발간논문

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The dynamic strain aging effect which had been commonly identified with the serrations on the stress-strain curve, was studied by observing the strain rate sensitivity, if., the stress change due to the sudden change of strain rate during deformation. The new experimental method attempted in this study was to change the strain rate at the same stress and strain on the cyclically stabilized stress-strain curve so that the work hardening effect could be eliminated. The initial stress change and the gradual stress change accompanying the strain rate change were measured seperately identifing the form or with the viscous effect and the latter with the dynamic strain aging effect. The experiment was performed in a relatively large strain rate range of 3.2×10＾(-7) to 3.2×10^(-3) and a temperature range of 20℃ to 800℃. Among the substitutional alloys which were of our interests for the current study, 304 austenitic stainless steel was chosen as a test material because its dynamic strain aging temperature and strain rate ranges could be obtained with our experimental facilities. The main conclusions are that the dynamic strain aging effect could be studied quantitatively and the strain rate change method is very effective for the purpose, and that, for 304 austenitic stainless steel, the dynamic strain aging effect appears most strongly at the strain rates of 10＾(-6), 10＾(-5), 10^(-4) and 10^(-3) for the temperatures of 300℃, 400℃, 500℃, and 600℃, respectively.

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High temperature interfacial reactions in the Mo-fiber reinforced metal matrix composites were studied. The matrices were 304 stainless steel and pure nickel. For Mo/SUS 304 composites, thin brittle layers were formed at the interface. Molybdenum wires were recrystallized after heating for 50 hrs at 1150℃. For Mo/Ni composites, the circumferences of molybdenum wires were recrystallized at temperatures below 1050℃. The re-crystallization of molybdenum wires seems to be activated by the diffusion of nickel into the grain boundary of molybdenum The depth of the recrystallized region was proportional to the exposure time. Molybdenum wires were recrystallized completly at 1150℃ and two kinds of brittle layers were formed at the interface. The recrystallized grains of molybdenum wires were much larger than that in the Mo/SUS 304 composite.

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A study has been made of stress and strain distributions of the solidified shell in a moving slab near the straightening lone where the secondary water cooling is no more applied. The thermal load due to surface reheating and the mechanical load due to ferrostatic pressure were taken into account to calculate the stress and strain distributions based on thermo-elastoplastic-creep behavior of materials. The thermal stresses due to sur face reheating increased with decreasing initial surface temperature and increasing re heating temperature. Superposition of the thermal and mechanical loads increased the in terface strains at the solidification front, which, in turn, increased probability of crack formation, especially when the roll spacing increased and the solidified shell thickness decreased. The position of maximum strains were shifted forward the casting direction from the roll sites possibly due to the effects of creep and moving slabs.

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Solvent extraction of In(Ⅲ) with versatic acid-10 in sulfuric acid solutions was studied. The distribution coefficient was increased with pH and showed a transition at about pH of 2.4. In the pH range above 2.4 the slope was still less than 3 because of the formation of complexes between indium ions and sulfate ions, and below 2.4 it was much less than 3. The distribution coefficient had a maximum value when the concentration of indium ions in the aqueous phase was 1g/ℓ. It was higher with the versatic acid dissolved in benzene than in keroserne because of the stronger dissociation of the versatic acid in benzene, and was increased with the concentration of versatic acid-10. The enthalpy change of the extraction reaction was found to be 12 Kcal per mole. There was antisynergistic effect in the mixed solvent with TBP but synergism with T.O.P.O. in the pH range of above 24. The McCabe-Thiele diagram showed that five extraction stages with the A/O ratio of 1/4 was required to extract about 99% of indium.

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The nonlinear behaviors of the load-crack opening displacement in compact tension test were investigated in relation to the microcrack appearances and acoustic e mission wave forms under stepped increased load by changing the chemical compostions, Cu and Sn weight %. In case of Sn weight % is 0.05 and Cu weight % is 0.5, the Baushinger effect did not appear and fracturing load was decreased in the stepped incrpased CT tensile test. As the Cu and Sn weisht %, is increased, the AE was detected from the initial loading in the stepped increased CT tensile test, but the AE was not detected at the initial loading when the Cu and Sn was not applied.

