발간논문

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As-cast state of Al-7% Mg alloy was prepared by casting the melt into metallic mold. Then β-phase solution behaviors and mechanical properties of this alloy have been investigated. The activation energy of β-phase solution into matrix is about 25 ㎉/㏖, independent of β-phase content. This value is in good agreement with activation energy for diffusion of magnesium in Al-Mg alloy, and hence the solution process of β-phase is considered to be controlled by diffusion of magnesium in the alloy. Ultimate tensile strength increases linearly with decreasing β-phase content. Elongation, however, increases with decreasing β-phase content up to about 50%β, and then shows a nearly constant value, regardless of β content below 50% β. Impact toughness increases with decreasing β-phase content up to about 50%β, and then keeps a nearly constant value, irrespective of β content below 50% β, in the same way as was exhibited in the elongation. It is noted that the impact toughness shows a same value at the same level of β-phase content, irrespective of solution treatment temperature.

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The formation of magnesium hydride by the reaction of high pressure hydrogen (23 to-27 atm) with magnesium and Mg-25Al alloy powdered was studied in the temperature range of 346℃ to 400℃. And reaction products were identified by using a X-ray diffractometer. For both magnesium and Mg-25Al alloy, hydride phase formed was found to exist in the form of MgH₂ only. Yields (weight percent of magnesium reacted) were increased with the decrease of average particle size of the sample, and were much higher for Mg-25Al Alloy than for magnesium. Decompositions of magnesium hydride and Mg-25Al alloy hydride were also carride out under the dynamic argon atmosphere using a thermogravimetric technique, in the temperature range of 287℃ to 397℃. In both cases, the reaction rates of the initial stage were controlled by the chemical reaction at the interface. The rate equation for the initial stage was expressed, in terms of the fraction decomposed X and reaction time t, as 1-(1-X)=k·t. The apparent activation energies for the decomposition reactions of magnesium hydride and Mg-25Al alloy hydride were 19.5 Kcal/mole and 15.9 Kcal/mole, respectively. And the activation energies for the dehydriding reactions in Mg-Al alloys were considered to decrease with aluminum content.

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An investigation has been performed to compare the effects of Co-and Ni-addition on the shrinkage behavior, the hardness, the contact resistance and the contact erosion of W-Cu contact materials in order to obtain a contact material of a lower contact resistance and higher arc resistance. For a smaller amount of liquid phase Co seems to be considerably more effective in promoting shrinkage than Ni, while for the higher Cu concentration the beneficial effect of both dopants is negligible. After the infiltration process the large volume shrinkage was found in all specimens and their densities were between 96% and 97% of the theoretical density. The hardness of W-Cu-Co alloy showed the higher value than that of W-Cu-Ni or that of simple W-Cu. The W-Cu-Co system showed much lower value of the contact resistance and the contact erosion than the other systems. Therefore Co can substitute for Ni in W-Cu-Ni electrical contact.

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The martensitic transformation temperature is believed to be lowered when Co is substituted for Ni in NiTi intermetallic compound. Therefore the transformation temperature can be lowered without change in its good properties by substituting a small amount of Co for Ni. To obtain the extent of decrease in transformation temperature caused by Co addition, NiTi and those with 3% and 5% Co were prepared by induction melting in Ar atmosphere. After machining, they were studied by means of, dilatation and electrical resistivity measurement through a temperature range of -100 to +150℃. It is certain that the martensitic transformation temperature is lowered by a small amount of Co addition and it is lowered by 20℃ with the increase of 1% of Co concentration. It is also believed that there exits an intermediate phase when Co is added.

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The reduction of Nickel oxide powder mixed with the graphite in solid state has been studied in the temperature range of 820-920℃ by Thermogravimetric method under the flowing CO₂ and N₂ gas. The reduction rate shows rapid increase suddenly after initial slow rate in CO₂ gas flowing system. This rapid increase of reduction rate has been explained in terms of cyclic reduction due to the generation of CO gas and autocatalytic effect of the product Ni powder. The similar pattern of the reduction rate in less than stoichiometric carbon content in the N₂ gas flowing system has been observed in the later part of the reduction. The mechanism of the anomalous reaction rate has been also explained in terms of reaction contact area and the cyclic reduction of CO generation. The activation energy of the N₂ gas flowing system has been determined to be about 55±5 ㎉/mole.

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The present investigation was undertaken to study the effect of centrifugal force during centrifuged casting on the soundness of Al alloy investment castings, including studies on gating system, pouring temperative, age hardening, etc. The effect were evaluated by the test of specific gravity, microstructure and mechanical properties of Al alloy investment castings. The alloy selected for this investigation were three Al-base alloys: Al-10%Mg, Al-4%Cu & Al-7%Si. These pre-alloyed pigs were melted in electric resistance crucible furnace and each melt was degassed thoroughly with chlorine before pouring. On the basis of these experimental results, the conclusion as follows can be drawn. 1. Down-tapered sprue would be more effective to eliminate the turbulance of metal stream and non-metalic inclusion. 2. Surface smootheness and dimensional accuracy of Al-alloy centrifugal castings were much better than the gravity investment castings, but they were much more effected by finess of investment material than centrifugal force in centrifuged casting of Al-base alloys. 3. The soundness and mechanical properties was promoted with increase of centrifugal force in Al-alloy investment castings. 4. The optimum pouring temperature was found to be 720℃. 5. Age hardening was accelerated with applied centrifugal force up to 50G.

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In the spheroidal graphite cast iron, the effects of graphite size were studied on the mechanical properties such as tensile strength, yield strength, elongation, reduction of area and hardness. The important results obtained are as follows. 1. As the graphite size increased, the tensile strength decreased lineally little by little in case of less than 3.0% of Si and varied little in case of 3.0% and 3.3% of Si. For example, with 0.1㎜ increase of the graphite size in case of 1.8% of Si, the tensile strength decreased as much as 0.4 kg/㎟. 2. The elongation incresed in proportion the increase of graphite size. For example, with 0.1㎜ increase of the graphite size, the elongation increased as much as 0.5% in case of 1.8% of Si. 3. The graphite size did not influence the hardness of the spheroidal graphite cast iron which had the same matrix structure and composition and the Brinell hardness was about 148 in case of 1.8% of Si.

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