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To clarify the effect of centrifugal force in unidirectionally solidified metals, Al-0.7wt% Cu alloy system was selected as test material. Centrifugal force was added from 1G(G: territorial acceleration=9.8m/s²) to 20G ; test pieces were quenched at about 50% solid fraction. Planar interface morphology became unstable with increasing centrifugal force, but at the centrifugal force is about 15G, the interface morphology of this alloy became stable again. In order to understand this interface stability under the high centrifugal force apparent diffusion coefficient were measured and also flow modes over solid-liquid interface were analyzed by Favier Method. The Be´rnard cell formed by centrifugal force enhances the heat flow from top of test piece to solid-liquid interface but it and gravitational segregation also restrict enhancement of diffusion in some extant. We could infer qualitatively that solid-liquid interface stability was due to this enhancement of heat flux by centrifugal force at solid-liquid interface.

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This study is to investigate the effect of load ratio on fatigue crack of die-cast SiC particulate aluminum composites. Fundamental fatigue crack propagation tests were performed with sub-sized C-T specimens at a load ratio of 0.1, 0.3 and 0.5 respectively. Crack closure measurements were conducted for explaning the effect of load ratio on fatigue crack growth. The experimental results showed that the fatigue crack growth rate increases with the load ratio, especially at near threshold. The composite reinforced with 20 vol.% SiC particles was better in fatigue crack growth resistance than that with 10 vol.% SiC particles. Through the measurement of Kop and ΔK at various R ratios the concept of effective stress intensity factor range ratio. U was reviewed to evaluate the stress ratio effect on fatigue crack growth. Relationships between U and variables such as ΔK and R were obtained empirically so that we could predict ΔK_(eff) that is of critical importance for the prediction of fatigue crack propagation rate.

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The transition of hot restoration behavior of the SiC_p(15Vol.%)/Al 2024 composites reinforced with 1 and 8㎛ SiC_p reinforcements was investigated by hot torsion tests. Flow curves and activation energy of the composites were studied in order to understand the transition behavior of hot restoration mechanism such as dynamic recrystallization(DRX) or dynamic recovery(DRV) during hot deformation. Hot deformed microstructures and dislocation density of the composites were analyzed by TEM. The composite reinforced with 1 ㎛ SiC_p deformed at 320∼460℃ showed a flow curve representing DRX, while the composite deformed at 460∼520℃ showed a flow curve representing DRV. The composite reinforced with 1 ㎛ SiC_p showed the highest failure strain(ε_f=3.65) at ∼470℃, which is a transition temperature of hot restoration mechanism from DRX to DRV.

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The Al₂O₃-SiC composite powder was prepared by Self-propagating High-Temperature Synthesis(SHS) process using SiO₂ powder as raw material instead of silicon. Aluminum powder was used as reducing agent of SiO₂ and graphite was used as carbon source. The effects of the molar ratio in raw material, compaction pressure, initial temperature of reactants on the products and combustion process were studied. Self-propagating high temperature synthesis of SiO₂/Al/C system should be preheated above 400℃. As the result of the combustion reaction, the purity of final product became better than that of reactants. When Al₂O₃-SiO₂ prepared from SiO₂/Al/C system, the optimum molar ratio of SiO₂ : Al : C was 3.0 : 4.0 : 6.0. The free carbon was removed by roasting at 650℃ for 30min.

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The growth mechanism of MoSi₂ coatings formed by the pack siliconizing of molybdenum was investigated in the temperature range of 1223K∼1473K. Experiments were conducted under an argon atmosphere using 40Si-5NaF-55Al₂O₃ in weight percents packs. The results clearly revealed the following novel phenomenon : when packs were kept at a constant temperature, the growth rate of MoSi₂ coating was proportional to time at early stage, but at later stage the growth was changed to parabolic function of time. In order to interpret the phenomenon, a model was proposed under the assumption that chemical reaction for MoSi₂ formation at MoSi₂/Mo interface were responsible for the early stage growth. As a result, it was understood that the growth at early stage was controlled by chemical reaction and the later stage growth controlled by the mass balance conditions of gas and solid diffusions. Thus, two kinds of activation energy were obtained ; one with the value of 240 KJ/mole was considered as the linear growth rate constant, k₁, (㎝/sec), and the second with the value of 1.93 KJ/mole was about the parabolic growth rate constant, k_p(㎠/sec).

