

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The purpose of this study is to make clear the effects of Ni on the behavior of precipitate formation in Vi added galvanizing bath, which has various Ni contents from 0.03wt% to 0.5wt%. The morphology of the precipitate changed with Ni addition from a needle like rectangular ζ-phase to a globular polygonal Zn-Fe-Ni phase. Al addition in Ni containing Zn-bath resulted in forming the Al-Ni intermetallic compounds such as Al₃Ni₂ and Al₂Ni which consist most of top precipitates. In the range of higher content of 0.07wt% Vi in Zn-bath, Al-Ni intermetallic compound was formed sensitively with small amount of Al addition.

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Alloys of TiAl+(0, 2, 6. 10%)Cr manufactured by a mechanical alloying- plasma active sintering method (MAPAS) were oxidized at 800. 900 and 1000℃ isothermally and cyclically in air up to 72 hrs. It was found that the oxidation resistance increased in the order of TiAl+2%Cr, pure TiAl, TiAl+6%Cr and TiAl+10%Cr. The increase in the oxidation rate of TiAl+2%Cr compared with pure TiAl is considered to be due to the doping effect of Cr₂O₃ dissolved in TiO₂, which increases the overall growth rate of scales formed. In contrast with this deleterious Cr effect in TiAl+2%Cr, the decrease in the oxidation rate of TiAl+(6, 10%)Cr compared with pure TiAl is considered to be due to the formation of a sufficient amount of Cr₂O₃ beyond the solubility limit in TiO₂. Chromic was mainly present inside the Al₂O₃ rich. TiO₂-deficient inner scale.

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