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The Y-coated Ni₃Al with post heat treatment shows much better oxidation resistance than the Y-coated Ni₃Al without post heat treatment. In order to explain the effect of post heat treatment at low oxygen pressure, post heat treatment after Y-ion plating is performed in flowing hydrogen as a function of time. During post heat treatment, the Y-coated layer is modified into (Y, Al)O-type oxide by reaction between Y₂O₃, and Al₂O₃. Thickness of the modified Y-layer is related to oxygen pressure and time. The Y-modified layer formed during post heat treatment acts as a barrier to the transport of oxygen. The fine (Y,Al)O-type oxide can relieve easily growth stress by permitting easy plastic deformation, and can absorb the thermal stress developed in the Al₂O₃ layer. The tensile stress generated by the difference in thermal expansion coefficient of the (Y,Al)O-type oxide and Al₂O₃, layer compensates the large compressive stress generated by the difference in thermal expansion coefficient of the Al₂O₃layer and Ni3Al alloy.

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Inconel alloys which have good high temperature mechanical properties and corrosion resistance have been used extensively as steam generator tube of nuclear power plants. There have been some reports on the intergranular stress corrosion cracking (IGSCC) failure problems in steam generator tubes of nuclear reactors. In order to evaluate the effects of heat treatment and composition on the IGSCC behavior of Inconel alloys in simulated nuclear reactor environment, four different specimens(Inconel 600 MA, 600TT, 690 MA and 690 TT) were prepared and tested by electrochemical method. Static potential tests for stressed C-ring type Inconel specimens were carried out in 10% NaOH solution at 300℃(75atm). It was found that IGSCC was initiated in Inconel 600 MA specimen, but the other three specimens were not cracked. Based on the gradients of corrosion current density of the four specimens as a function of test time, thermally treated alloys show better IGSCC resistance than mill-annealed alloys, and Inconel 690TT has better passivation characteristic than Inconel 600 MA. Inconel 690 TT shows clear periodic passivation that indicates good SCC resistance. The good IGSCC resistance of Inconel 690TT is due to periodic passivation characteristics of surface layer.

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The effect of supercooling on surface morphology of garnet single crystal films grown by liquid phase epitaxy(LPE) using a PbO/B₂O₃ flux was investigated. At the degree of supercooling smaller than 52℃, triangular hillock which has atomically smooth plane of {100} was observed. Mirror face with roughness less than 0.1㎛ was obtained at the supercooling more than 132℃. Faceting and mirror face, or faceting and hillocks were occurred simultaneously at the range of supercooling of 112℃-72℃. The change of surface morphology from hillock to mirror face with increasing supercooling is related with decreasing lattice mismatch between substrate and film, and the change of growth mechanism with increasing supercooling. Surface morphology changed from 3 dimensional nucleation and growth into 2 dimensional nucleation and lateral growth with increasing the supercooling.

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This paper is a second part of the study on the effects of processing parameters on characteristics of the laser surface alloyed chromium carbide layer on the stainless steel. This Part II correlates surface properties (hardness, erosion and corrosion) with microstructures of alloyed layer. Further, a possible operation mechanism in particle erosion of the substrate and alloyed layer is also proposed based on the observation of erroded surface. Microstructures of the laser alloyed layer consisted of three different layers. In the layer I , adjacent to the bond line, applied Cr₃C₂ completely dissolved and mixed with the melted substrate and formed C saturated austenite matrix and carbides (Cr_7C₃ and Cr_(23)C_6) along dendrite boundaries during rapid cooling. In the layer II which is a mid-region of the alloyed layer, Cr₃C₂ also dissolved but did not appear to mix with the substrate completely, and transformed as metastable phase of M₃C₂ during cooling, In the layer III near surface, some of the Cr₃C₂ were not dissolved and embedded in the C, Fe enriched austenitic solid solution. Erosion and Cl^- corrosion resistance of the laser alloyed layer was found to be far superior to that of martensitic strainless steel substrate and plasma coating. Small amount of pores in the Layer III did not influence on the pitting corrosion resistance.

