발간논문

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The solubilty products of La₂O₃ and Ce₂O₃ were measured by the potentiometric titration method in molten KCl-LiCl eutectic at 743K. Measured titration curves clearly showed formation of soluble intermediate oxychloride complex ions, LaOCl_x^(x-1)- and CeOCl_x^(x-1)-, before in situ precipitation of those rare earth oxides. Oxides thus formed seemed to be far more reactive than ordinary oxides, thereby explaining the discrepencies between measured and calculated solubility products. The equilibrium formation constants for the intermediate oxychloride complex ions, LaOCl_x^(x-1)- and CeOCl_x^(x-1)- were 10^(-5.09) and 10^(-6.39), respectively. The solubility products for La₂O₃ and, Ce₂O₃ were also determined, being 10^(-13.72) and 10^(-18.25), respectively. From those results the potential-pO^(2-) diagrams for La and Ce were constructed and new methods for the separation of the rare earth oxides have been suggested.

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The structure parameters of gas transport for the oxide pellet were obtained by the modified Wicke-Kallenbach method(abbreviated as W-K method in the present paper). The specimens are acid pellet, basic pellet. Reduction was conducted in CO-CO_2 gas mixtures in stepwise manner. The change of pore structure of the sample during the stepwise reduction was observed with the mercury porosimetry. Samples in a single particle were also reduced to observe the weight change during the stepwise reduction and the effective diffusivity was derived by applying the unreacted core model. Thus obtained values by different methods was compared with that by W-K method. The results are summarized as follows 1. Temperature dependence of parameters for the reduced phases as well as original sample of acid and basic pellets were determined at 850, 950, 1050℃ Thus obtained values display not the clear relation with temperatures but a tendency to a gradual increase with increasing temperature. 2. The porosity and average radius of the original and reduced phase of pellets were observed with the mercury porosimetry. These obtained values do display a tendency to a gradual increase with increasing temperature. 3. Good accordance was found between the effective diffusivity of pellets by using the unreacted core model and that obtained from W-K method except some cases in acid pellets.

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The effect of thermomechanical processing conditions on grain refining and superplastic properties of Al-3Cu-2Li-1Mg-0.15Zr alloy was studied. A large superplastic elongation of about 1200% was obtained at high strain rate of 10^(-2)/s, ten times faster than that of previously reported values for this alloy. The attainment of such a large elongation at this faster strain rate is attributed to the formation of fine subgrains with the size ranging from 0.5㎛ to 1.0㎛, that are statically stable up to the test temperature but dynamically recrystallize into very fine grains of 6∼7㎛ in size during high temperature(530℃) tensile deformation. The statically stable subgrains are thought to be formed due to the combined thermomechanical processing of high temperature homogenization and warm rolling. The solute atoms dissolved from coarse eutectic compounds may be reprecipitated as fine particles stabilizing subgrains during a subsequent warm rolling process.

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The present research was performed to investigate the effect of precipitates on the fatigue properties of 8090 Al-Li alloy. By applying different heat treatment paths, two types of microstructures were produced : one with the δ` phase dominant microstructure and the other with δ` + S` microstructure. It was observed that both the fatigue crack initiation resistance and fatigue crack propagation resistance of δ` microstructure were considerably higher than those of δ` + S` microstructure. Excellent resistance for fatigue crack initiation in δ` microstructure was mainly attributed to the extrinsic nature such as crack path tortuosity and higher level of crack closure, as well as to the intrinsic nature of greater slip reversibility in δ` microstructure. However, δ` + S` microstructure showed greater intrinsic resistance for crack propagation at intermediate ΔK region, probably due to the effect of slip dispersion by the presence of S` phase. It was also observed that the relative amount of crack closure was maximum at the near threshold regime and decreased rapidly as ΔK increased.

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The transition from single- to multiple-peak dynamic recrystallization flow curves of V-microalloyed steel was studied by torsion test in the temperature range of 900℃ to 1000℃ and over a strain rate range of 2.0×10^(-2) to 1.0×10^0//sec. The grain size of the dynamically recrystallized specimens decreased with increasing Zener-Holloman parameter, Z without exhibiting any dependence on the initial grain size. The real transition occurred at the upper Z values, 2.0×10^(11)∼2.0×10^(10) than that determined by ε_x = ε_p. On the initial grain size D_0, dependence of the critical condition for the flow curve transition, Z_c, the equivalency with Z vs. D, relation was not observed. Therefore, the criterion D_o=2D_s has an applicability for only a limited range of Z. The constitutive equation of D, vs. Z could be described by D_s =897.865 Z^(-1.04)(㎛)

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Multistage torsion tests in the temperature range of 850 to 1150℃ and over a strain rate range of 1.89×10^(-2) to 3.78×10^(-1)/sec were performed to study the static softening of V-microalloyed steel under isothermal conditions. The fractional softening FS decreased with reduction in temperature, T, pass, ε, and strain rate, ε˙. Due to the occurance of FS, the multistage curves fall below the peak of the continuous curve, but are above the steady state regime after the peak. The multistage flow stress envelope of pass 2 to 10 is flat because they become almost identical due to similar softening in the intervals. During the intervals of multistage schedules, the following microstructural changes take place : static recovery, static recrystallization and grain growth. Therefore, the multistage grain size, being a strong function of temperature and strain rate, is larger than that produced by dynamic recrystallization, and being still smaller than the original grain size.

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