발간논문

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In-situ tensile deformation tests of molybdenum were carried out in a transmission electron microscope at temperatures ranging from 120K to room temperature. At all test temperatures, brittle crack propagation was observed to precede plastic deformation. The characteristics of cracks, either ductile or brittle, was found to vary depending on crack geometry and loading condition. In some cases the coexistence of brittle and ductile crack was observed and in other instances crack propagation changed from brittle to ductile mode. Such phenomena could be successfully explained by the concept of local stress intensity factor for dislocation emission and brittle crack propagation. Dislocations generated at a crack tip are thought to shield the crack. The shielding of a crack by emitted dislocations would then increase the fracture toughness of materials. On the other hand, the anti-shielding of a crack by externally generated dislocations could occur, just like the shielding due to dislocations generated at the crack tip to reduce the toughness of materials.

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Solubility products of Y₂O₃ and Eu₂O₃ were measured by the potentiometric titration method in molten KCl-LiCl eutectic at 743K. Measured titration curves clearly showed the formation of new types of soluble intermediate oxychloride complex ions, Y₄O_5Cl_(x+1)^-(x-1) and Eu₂OCl_(x+3)^-(x-1), before in situ precipitation of those rare earth oxides. Oxides thus formed seemed to be far more reactive than Chose in their standard state, thereby explaining the discrepancies between measured and calculated solubility products. The equilibrium formation constants for the intermediate oxychloride complex ions, Y₄O_5Cl_(x-1)^-(x-1) and Eu₂OCl_(x+3)^-(x-1) were 10^(-35.01) and 10^(-3.88), respectively. The solubility products for Y₂O₃ and Eu₂O₃ were also determined, being 10^(-19.73) and 10^(-6.63), respectively. From those results the potential pO^(2-) diagrams for Y and Eu were constructed.

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Based on the stress-modified critical strain fracture criterion, a new analytic model of ductile fracture of a blunt notched specimen is proposed. With the aid of a notch tip stress and strain field in power-law hardening material given by Bates et al., the interrelatonship between the local stress and strain state and the microstructural characteristic distance is analyzed in terms of the notch root radius. From this model, the K_(IC)(ρ) values are predicted to increase with increasing ρ, but a minimum point of K_(IC)(ρ) exists at a small ρ value. Also, the material parameters which control the ductile fracture criterion, namely the microstructural characteristic distance and the reference critical strain, can be quantitatively estimated by fitting the K_(IC)(ρ) vs. ρ data on the model equation. These parameters enable us to understand more ststematically the micromechanism of ductile fracture.

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The optimum conditions for the processing of WC/Co reclaimed powders with $quot;virgin$quot; properties has been studied. The experiments were carried out by contacting three kinds of cemented carbide scraps with molten zinc in an electric furnace under about one atmosphere of inert gas at temperatures from 650 to 800℃ and subsequently distilling the zinc from the reaction mass at temperatures from 700 to 950℃ From the experimental results, it has been found that the set of optimum conditions is a dissolution temperature of about 750-800℃ depending upon the cobalt content in the scrap, a distilling temperature of 850-950℃, a Zn/Co ratio of about 15 in the case of 7% Co-containing scrap or about 12.5 in the case of more than 8.5% Co-containing scrap, a dissolution time for about 2 hours, and a distillation time for about 3 hours. The $quot;virgin$quot; properties of reclaimed powders were confirmed in the case of the sintered products from 50% virgin- 50% reclaimed powders by the chemical analysis, the measurements of physical properties, and the tool wear tests.

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Parameters needed in model calculations of the α/γ phase equilibrium in Fe-C alloys are calculated by using new Monte Carlo procedures. The parameters include the carbon-carbon interaction energies and standard carbon potentials in the αand γ phases and the relative lattice stability of pure bcc and fcc Fe. Accurate values of the model parameters are determined by a comprehensive analysis of the existing experimental carbon activity data in both the αand γ phases and by utilizing the Fe-C phase diagram. The parameters determined in the present work together with the new Monte Carlo procedures will surely provide more confidence in the low temperature extrapolation of the metastable α/γ phase equilibrium.

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The texture change accompanied by α`→γ transformation has been studied in Fe-15Mn-5Cr5Co-3Si alloy. After hot rolling, the alloy was composed mainly of γ phase with a small amount of ε martensite formed during cooling. The γ phase, by cold rolling, transformed into εwhich later transformed to α` martensite by further deformation. After 75% cold rolling, the γ phase has completely transformed to α` with the texture of (010) α` [101￣]α` and (11￣2)α` [110]α` orientations. By the heat-treatment at 630℃ for 1hr, the stress-induced α` martensite was completely transformed into γ, forming the texture of (011) γ [(21)￣1] γ and (11￣0) γ [001] γ orientations. The change in the texture was explained by the orientation relationship of [110] α`∥ [001]γ and (111)α`∥(110) γ. The yield stress in rolling direction was 100 ㎫ lower than that of transverse direction, which was attributed to the texture in γ phase.

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