발간논문

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Alumina dispersed copper alloy was manufactured as a material requiring high temperature strength, endurance to thermal fatigue, and good thermal and electrical conductivity. The material was fabricated by mixing, compaction, sintering at hydrogen atmosphere, and hot pressing, where the size of Cu powder was 3∼5㎛ and those of Al₂O₃powder were 3∼5㎛, 0.06 and 0.30㎛. The amount of alumina in the specimen varied from 0.5 to 5 percent. The optimum sintering was obtained at 1,000℃ for 1hr, and micro vickers hardness at room temperature was maximum at Cu-3.0w/o Al₂O₃. Dispersion strengthening effects were found considerable at high temperature. When the amount of alumina in the dispersion strengthened alloy increase more than 3%, the hardness and strength decreased due to coalescence of dispersion, and decrease in density. Cu-3.0 w/o Al₂O₃was discovered to increase the tensile strength by 67% compared with pure Cu.

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The magnetic properties and microstructure of sintered iron-phosphorus and iron-silicon-phosphorus compacts have been investigated. Measurements of maximum permeability (μ_max) and coercive force (Hc) were performed with a hysteresisgraph tracer. The densification rate of iron powder with phosphorus was greater than that of pure iron powder. At the same sintering temperature, the increased phosphorus content in iron caused B_(10), Br, μ_(max) to increase and He to decrease. In Fe-3w/o Si-P system, the addition of phosphorus enhanced the sinterability but decreased μ-max and increased Hc. These phenomena were discussed in terms of pore distribution, the formation of secondary phases in the grain and the formation of amorphous phase in the grain boundary. The resistivities of sintered compacts were increased with the increase of the amount of phosphorus in both the Fe-P and Fe-3w/o Si-P systems.

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To obtain suitable process conditions for diffusion bonding of two types of steel, Hot-pressure bonding and Hot-roll bonding techniques are studied. High carbon steel and low carbon steel are bonded by Hot-pressure bonding and Hot-roll bonding with Auto-vac. cleaning process. GTAW welding method was employed to seal the bond surface in Hot-pressure bonding and SMAW welding was employed in Hot-roll bonding. The bond strength was increased by the alloying effect of the diffusion of carbon. In Hot-pressure bonding, the bond strength was increased with the bonding temperature and it was related to bonding time by the following equation. ln(bond strength)＝A·t^(½)＋B In Hot-roll bonding, the bond strength was related to rolling reduction ratio(R) by the following equation. bond strength＝C·R(2-R) The bonding process variables to obtain suitable bond strength are summarized and the possible bond mechanisms are discussed.

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Sievert`s type apparatus using high pressure autoclave was employed to study the hydrogen storage properties of intermetallic compound FeTi. The effect of the alloying element, Mn, on the hydrogen storage properties of FeTi alloy was investigated by substituting Mn for Fe in FeTi alloy. The alloy Fe_(0.98)Ti_(1.02) reacted directly with hydrogen to form, in succession, hydrides of the compositions Fe_(0.98)Ti_(1.02)H_(1.04) and Fe_(0.98)Ti_(1.02)H_(1.92) at 0℃. Both hydrides had dissociation pressures of over 1 atmosphere at 0℃. The relative partial molal enthalpies of hydrogen had the values of －3.11 ㎉/g.atom at the composition of Fe_(0.98)Ti_(1.02)H_(0.8) and －3.89 ㎉/g.atom at the composition of Fe_(0.98)Ti_(1.02)H_(1.4). The hysteresis effect was observed, the absorption isotherm of pressure vs. composition being about 10 atmospheres higher than the desorption isotherm at the composition of Fe_(0.98)Ti_(1.02)H_(0.8). The Mn alloyed specimen tended to be more easily cracked than FeTi alloy after hydriding. But the addition of Mn in FeTi alloy faciliated activation process without seriously reducing the hydrogen storage capacity of this alloy.

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The effect of Si addition on the stress corrosion cracking (SCC) of austenitic stainless steel was studied in the boiling aqueous solution of 42% MgCl₂. Failutre time, under constant load, remained nearly constant for the specimen with Si content up to 1.42 wt.%, however, it increased drastically as Si content increased from 1.42 to 2.21 wt.%, and further increase in Si content did not affect the failure time. Metallographic examination showed that the specimen with Si <1.42 wt.% and Si> 2.21 wt.% had austenitic single phase and austenitic-ferritic duplex phase respectively. The increased failure time caused by Si addition was explained in terms of retardation of SCC growth due to ferrite phase and formation of stable passive film. No change in pitting potential(breakdown) was observed for both unstressed and stressed specimen regardless of its Si content respectively. The stressed specimen showed lower (more anodic) pitting potential than the unstressed. Critical passivation current density (critical anodic partial current density), in pass estimated from electrochemical polarization curve and weight loss measurement decreased with Si addition and higher passivation current density was observed with the stressed specimen compared with the unstressed.

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Alkaline leaching of an uranium containing coaly shale in Okchon Series was investigated under the various oxygen partial pressure and different alkaline concentrations. The relationship between oxygen partial pressure, P, and uranium leachability, X, was determined as follows: X＝kP^(0.34), X＝kp^(0.11) for carbonate and carbonate-bicarbonate solutions, respectively, where k is an overall rate constant. As the carbonate concentration increased the leachability of uranium increased up to 80 %. With the mixture of carbonate-bicarbonate solutions higher leachability was obtained.

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