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The phase transformation of TiAl-X (X = Mo, W, Re) system at elevated temperatures was investigated by analyzing the dendritic morphologies of TiAl-X alloys and by constructing pseudo-binary phase diagrams of TiAl-2X ternary alloys. It was found that the addition of β stabilizers such as Mo, W, Re shifted the high temperature region of the binary Ti-Al phase diagram to the high Al composition side so that β phase forms as a primary crystal even at higher Al composition compared with the binary Ti-Al system. It also appeared that those β stabilizers shifted melting temperature and β phase region to low temperature and high Al composition side comparing the TiAl-2X (X = Mo, W, Re) pseudo binary phase diagram with the binary TiAl phase diagram. From the partial liquidus projections and pseudo-binary phase diagrams, the degree of `β stabilizing effect` was found to be in the following order : W$gt;Re≥Mo.

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The lamellar orientation of TiAl/Ti₃Al two phase alloy containing W was controlled to align at 0° and 45° to the growth direction by directional solidification (DS). The composition of β solidification in Ti-Al-W ternary system could be determined from the partial liquidus projection which was constructed by observing the microstructure of arc-melted buttons. The Ti-47Al-2W composition was selected for β solidification and the directional solidification was performed by using a Bridgman-type DS apparatus at the growth rate of 90 ㎜/hr. At the beginning part of the as-grown DS ingot, the columnar grains with the lamellar orientation at an angle of 0° (A orientation) and 45° (B orientation) to the growth direction were both observed, but the grains with A orientation disappeared increasingly. Finally, the columnar grains with B orientation coalesced to whole DS ingot. In the results of room temperature tensile test, Ti-47Al-2W DS alloy exhibited superior tensile elongation to the polycrystalline alloy having the same composition.

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The effects of alloying elements in the TiAl₃ coating layer are studied with respect to mechanical property and oxidation resistance. All the TiAl alloys are pack aluminized in the same condition of high activity process. The mechanical property of TiAl₃ coating layer formed on the surface by pack aluminizing depends considerably on the third alloying element in the TiAl matrix. The TiAl₃ layer formed on the TiAl alloy becomes thicker and has the finer micostructure as the content of Nb or Cr is increased. Micnhardness test reveals that addition of alloying element of Nb or Cr improves the ductility of the coating layer and thus improves the cracking resistance. Cyclic oxidation test shows that the TiAl₃ coating layer formed on the TiAl alloy has a better oxidation resistance as the content of Nb is increased. The ductility and oxidation resistance of the TiAl₃ coating layers are increased by Nb addition, which contributes to the grain refinement of TiAl₃ structure.

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The effects of third elements in the TiAl on the mechanical property and the oxidation resistance of aluminized coating layer formed on the TiAl have been studied. With the basic alloy of TiAl, the ternary alloys of TiAl-2 at%X were prepared X being Nb, Cr, Fe and V. All the specimens were pack aluminized in the same condition of high activity process. The toughness of the coating layer was evaluated by vicker`s hardness test, and the oxidation resistance was evaluated by isothermal and cyclic oxidation tests. The third element added in the TiAl affected the property of the TiAl₃ coating layer. Among the alloying elements evaluated, Nb showed the moat positive effects on the coating thickness, coating microstructure, and toughness and stability of the coating layer. The oxidation resistance of aluminized TiAl alloy was influenced by the property of coating layer. The best oxidation resistance was obtained in the aluminized TiAl alloy with Nb which has most tough and thick coating layer. However, further studies are required to clarify the exact role of Nb on the mechanism of coating process and on the mechanical property of coating layer.

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Ductile cast iron was treated with the so-called duplex surface treatment process, which is a combined plasma nitriding and oxidation process. The surface and cross-sectional microstructures and corrosion characteristics were analyzed. Potentiodynamic polarization curves were obtained at room temperature from the samples immersed in 3% NaCl solution. In case of the oxidation after nitriding, oxide layer(double layers of Fe₂O₃ and Fe₃O₄was found to be formed on the top of the compound layer(γ`- Fe₄N). The value of the measured parameter Ra(roughness average) of plasma nitrided sample was higher than that of plasma oxidation treated sample. The untreated sample showed typical graphitic corrosion, and the plasma treatment was found to increase the corrosion resistance.