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Changes of the electronic structure as functions of oxygen content and composition of the co-evaporated amorphous Gd-Fe alloy films have been studied using an Auger electron spectroscopy and a x-ray photo-electron spectroscopy. A comparison of the spectra for oxidized species to trose of pure Gd element or alloy films showed characteristic differences in the low energy Auger transitions, usually incorporating the valence level and the L₁or L₂.₃ levels of oxygen. The disappearance of the N₂N_(4.5)O₂.₃transition at 122ev in an oxidized Gd was speculated to be due to a cross transition such as N₂N_(4.5)L₁(O). The charge transfer from a Cd atom to the 3d levee of Fe was less than leV per Gd atom, and binding energies of core levels were barely affected by the change of chemical composition of the alloy film, No, chemical effect was observed from the Auger spectra of the amorphous Gd-Fe films. SiO layers thicker than 200Å coated on both sides of the film were effective in preventing oxidation of the amorphous film.

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This paper describes a work performed as a part of numerical simulations to explore computational procedures incorporating fluid dynamics, and rebated turbulences. The work was intended to understand and to improve gas combustion in a reheating fur nace of a steelmaking procedure. To solve for the swirl flow field, a two-equation k-εturbulence model was used to predict variation of the turbulent eddy viscosity in the chamber. Numerical computation in this study predicts that a key factor of the combustion performance is the strength of the recirculating eddy in central toroids caused by the swirl entertainment.

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The stress corrosion cracking (SCC) characteristics of Zircaloy-4 sheet in iodine vapour environment were studied by three point bending method. SCC tests were conducted in an iodine vapour evironment with the concentration of 20 ㎎/㎤ at 300℃ in which the crack growth rate, fracture surface morphology and the effect of crystallorgraphic texture on the SCC susceptibility were determined. Specimens of as-received, furnace cooled and oil quenched from the β-phase region, respectively, were precracked in an iodine-methanol solution. The crack growth rate formula after Kreyns, da/dt=c.K＾n, were applicable to the test results. The vague of the exponent n ranged from 1.8 to 2.8. The varue of n decreased with the increase of the intensity of the normal basal pole texture, which suggested that the crystallographic texture influenced the SCC susceptibility. A mixed mode of transgranular and intergranular fracture was observed in the as-received specimen. Transgranular fracture mixed with cleavage was predominant in the heat-treated specimens.

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The effect of low cycle fatigue at 871℃ on the microstructure of Rene´ 80 was studied. The fatigue test was conducted in a push-pull, strain controlled mode with a frequency of 0.05 Hz. The relationship between the plastic strain range and the fatigue life was fitted with the Coffin-Manson model. The alloy showed a cyclic softening initially, which was followed by a steady state until failure. The softening was attributed to a preferrential shearing of secondary γ` precipitates. The dislocation characteristics varied from γ`/γ` localization to γ`/γ` interface misfit dislocations depending on the test condition. A local coarsening of γ` precipitates was also observes. The fatigue crack initiation at surface was accompanied by oxidation, and the mode of crack propagation was predominantly transgranular. The carbides and γ` precipitates were fairly stable against thermal exposures of up to 982℃/70 hrs.

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The effect of transition metal substitution instead of Co on the magnetic properties of (Co_(0.94) Fe_(0.06))_(78)Si₄B_(18) amorphous alloy ribbon was investigated. Mean magnetic moment per metal atom(μ￣) at room temperature decreased linearly with transition metal substitution and the decrease rate was larger for group Va elements than for group Ⅳa and Ⅵ_a elements Curie temperature decreased linearly with the same trend of μ￣ by transition metal substitution. The soft magnetic properties were improved by transition metal substitution except for Mn or Ni and the best properties were obtained at X=2 at% in asquenched state, whereas at X=8 at% in annealed state.

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