발간논문

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We developed the Biased Double D. C. Arc Plasma apparatus and synthesized diamond thin films on silicon wafer using it. We named this process Biased Double Arc Plasma CVD process (B. D. A. Plasma CVD). It was possible to obtain a high growth rate of diamond films because of the effective decomposition of reactant by applying bias voltage between plasma and substrate and also, the degree of freedom of the selection of reaction conditions to get a good diamond films was broadened compare with the HFCVD on MWCVD process.

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For SOI(semiconductor on insulator) application, recrystallization of poly-Si thin films by ZMR(zone melting recrystallization) was carried out with the halogen lamp line heating system In this study, the effects of scanning speed, energy density, and the existence of the single crystal seed on ZMR were mainly investigated. Several mm sized grains containing many subgrain boundaries aligned along the scanning direction were obtained at the scanning speed of 1∼2㎜/sec and halogen lamp power of 2500W, which was identified with an optical microscope and the cross-sectional TEM. As the scanning speed was increased the grain size was slightly increased. Through thin film X-ray diffraction analysis, development of (100) texture after ZMR was found and the specimen with the single crystal seed showed very strong development of the texture.

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In recent years, manufacturing processes for the Fe-25Cr-20Ni-0.4C heat resistant alloy such as hot rolling and hot extrusion, were tried to improve the shape and surface quality of the centrifugally casted product although these hot worked ones have poor creep-rupture properties. So heat treatment and addition of alloying element were needed to improve the poor creep-rupture properties of the hot worked product. The effects of heat treating temperatures and addition of Ti on the creep-rupture properties of the hot worked alloy were investigated. The results obtained are as follows : 1) The times to rupture of the alloy heat treated at 1250℃ were greatly increased than those of hot worked one due to the change of grain size and distrubution of M_(23)C_6 at the grain boundary. 2) Heat treating at 1290℃ was more effective to increasing of the rupture time than those at 1250`C due to the formation of M_7C₃ carbide at the grain boundary besides M_(23)C_6. 3) The fracture modes of hot worked and solution treated alloy at 1250℃ were mixture of transgranular and intergranular. But those of solution treated alloy at 12901℃ were intergranular. 4)Titanium is a useful element to increase the rupture time of the alloy due to the formation of TiC at the grain boundary.

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The effects of Hf on microstructures and high temperature low cycle fatigue(LCF) behavior of directionally solidified Rene 80 (DS Rene 80) alloy were studied. LCF tests were conducted at 760℃, 871℃ and 982℃, respectively. The fatigue properties of Elf -modified DS Rene 80H alloy were superior to those of DS Rene 80 alloy under all experimental conditions. The beneficial effect of Elf was found to be twofold : it increased the resistance to crack propagation by changing the morphology of carbides from script-type to blocky-type, and suppressed the crack initiation by increasing the resistance to oxidation. Cyclic softening was observed during high temperature LCF tests. At 760℃, the mechanism for the cyclic softening was disordering of γ` preciptates due to dislocation shearing; at 982℃, it appeared to be mainly the loss of coherency between matrix and T`s and partly the coarsening of γ`s.

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Effects of bending temperature and thermal cycle on SME in Fe-X%Mn alloy were studied. As the bending temperature was decreased from room temperature to-196℃, the SME of Fe-17%Mn, Fe-21%Mn, and Fe-24%Mn alloys were increased. The reason is considered that the mechanical driving force required for stress induced a martensite transformation is lowered due to the increase in chemical driving force for the martensite transformation in accordance with decrease in the bending temperature. However, since Fe-28%Mn alloy is stable, the SME of the alloy was very small regardless of the bending temperature up to-196℃. The SME was increased with increase in number of thermal cycle, showing a peak value at around 5 cycles, and then decreased with further thermal cycle. This effect of thermal cycle on the SME became greater with decrease in Mn content. After the 17%Mn alloy was controlled to have various a martensite contents by holding at temperatures below Ms temperature, the SME of the alloy was measured with respect to a martensite content after bending at 170℃ and heating to 350℃. The measured SME of the alloy showed tendency to increase with increase in a martensite content. From this result, it is concluded that the presence of more or less a martensite content improves the SME at high bending temperature such as 170℃, because the yield strength of γ is increased with the presence of some ε martensite content and the usual slip off {111} $lt;110$gt;γ is γ estrained during bending.