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Critical to the electric arc spray forming(EASF) including Osprey forming and plasma spray forming is the thermal condition of the deposit, which determines the microstructure and mechanical properties of the deposits. In the EASF of Fe-0.06%C steel ring, the microhardness of the deposit varied in the range from 160 through 230 Hv(0.2) with various EASF conditions. It was found out that the microhardness of the deposit was closely connected with the thermal condition of the deposit. Temperature profiles in an EAS formed ring of low carbon steel(Fe-0.06%C) has been calculated by using a growth model based on the vector calculus and heat transfer model based on the body fitted coordinate method. The effects of EASF parameters such as substrate rotation velocity, droplet flow rate and average droplet temperature on the preform temperature profile were investigated. The calculation results were compared with experimentals. Fe-0.06%C steel was arc-melted and sprayed onto a rotating tubular steel substrate. During spray forming, the surface temperature of the deposit was measured using an infrared thermal imaging camera that was accurately calibrated. The effect of deposition temperature on the deposit microstructure was also investigated. The calculation results showed a good agreement with experimental results.

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During the electric arc spray forming(EASF) including Osprey forming, thermal spray forming and plasma spray forming, critical is the thermal condition of the deposit, especially at the deposition domain (footprint), where droplets impinge on a substrate or surface to form a deposit. Many experimentals show that the microstructure of the deposit depends on the thermal conditions at the footprint. In thin paper, microscopic model was presented to describe the splat boundary coalescence at the footprint, which estimates the fractional quantity of splat boundary coalescence at the footprint under the assumptions of average droplet size impinging on the deposit surface. This model was adopted to describe the microstructural evolution of the Fe-0.06%C steel ring EAS formed with a law substrate rotation velocity. The contents of splat boundary coalescence of the deposit, EAS formed on the substrate with a high rotation velocity or on the stationary substrate, were also estimated using the quantitative model.

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The purpose of this research is to find out the dominant factor determining exchange biased field and characteristics of Ni-Fe/Mn-Ir/buffer/Si bottom structure. We found out that the H_(ex) and the H_c of Ni-Fe/Mn-Ir multilayers depended on the grain size of Mn-Ir layer and the grain-to-grain epitaxy relationship at the interface between Mn-Ir and Ni-Fe layers. The blocking temperature, at which the exchange biased field disappears, of top structure was about 220℃ and that of bottom structure was about 180℃. The grain size of Mn-Ir layer in bottom structure is smaller than that in top structure. The bottom structure has more magnetic dead layer and less alignment of spin of the Mn-Ir layer than top structure.

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Schottky enhancement was demonstrated experimentally by using MESFET with Ni/Al/Ni gate. Forward I-V characteristics showed that Schottky barrier height increased from 0.77 eV to 0.89 eV when the contacts were annealed at 350℃ for 15 min. From XRD analysis, we found that the ternary phases, Ni_xGaAs, were formed at 200℃ and NiAl₃ and Ni₂Al₃ phase were dominant at 325℃. As the Sohottky barrier height increased, current density level deceased at same gate voltage and the maximum transconductance, ｇ_m , was shifted to the higher gate voltage. In addition, pinch-off voltage and threshold voltage were increased by 0.1 voltage.

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Atomic bonding states of the surface of (NH₄)₂S_x-treated AlGaAs and their change with annealing temperature were investigated by synchrotron radiation photoemission spectroscopy. The surface of AlGaAs sample was passivated using (NH₄)₂S_x solution after the removal of native oxide by NH₄OH solution. This (NH₄)₄S_x passivation was effective in removing Ga-O and As-O bondings, and resulted in the formation of Ga-S, As-S and Al-S bondings. Sulfur atoms migrated from As and Ga to Al atoms when the sample was annealed in ultra-high vacuum. The valence band spectra for the (NH₄)₂S_x treated AlGaAs surface showed that the Fermi level shifted by 0.21 eV after annealing at 550℃. This resulted from the increase of Ga or Al vacancies which energy states located near the valence band maximum.

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