발간논문

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In order to investigate the cyclic creep acceleration phenomenon, pure aluminium specimens are crept under static and cyclic stress of 54 ㎫ at 308 K. Macroscopically, cyclic creep has faster creep deformation rate and lower creep activation energy than those of static creep. Dislocation structures developed in these specimens are investigated under TEM and some differences are observed. Cell sizes are similar in both cases but thinner cell boundary width, lower tangled boundary fraction, smaller mobile dislocation density and more simple configuration of mobile dislocation are observed in cyclically crept specimen. These results of investigation (faster creep rate, lower activation energy and more recovered dislocation structures under cyclic stress) are interpreated with the roll of excess vacancies which are created by the nonconservative motion of jogged dislocation under cyclic stress.

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Desulfurization of molten steel was carried out in a high frequency induction furnace at temperatures between 1550℃ and 1705℃. Rates of desulfurization by CaO powder injection with Ar-gas were measured with varying the amount of silicon added, the initial sulfur content, and the temperature, respectively. Results showed that the desulfurization rate was increased with increasing the amount of silicon and also the temperature. Rate equation for desulfurization was developed to be the 1st order with respect to dissolved sulfur content and also the reaction mechanism was proposed. The apparent activation energy of the reaction was found to be about 40.7㎉/mole.

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The effects of substituting Ti for V on mechanical properties in weathering type high-strength low-alloy steel were investigated. Two different final rolling temperatures were employed in the controlled rolling. Air cooling, accelerated cooling by water spraying, and normalizing were applied as post rolling treatments. Accelerated cooling greatly enhanced the strength and better toughness in the Ti-bearing steel. Ti-steel exhibited better isotropic impact behavior compared with V-steel due to spheroidization of non-metallic inclusions by Ti. Normalizing reduced the strength of Ti-steel. The high cycle fatigue strength of air cooled Ti-steel was 18% higher than that of air cooled V-steel owing to the higher tensile strength of Ti-steel. Accelerated cooling also increased the fatigue strength.

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A rotating disc geometry and Zn-Sb alloy powder were used to study the kinetics of cobalt cementation with zinc. In the absence of initial zinc ions and above 55℃, it is considered that the overall cementation rate would be controlled by mass transfer rate through the aqeous diffusion boundary layer, however, below 55℃ the reaction mechanism would be changed. In the presence of 100 gr/l Zn^(2+), the rate of cementation was very slow, which means that the reaction mechanism would be changed. By using Zn-Sb alloy instead of zinc metal as a precipitant, the rate was much increased even in the 100 gr/l Zn^(2+) solution and hence overall reaction rate would be controlled by mass transfer rate above 85℃. The similar phenomenon was found by using powdered Zn-Sb alloy as a precipitant.

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This study was carried out on the effect of carbon on the stability of ferromagnetic τ-phase in Mn-Al-C alloys of three different carbon contents (0, 0.18 and 0.27 wt.% at a fixed Mn of 70 wt.%) by means of the isothermal heat-treatment at temperatures ranging 450℃-600℃. The results obtained can be summarized as follows; i. The values of magnetic properties (Br, bHc, (BH)_max) increased with the amount of τphase present, and then these values decreased gradually upon the decomposition of τphase after each of them reached its maximum value. The optimum temperature of isothermal heat-treatment for the highest magnetic properties ranged from 500℃ to 600℃ for varying heat-treating time depending on carbon contents. 2. A T.T.T. diagram calculated from the data for the formation and decomposition of τphase was possible to obtain by means of the quantitative metallography, X-ray diffraction, and the measurement of magnetic properties at each step during the heat-treatment. 3. The higher the carbon content, the more was retarded in the rate of formation and decomposition of τphase. However the effectiveness of this was analyzed to be much great for the decomposition compared with the formation rate of τphase. Consequently, the τphase region in T.T.T. diagram was expanded as the carbon contents increased.

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The effects of neutron-irradiation and post-irradiation annealing on the mechanical properties of interstitial free Mo and Mo-0.5V alloy was studied. Specimens were irradiated at a temperature of approximately 573 K to a fast neutron fluence of 1.3×10^(24) n/㎡ (E≥1 MeV) and subsequently annealed at 873 K and 1073 K. The results obtained are as follows; 1. Ductile-brittle transition temperature (DBTT) was raised about 130 K by fast neutron irradiation of 1.3×10^(24) n/㎡ at 573 K. 2. Post-irradiation annealing at 873 K increased the strength, while annealing at 1073 K decreased the strength. 3. The relationship between yield stress increment, Δδ_y, and hardness increment, ΔH_v, can be expressed by Δσ_y ○ 3.4 (ΔH_v) MPa.

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In this paper decomposition and thermal stability of the supersaturated solid solution of Al-Cr alloys were investigated. Al-1.2 wt%Cr alloys were rapidly solidified by using a small droplet chill quenching method. The decomposition process of the supersaturated Al-Cr solid solution by annealing at 350℃ to 600℃ and up to 60 hours was examined by means of microhardness and X-ray diffraction analysis. Hardness measurements on supersaturated Al-Cr solid solution annealed isothermally showed no sign of age hardening. The activation energy for decomposition was estimated to be 52.6㎉ and the thermal stability to be 0.76. The microstructural change during isothermal aging was that the fine, globular particles, which were nucleated heterogeneously along the cell boundary, were grown into chunky particles. The decomposition process was considered by establishing a cylindrical cell model and by using Johnson-Mehl equation. At the initial stage of the decomposition process the decomposition was governed by the diffusion controlled growth of a fixed number of particles (that was, the time exponent of Johnson-Mehl equation n＝3/2), and at the middle and final stage the decomposition was governed by the combination of the diffusion controlled growth of cylinders in axial direction only and growth on dislocation (that was, n＝1∼2/3).

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