발간논문

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Aluminum alloy matrix composites containing 1 or 3 w/o of Al₂O₃ and SiC particles range in size from 0.3㎛ to 53㎛ were prepared by "Compocasting process." The non-metallic particles were uniformly distributed in the cast composites. The cast composite ingots can be fabricated to thick plate with 6㎜ thickness by hot forging and rolling, and the non-metallic particles initially uniformly distributed in matrix alloy were aligned along the working direction. Tensile tests conducted at room temperature and 250℃ show that in samples with 0.3㎛, l㎛ Al₂O₃ both yield and tensile strengths have increased, but in samples with 38㎛, 58㎛ SiC only high temperature tensile strength has increased over those of matrix alloy. Especially, the increase of room temperature yield strength and high temperature UTS was noticeable.

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The kinetics of desul phurization of carbon saturated iron by CaO-MgO-SiO₂ slag was studied at 1460℃ under CO gas atmosphere varying the slag composition, melt geometry, Si and MnO additions, and ambient pressure. It was shown that the desulphurization reaction is not controlled by either diffusion of sulphur in the metal or the slag but by an electrochemical reaction. Desulphurization takes place in three different reaction stages; the intial fastest stage is dependent anodic reaction of iron transfer and CO evolution, the second medium rate stage is controlled by CO evolution, and the final slowest stage controlled by silica reduction from the silicate slag.

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Reduction kinetics of synthetic hematite has been studied in hydrogen and carbon monoxide atmosphere. The morphology of reduced layer was throughly investigated by means of optical and electronic microscopic technique. A general kinetic formula incorporated with the morphology was suggested, and computer simulation of reduction kinetics was attempted. Results withdrawn from experiments are as follows; 1) At temperatures lower than 700℃, the reduction is controlled by the pore-diffusion process, while at higher temperatures by oxygen diffusion process through oxide matrix. It was also found that overall reduction rate of iron oxide was controlled by the reaction stage wustite to iron. 2) Relative size of pore as well as number play an important role in the reduction process. Those relations could be well expressed by Pepper`s empirical equation Δε∝t^0.4 where Δε and t are changes of porosity and reaction time, respectively. 3) The parallel kinetic model in which both chemical reaction and gaseous diffusion process proceed in a parallel fashion was found to be the most adequate one, describing for the present system. The model equation was solved by means of Runge Kutta method. For boundary conditions given, the model equation was proved to be satisfactory, especially in the reduction of iron oxide by hydrogen.

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The Production of materials with composite structure from off-eutectic alloys was investigated. In this work, commercially pure aluminum-copper alloys with compositions on the aluminum-rich side of the eutectic point were solidified vertically under the conditions of controlled crystal growth rate (R) and temperature gradient (G). The value of G/R ratio was between 10²∼10⁴℃/㎠/sec. In the range of the composition from 33 to 33.5 wt. percent copper, `coupled growth` was fully achieved even when the value of G/R approached to zero. The relationship between the interlamellar spacing (λ) within the colony structure and the crystal growth rate was represented by λ²R=1.8x10^(-10)㎤/sec. And the off-eutectic alloys which exhibit eutectic or colony structure behaved as reinforcing composite materials, with the increasing fracture strength and fracture strain as the interlamellar spacing decreases.

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An investigation into the effect of A₃-transformation on sintering has been carried out for the compacts prepared from powder s of pure Fe and Fe+Cu mixture, and for the Cu-infiltrated Fe powder compacts. The approach has been to employ the techniques of density and porosity measurement, and metallographic examination for the specimens sintered at various stages under isothermal (at 895℃: α-range and 925℃: γ-range) and non-isothermal (cyclic between 895℃ and 925℃) conditions. Results obtained can be summarized as follows; (1) For pure Fe powder compacts, the repetition of A₃-transformation during sintering caused a negative effect on densification while isothermal heating at the temperature of α-range showed a relatively high densification. Isothermal sintering at γ-range, however, was found to be similar to that of cyclic sintering. (2) In the cases for mixed and infiltrated specimens, densification was considerably improved by the cyclic heating as compared to either of isothermal sintering at α-or γ-range. This enhancement in densification was high especially for the case of infiltrated specimen. The above mentioned negative effect on densification is believed to be resulted from the irreversible destruction of contact points due to the volume changes at the moments of allotropic transformation α⇔γ, and the lack of a suitable constraint to maintain or to improve sintering condition during the cyclic treatment. And the improved sinterability upon the cyclic heating for mixed and infiltrated specimens can be explained as follows: (1) Bonding between Fe and Cu at early stage could provide the proper constraint to the particles in a compact, (2) under this constraint, particles in the sinter mass may experience the repeating local stress at the contact points, and (3) these stresses could be responsible for the transformational superplasticity and also for the enhanced diffusion mechanisms, such as grain-boundary-short-circuiting, viscous creep, and so on.

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