발간논문

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The relation between strength and second phase volume fraction in two-phase iron alloys was investigated. The alloys used in this study were classified into two groups; one was Fe-C alloy system composed of ferrite, and martensite, and the other was Fe-Ni-C alloy composed of austenite and martensite. The relation between strength and second phase volume fraction in two-phase iron alloys was better represented by rule of mixture, as the ratio of strength of second phase to that of matrix became smaller. As the strength ratio became larger, the relation between strength and second phase volume fraction deviated more from the linear relationship expressed by the rule of mixture. This may be caused by the difference in strain between the second phase and the matrix, which becomes larger with an increase of the strength ratio.

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Effect of oxygen on the corrosion of Armco iron in molten LiCl-KCl eutectic salt at 450℃ was studied by electrochemical techniques. All the experimental work was carried out in an inert atmosphere glove box, equipped with a specially designed electrolysis cell system. Results clearly showed the existence of active-passive transition for Armco iron corrosion in the melt and also the dependence of corrosion rate and potential upon the oxygen limiting diffusion current. Above 0.25V of electrode potential refered to Ni/Ni₃S₂/S^(--) reference electrode, main partial anodic reaction appeared to be Fe →Fe^(+3)＋3e and showed clear Tafel linearity with slope of about 80 mV/dec. Corrosion rate was increased in proportion to 1/5 power of oxygen partial pressure. Other electrochemical parameters related to the corrosion reaction were also discussed based on the mixed potential theory and electrode kinetics.

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The theoretical basis of Ostwald ripening has been reviewed with particular reference to the effect of medium grain fraction on the size distribution. And the procedure to obtain a reliable size distribution by linear intercept method has been specified in detail for cobalt grains in 50% Co-50% Cu alloy at 1,200℃. Systematic tests are performed to confirm the validity of the observed grain intercept distributions. The shortcomings of chi-square test are discussed. The grain intercept distribution approaches to a stabilized form when the number of intercepts reaches about 1,000 and that of photo-micrographs 5 to 6. This result does not vary with the grouping method of intercept size classes. During coarsening, the size distribution maintains a stationary form, which is consistent with the theoretical predictions for diffusion controlled growth when the grain volume fraction exceeds about 0.5.

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The stress corrosion behaviour of mild steel (0.03-0.04 wt.% C) in a boiling aqueous solution of 860g Ca(NO₃)₂·4H₂O and 30g NH₄NO₃/ℓ was studied with specimens under constant load, related to Al content and grain size of specimen. Grain size of Al steel was controlled by annealing temperature and time of the specimens. Grain size remained nearly constant for the specimen with Al content up to 0.37 wt.%, however, grain became coarser for Al content from 0.37 to 1.01 wt.%, and a further increase in Al content did not affect grain size. The decreased grain size of specimen annealed in the γrange was explained in terms of delay of grain growth due to γto αphase change. And grain size of ferrite phase increased with increasing annealing temperature and time. Failure time, under constant load, as a whole increased with increasing grain size. Variation of failure time was discussed in terms of segregation of carbon at the grain boundary, and precipitation of Al₂O₃and cementite in the grain.

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The crystallization mechanism of amorphous Fe-P alloys has been investigated through the differential thermal analysis, the X-ray diffraction, and the change of the electrical resistance to temperature and time variations. 1) The crystallized products were α-Fe(bcc, a=2.86Å) and Fe₃P(bct, a=9.llÅ, c=4.46Å, c/a=0.490) regardless of the concentration range of amorphous Fe-P alloys. 2) Crystallization temperature of the three amorphous samples, Fe_(84)P_(16), Fe_(83)P_(17) and Fe-(82)P_(18) measured by the DTA and resistance methods were 368℃, 373℃, and 377℃ respectively. 3) The crystallization process of the amorphous Fe-P alloy was different according to the phosphorous concentration. The process of Fe_(84)P_(16) and Fe_(83)P_(17) was based on the auto-catalysis but that of Fe_(82)P_(18) was on the nucleation and grain growth. Activation energies for the crystallization were computed as 37.3 ㎉/㏖ for Fe_(84)P_(16), 38.3 ㎉/㏖ for Fe_(83)P_(17) and 40.6 ㎉/㏖ for Fe_(82)P_(18), individually.

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Trials were made to develop gamma prime precipitation hardened nickel-base superalloys which have both excellent creep rupture strength and hot corrosion resistance. The factors which influence high temperature characteristics were expressed a function of alloy composition by means of multiple regression analysis. Setting certain conditions on the factors, alloy composition was calculated with use of the computer. The alloys developed by this method are proved to be excellent in creep rupture strength and sulfidation resistance.

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