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Lap joint welding of 1.2 ㎜ thick Inconel 600 plate was performed by Nd:YAG laser. Welding parameters such as pulse width, repetition rate, peak power and scanning speed, were varied in this experiment. The optimum welding condition observed were 6-12ms pulse width, 15-50㎐ repetition rate, 1.5-3.0㎾ peak power and 288-360㎜/min scanning speed. Cracks were not observed in the weldment. An aspect ratio of 0.7-1.4 was obtained by means of controlling the above parameters. Microscopic analysis revealed that the weldment consisted of a planar front solidification zone which formed at the initial stage of solidification, and a cellular-dendrite solidification zone produced by rapid cooling. Epitaxial growth from base metal was also observed. Post-welding heat treatment was performed in order to relieve residual stresses induced by welding. Results obtained from the Huey test and ABI test, the optimum heat treatment condition was observed at 5 min. annealing at 640-700℃. It was found that the susceptibility to intergranular corrosion attack of the weldment was approximately three times higher than that of the base metal and the base metal corroded more severely than the weldment. This occurrence is believed to be caused by the sensitization of the base metal by heat input during welding. It was also observed that the intergranular corrosion rate was also accelerated as heat treatment temperature was increased. ABI tests showed that residual tensile stress was present in the weldment and was subsequently relieved by post-welding heat treatment.

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In a smelting reduction furnace, slag viscosity has much influence on the smooth tapping operation. In this study, the high temperature properties of slags sampled from a melter-gasifier were examined by the viscosity and melting temperature measurements. The effects of chemical composition changes and high temperature slag phases were considered. The effect of char or TiN as a second phase on the slag viscosity was also discussed. In the ironmaking slag system such as the CaO-MgO-SiO₂-Al₂O₃, the slag temperature played an important role in high temperature slag properties in terms of slag viscosity. The modified basicity, i.e., [(CaO+MgO)/(SiO₂+Al₂O₃)] turned out to be a better guidance for slag chemistry control, compared with the conventional basicies like CaO/SiO₂ and (CaO+ MgO)/SiO₂.

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The objective of this study is to find out the optimum conditions and characteristic of zinc-nickel alloy electrodeposition in an alkaine bath. The results show that it is known from the polarization curve that nickel in the zinc-nickel bath is deposited at -1.0V. Also, increasing the nickel concentration in the bath increases the nickel content in the alloy deposit. Increase in current density from 0.2 to 0.5A/d㎡ decreases the nickel content in the alloy deposit, significantly. Electroplating of zinc-nickel alloy in an alkaine bath appears anomalous codeposition, zinc being the less noble metal deposit in preference to the more ncble nickel. Temperature within the range of 50℃ has no appreciable effect on the content of nickel in the alloy deposit. But the nickel content in the alloy deposit increase dramatically in the range of more than 60℃. Generally, the nickel content of alloy deposit obtained from an alkaline bath maintains below 10 wt% under the following optimum conditions; Zn conc. 10g/l·Ni conc. 0.8-1g/l·Triethanolamine 40g/l·Room temperature Current density 6-10A/dm₂.

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Ti_5Si₃, was synthesized by arc melting of Ti and Si elemental powders. The synthesized Ti_5Si₃ buttons were milled and sieved up to -325 mesh size powders, which were densified using hot isostatic pressing. Microcracks in larger grains were observed for the HIPed specimens with initial powder size of -325 mesh. Six hour milling of -325 mesh powder resulted in fine grain size (about 1-2㎛ in diameter) with no microcracks after HIPing. The thermal expansion coefficients along the a and c axis were measured as a function of temperature using high temperature X-ray diffractometry. The anisotropy in the thermal expansion coefficient is believed to be one of the major causes for microcracks in larger grains. Mechanical properties, such as hardness and fracture toughness, were measured for HIPed specimens. The oxidation behaviors of consolidated Ti_5Si₃, were also investigated in terms of weight changes at elevated temperatures in air.

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Heat transfer, residual stresses and deformations during a multi-pass butt-welding of natural gas(NG) pipes have been analyzed using a 2-dimensional thermo-elastoplastic finite element method. A double ellipsoidal power density distribution model has been used to take into account the heat distribution by a moving arc, and an element birth technique has keen also used to model the multi-pass welding. The calculated sizes of FZ and HAZ are in good agreement with the experimentally measured one. The maximum residual effective stresses and effective plastic strain are found to be developed in the boundary between FZ and HAZ. The hoop residual stresses of inner surface of pipe are determined at the 1 and 2-pass of welding and those of outer surface are determined at the 3-pass of welding.

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