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Diffusion kinetics for growth of NiTi at Ni/Ti interface has been investigated. The temperature dependence of the intrinsic diffusion coefficient of Ni, Ti in NiTi intermetallic compound was obtained by D_(Ni)(NiTi) (㎠/sec) = 9.8401×10^(-6)·exp(-95950/RT) and D_(Ti)(NiTi)(㎠/sec) = 1.9111×10^(-5)·exp(-101730/RT). The activation energies of the intrinsic diffusion coefficients of Ni, Ti are 9.5950(±0.84)×10⁴(J/㏖) and 10.173(±3.02)×10⁴(J/㏖) respectively. The parabolic growth constant(k_p^(NiTi)(㎠/sec)) of NiTi calculated by using the equations given by Shatynski. Hirth and Rapp was 1.0797×10^(-5)·exp(-109980/RT) for the temperature range between 800 and 950℃. The activation energy of the growth rate constant is 1.0998(±0.17)×10^5(J/㏖). Present study suggests that the calculation of growth rate constants by the method of Shatynski, Hirth and Rapp can be applied successfully when the interface layer growth follows the parabolic law.

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The p-type 4wt% Te-doped Bi_(0.5)Sb_(1.5)Te₃ compounds were extruded under ratio of 10:1, 20:1 and 30:1 at 420℃. The effect of extrusion ratio on the microstructure and thermoelectric properties has been studied. With increasing the extrusion ratio, grains are slightly refined due to the dynamic recrystallization and also slightly oriented along the extrusion direction. The Seebeck coefficient α and electrical resistivity ρ increased with increasing extrusion ratio, which might be due to the decrease of the carrier concentration. The thermal conductivity k is gradually decreased with increasing the extrusion ratio due to the grain refinement. The compounds hot extruded under ratio of 20:1 show the highest value of figure of merit Z (2.75×10^(-3)/K). The Seebeck coefficient α and electrical resistivity ρ increased after heat treatment at 400℃ for 3 hour. This indicates that Te vacancy formed by extrusion promote second phase Te atom to diffuse into matrix.

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Electrical and thermoelectric properties of the 0.2 wt% SbI₃- doped n-type 90% Bi₂Te₃-(10-x)% Sb₂Te₃-x% Sb₂Se₃ single crystals have been investigated at temperatures ranging from 77K to 600K. The carrier concentration and Seebeck coefficient were independent of the Sb₂Se₃ content. With increasing the Sb₂Se₃ content, the electrical resistivity increased and Hall mobility decreased, which was attributed to the lattice distortion. The thermal conductivity decreased with increasing Sb₂Se₃ content, which was mainly due to the decrease of the electronic thermal conductivity. With increasing the Sb₂Se₃ content, the maximum figure-of-merit decreased and was shifted to the lower temperature. The 0.2 wt% SbI₃-doped 90% Bi₂Te₃-5% Sb₂Te₃-5% Sb₂Se₃ single crystal showed the maximum figure-of-merit of 1.65×10^(-3)/K at 280K.

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In order to study systematically the notch toughness and microstructural alterations across the heat-affected zone (HAZ) as functions of position and weld conditions, a unit HAZ concept was applied to a quenched and tempered SA 508 Cl. 3 reactor pressure vessel steel multipass weld. The unit HAZ, which consists of the width of HAZ (W_H) and the width of interpass (W_I), could be determined by the physical constants and welding variables of the materials. Weld HAZ thermal cycles, which correspond to the various HAZ regions, were simulated with a Gleeble 1500 thermal simulator. Seven typical positions were selected to evaluate the spatial distribution of notch toughness and microstructure in the unit HAZ. Three coarse-grained regions and two fine-grained regions showed relatively good toughness. On the contrary, an intercritically reheated region and a subcritically reheated region showed lower toughness than the base metal. The causes for the variations in toughness were discussed with reference to the microstructures and thermal cycles.

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Small additions of microalloying elements such as niobium, vanadium and titanium have been known to play an important role in the grain refinement, but the role of molybdenum is not clearly known yet. On this study, the effects of molybdenum on the structure of the direct quenched plate steels were investigated. The effect of molybdenum on the prior austenite grain refinement was negligible, but it prevented the recrystallization and made T_R(Recrystallization finishing temperature) lower about 150℃(1150℃→1000℃), and Ar₃ temperature increased. The results were attributed to the pinning effect of the molybdenum precipitates and a broaden ferrite range with molybdenum. A fine-grained steel could be produced through the rolling at the temperature below the T_R, and direct quenching.

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