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Soft magnetic properties of 6.5wt% Si-Fe alloy prepared by single-roll quenching technique have been studied. As surface speed of roll decreases columnar cells are formed perpendicular to ribbon surface and size of the cells becomes coarse. Thickness of sample decreases with increasing surface speed of roll. Experimental equation of thickness versus surface speed of roll may be written as D=S^(-(0.5∼0.6)), Coercive force and power loss measured at 60㎐ increase with increasing surface speed of roll. However, power loss measured at high frequencies, i.e. above 100㎐, is smaller than that at 60㎐, when surface speed of roll increases.

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Pseudoelasticity and cyclic deformation effect in Ni-Ti alloy were studied. A method of predicting σ_M and σ_R - stress for martensitic and reverse transformation above Af temperature, respectively - by the ΔH and the temperature of transformation was newly proposed using Clausius-Clapeyron equation. By cyclic deformation above Af temperature, the σ_M and σ_R decreased due to anisotropic stress field of the dislocations generated during the deformation. With Δσ_M-the quantity of decreased in σ_M-three to four times larger than Δσ_R, Δσ_M increased with test temperature. But Δσ_M did not change with varying the amount of deformation strain. Heat treatment at 300℃ for 30sec after cyclic deformation could recover its original σ_M and σ_R by eliminating the stress field of the dislocations.

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The effect of β-heat treatment on the nodular corrosion behavior of Zircaloy-4 alloy was investigated from autoclave tests at 500℃ under 1500psi for the specimens ,β-treated at 1050℃ and cooled in various cooling processes such as liced brine, water, oil, air and furnace cooling. It was observed that the corrosion resistance of β-treated Zircaloy-4 specimens increased with increase in cooling rate, showing the worst corrosion resistance in the furnace-cooled specimen resulting in nodular corrosion. It was also found that the size of nodule on the surface of specimens increased with increase in exposure time in the autoclave, but the number of nodule remained unchanged. From the SEM observation in microstructure of oxide, precipitate and alloy elements, it was also observed that the corrosion resistance decreased with increase in the size of precipitate, and the corrosion of furnace-cooled specimens progressed mostly in the interior of grains where Fe and Cr alloying elements were largely depleted during the cooling process. However, the grain boundary showed relatively high corrosion resistance due to higher concentration of alloying elements. From combining the present results with other works, it is suggested that the nodules nucleate in the local region where some of alloying elements are depleted.

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A generalized treatment has been developed which determines an electrode potential in KCI-LiCl eutectic melt at higher temperatures. Entropy values for electrode reactions at 298° K could be extended to those for 723°K by using the correspondence principle. The entropy and free energy values enable one to determine electrode potentials in aqueous system at 723°K. The potentials were linearly related to experimental electrode potentials measured in the eutectic melt at the same temperature. The resulting equation becomes, E°_(exp)=0.751 E°_(cal) +0.332 where E°_(exp) and E°_(cal) stand for the experimental and theoretical electrode potentials, respectively. The potential equation was found to be applicable for the prediction of electrode potential in KCl-LiCl eutectic melt at 723°K in a reasonable accuracy.

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The steadite enveloping heat treatement and wear test were carried out in order to investigate the distribution of steadite and wear characteristics of phosphorus contained ductile cast iron. The steadite existed among eutectic cell boundaries was moved to the graphite and appeared to envelop graphite at the optimum steadite enveloping heat treatment condition of 1050℃ for 30 min. In the case of specimens not treated with steadite enveloping heat treatment, the maximum point of wear loss was moved to high velocity side with increasing phosphorus content, while the maximum point of wear loss did not appear clearly for the steadite enveloping heat treated specimens. The steadite enveloping heat treated specimen showed superior wear characteristics than the specimen not treated, due to the rein- forcement of hard steadite exists around soft graphite.

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