발간논문

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Gallium is a relatively rare element and its principal use in the manufacture of semiconducting compounds, mainly GaAs. Consequently, a large increase in the demand for gallium is anticipated in the future. Recycling of GaAs wafer scrap is required to solve the environmental problems caused by arsenic and to reutilize gallium which is an expensive metal. To recovery of gallium from the GaAs scrap, the basic study on the gallium leaching was performed with varying leaching agents and conditions. The important results were obtained such as followings; Nitric acid was the most effective leaching agent in comparison with HCl and H₂SO₄. More than 99% of Gallium was leached at conditions; 2N HNO₃, 500RPM, -100+200 mesh and at 60℃. The apparent activation energy of the leachuig reaction was 93.05 KJ/mole. The total reaction rate of this reaction was as follow : [dGa(III)/dt]_i=k′[GaAs]^(2.32)·[HNO₃]^(3.20)·Exp(-93.05/RT)

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The microstructural evolutions during the transition between the supersaturated solid solution and the solutal dendrite in rapid solidification were qualitatively examined using the model of banded structure formation proposed by Carrard et al., It was shown that the positive average temperature gradient at solid-liquid interface is a necessary condition for the banded structure formation, and the banded structure can not be formed during rapid undercooled solidification. Also, it was shown that the banded structure is hardly anticipated in melt-spun aluminum alloy and, on the other hand, it can be formed at the free surface side of the melt-spun Ag-Cu alloy ribbons. These predictions are in good agreement with the reported experimental results on banded structure formation.

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An amorhous Al_(88)Ni_(10)Nd₂ alloy was examined for its structure, thermal stability and mechanical properties at temperatures above precipitation onset temperature(Tx₁) of Al phase. The precipitates obtained were Al particles with grain diameter of about lO㎚. Tx₁ monotonously increases with increasing ageing temperature(T_a). This fact indicates an enhanced thermal stability of amorphous phase. The volume fraction of Al phase(V_f) increases with an increase of T_a, and reached a saturation value of 32% at 525K. The mixed phase alloy with V_f=18% exhibits an ultra-high tensile stength(σ_f) of 1980MPa which is about 1.6 times higher than that of the amorphous single phase alloy. A crystallization -induced plasticity was observed for an amorphous single phase or mixed phase Al_(88)Ni_(10)Nd₂ alloy. The mixed phase Al_(88)Ni_(10)Nd₂ alloy was found to exhibit considerably high value of tensile strength(σ_B) above 1000MPa in the temperature range from room temperature to 600K. On the other hand, σ_B for the mixed phase alloy was found to preserve valuees higher than that of an amorphous single phase alloy within this temperature range.

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Wear behavior of high temperature Al alloys (FVS0812 and ASCM16-5Fe) under dry condition has been investigated with variations of applied load and sliding speed at ambient (R.T) and elevated temperature (150℃). Wear rate was measured and worn surfaces and wear debris were observed to characterize the wear properties and the wear mechanism of these Al alloys. Results showed that higher wear resistance of the high temperature Al alloys comparing a conventionally cast ACBA Al alloy under the applied load, sliding speed and temperature conditions of the present study. Transitional sliding speed has been observed up to which wear rate decreased and after which increased as sliding speed increased. At R.T. wear mechanism changed from mechanical abrasive to plastic deformation controlled wear with appearance of the transition sliding speed. Wear rates of high temperature Al alloys at 150℃ were lower than those at R.T., especially at the lower sliding speed range due to oxidation layer formed at 150℃ on worn surfaces preventing the wearing of Al alloys.

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The effect of In and Be addition on the aging behaviors in Al-Cu-Li alloy has been studied with the differential scanning calorimetery(DSC) and the transmission electron microscopy(TEM). The addition of In in Al-Cu-Li alloy retards the formation of GP zone and δ′ phase, whereas it promotes the formation of T₁ and θ′ phases. However the formation of T₁ and θ′ phases as well as the formation of GP zone and δ′ phase are promoted by the addition of In and Be in Al-Cu-Li alloy.

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The surface of the polycrystalline diamond film has been flattened by the reaction with thick plate or powder of Mn and Fe at 900∼1000℃ for 0.5∼20 hours in hydrogen atmosphere. The effect of the shape and kind of the metals on the flattening process has been investigated in the perspective of the macroscopic flatness and the microscopic roughness. The macroscopically flat surface could not be obtained by Mn powder and Fe powder. In contrast. treatment with thick Fe plate gave macroscopically flat surface. However, ribbon-shaped protrusions were formed on the diamond surface flattened with Fe plate. It was due to the severe grain boundary grooving on the Fe plate surface developed during the huh-temperature reaction with diamond. The protrusion could be drastically reduced by using fine-grained sintered body of Fe. This phenomenon was interpretated in terms of a simple geometrical change involving the dihedral angle of the grainboundary groove and the grain size of the Fe plate. The microscopic roughness increased in the following order: mirror polished Fe plate, Mn powder or Mn flake, mirror-polished sintered body of Fe and Fe powder.

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The machinability of Ca-added microalloyed steel was characterized in terms of tool life, chip-breaking, and cutting force compared with that of Ca-free steel Microscopic examination revealed that inclusions have a globular shape of calcium aluminate surrounded by (Mn,Ca)S. The composition of calcium aluminate was 12CaO·7Al₂O₃+ CaO·Al₂O₃ through EPMA analysis, which had the lower melting point than alumina. The tool life for Ca-added steel was about twice as longer than that for Ca-free steel while chip-breaking and cutting force had no difference between. Based upon the above results, it is suggested that the enhancement of tool life is caused by lubrication of the inclusions, calcium aluminate, which were softened and sticked to tool surface during machining.

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The fractal geometry is a non-Euclidean geometry which describes the naturally irregular or fragmented shapes, so that it can be applied to fracture surface of a material to investigate the fracture process. The fractal dimension of irregular surface has fractal values ranging from 2 to 3, and that of irregular line profile has fractal values ranging from 1 to 2. In this investigation, this fractal geometry was appiled to the fracture surface of Al-Li alloy in order to correlate the fracture phenomena to microstructures of this alloy. Vertical section fracture profile methods were used to determine the fractal dimension, D_F. We found a correlation between fractal dimension and toughness of the alloy. More specifically, the logarithm values of toughness increased with those of (D_F-1), but not linearly when arbitrary lengths of measuring unit were used to depict the fracture line profile. However, a very good linear correlation was observed when the length of measuring unit was chosen such that its magnitude is of microstructural size. The microstructures affecting fracture processes were found to be grain size, slip step and fibering size in Al-Li alloy.

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The microstructual changes and strengthening behavior of pearlitic steel during cold drawing has been investigated. The strengthening behavior of pearlite concided with Embury-Fisher`s model with Hall-Petch constant, 1.9㎏/㎜^(372). The observed crack initiations were classified into two group. The crack initiation by necking of cementite was observed at the cementites aligned to the drawing direction. The other crack initiation was observed at the massive and round cementite particles. It could be explained by the behavior of cementite aligned perpendicular to the drawing direction. With increasing strains, the distance between buckled cementite plates were narrowed and the tip of buckled area formed massive and round cementite particles.

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