발간논문

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In the spheroidal graphite cast iron, the effect of carbon and silicon on the state of graphite were examined quantitatively at different levels of carbon and silicon contents. The principal results obtained from this study are as follows: The effect of carbon and silicon content on the size, shape and nodule counts of graphite are considerable; Carbon and silicon promotes the growth of graphite, whereas silicon contributes nucleation of graphite.

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Transmission electron microscopy and kinetic analysis of Ostwald ripening of S` phase were carried out on Al-2.0%Cu-1.1%Mg alloy aged at 200℃ and 230℃. The results obtained are summarized as follows: 1) The S` precipitates, most effective on the precipitation strengthening of alloy were coarsened by the Ostwald ripening, and its size distribution was similarly uniform. 2) The diffusion coefficient of the solute atoms for the coarsening of S` phase was deduced out by the kinetic analysis as D_(Cu-Mg)=7.03×10^(-14)㎠/sec at 200℃. This is the more increased value than 10³times the diffusion coefficient of Cu and Mg in the pure Al, respectively. The results showed that the diffusion coefficients of solute atoms for the coarsening of S` phase in the alloy state of precipitation are increased more than those in the pure Al. 3) The ratio of length/width of lath shaped S` phase was increased with the increment of ageing time and temperature.

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Reduction of Malay cassiterite by hydrogen gas has been carried out to investigate the effects of hydrogen flow rate, temperature, cassiterite size, and addition of lime and silica. Reduction degree was determined by thermal balance and the reduced cassiterite was examined with microscope and X-ray diffraction. The results from this experimental work were summarized as follows. 1. Reduction was accelerated with increased hydrogen flow rate, but not so much affected above certain limit. 2. The finer cassiterite size was the higher reduction rate and the kinetic equation, r_0(1-(1-R)^(1/3)) =kt, was applied well to the reduction of cassiterite. 3. As temperature was raised, reduction speed was increased, and the activation energy for the hydrogen reduction of Malay cassiterite was 18.2 Kcal/mole. 4. Addition of lime or silica to cassiterite inhibited the reducibility of the ore.

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Superplastic behaviors have been investigated in Duralumin 2017 alloy at 280-450℃ over the strain rate range 10^(-4)/min-10^(-1)/min. The usual method of measuring the strain rate sensitivity m of superplastic materials showed that m and flow stress in 2017 alloy are dependent upon temperature, strain rate, and grain size of recrystallized grains. The flow stress of 2017 alloy at 450℃ was proportional to L^a (a∼1) at the different strain rate and the apparent activation energy for superplastic deformation was estimated to be about a half of the one for lattice diffusion of Al. It may be concluded that the structural requirements for superplasticity in 2017 Al alloy can be produced by appropriate combination of severe hot-extrusion, coldrolling, and recrystallization, and that grain boundary slip is the rate controlling process for the superplastic deformation of this alloy.

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The effect of Si content on microstructures and mechanical properties in Cu-7ωt% Al alloy has been investigated experimentally for the purpose of establishing industrial application of Cu-Al-Si alloy casting (high silicon aluminum bronze) which is employable under the severely corrosive condition. The results obtained from this study are summerized as follows. 1. As-cast microstructures of Cu- 7ωt% Al alloy are refined markedly with the addition of Si more than 1.5%. 2. Tensile strength and hardness of Cu-7ωt% Al alloy are increased from 35㎏/㎟ to 56㎏/㎟ , from HR_B 20 to HR_B 80, respectively, but elongation is decreased from 70% to 20% with the addition of 2% Si. 3. Tensile strength and hardness of α-aluminum bronze casting which has silicon content of 2% are increased linearly with increasing Al content, but elongation is decreased. 4. Tensile strength and hardness of Cu-Al-Si alloy casting are increased, but elongation is decreased with increasing cooling rate which refine the microstructures of these alloys and suppress the precipitation of γ² phase. 5. Shrinkage of Cu-7ωt% Al alloy is decreased markedely with the addition of (1.5∼2.0%) Si.

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