발간논문

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The mechanism for the amorphization induced by mechanical alloying has been studied in the immiscible Cu-Ta system characterized by a positive heat of mixing of 2kJ/㏖. Here a chemical driving force is no longer expected and instead an energetically up-hill process has been proposed for the formation of the amorphous phase. From the studies on the atomic-level structure through the neutron diffraction and extended X-ray absorption fine structure coupled with thermodynamical consideration, we provide various evidences for the amorphization due to mechanical alloying for a mixture of Cu and Ta powders, The coordination number around Cu and Ta atoms is deduced by assuming the first neighbors of the total pair distribution function g(r) to consist of the three partial ones. We conclude that the amorphization proceeds such that the smaller Cu atoms preferentially enter into the bcc Ta lattice and destroy the long range order edtructure of a crystalline phase.

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The effects of mechanical and operating parameters on the radius of curvature in the thickness direction of layered composites under uniaxial tension have been investigated through the analysis based on bending theory. The three dimensional finite element method has been used to verify analytic results. Both of the methods yield the same results. The radius of curvature is proportional to the elastic modulus(E), the thickness and the reciprocal of the difference between the yield stresses of top and bottom layers of a tensile specimen, which vary linearly across the thickness. On the other hand, the applied strain and tangential hardening modulus have little influence on the radius of curvature.

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Microstructure, wettability, tensile strength and wear properties were examined in AC8A/SiC_p composites. Increasing Mg content in matrix improved wettability probably due to decreasing surface tension of the melt. By increasing the stirring time and particle size, the particles were more uniformly distributed in the matrix. And the particle distribution of as-cast state had a significant influence on that of extruded state, that is, particles were rarely aligned in the extrusion direction in the composites stirred for 20 mimutes. Tensile strength was increased as stirring time increased and particle size decreased, but significant increase of high temperature strength due to the presence of particles was not obtained. Also wear resistance did not vary significantly with the particle size. It is clear that SiC particle reinforcement improved both tensile strength and wear resistance.

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The effect of deposition variables in DC PACVD method on diamond synthesis was investigated. The main variables were deposition temperature, pressure, methane and oxygen concentration. Plasma characteristics of DC PACVD used in this study was close to those of thermal plasma owing to the high deposition pressure around 200Torr. This could make the growth rate and the diamond quality better than other low pressure deposition method. The film thickness increased linearly with time. Similar to the previous result, the growth rate increased with increasing methane, or decreasing oxygen concentration. The maximum methane concentration showing a clear diamond crystal facet appeared to be above 4%. The crystallinity of diamond depended on both methane and oxygen concentration. However, even at high methane concentration of 4%, the full width at half maximum(FWHM) value of diamond peak has shown a value smaller than those at lower methane concentrations, This was due to a difference in the growth plane. Other variables such as temperature, pressure etc., also affected the deposition characteristic, but with a less degree.

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The purpose of this paper is to evaluate the parameter A(t) which can characterize the local creep strain distribution in 2024-T6Al alloy. The evaluation of creep crack initiation and dependence of temperatures were investigated by A(t). The results showed that the instantaneous strain distribution ε_(y,i) near the crack tip could be characterized by the parameter A(t). The parameter A(t) was applied to evaluate the creep crack initiation from pre-crack. It was found that creep cracks initiated when A(t) was reached at some critical values. These critical values were independent of loading levels but changed in propotion to 1/T (T : testing temperature). It is possible to estimate the crack growth initiation time using the instantaneous strain distribution parameter A(0).

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Effects of thermo-mechanical treatments and thermal cycle fatigue on shape memory charac-teristics of Ti-50.2Ni and Ti-40Ni-10Cu(at %) alloys have been investigated by means of electrical resistivity measurements and thermal cycling tests under constant load. Transformation temperatures of the Ti-40Ni-l0Cu ternary alloy was increased by applying thermo-mechanical treatments to the solution treated alloy, while those of Ti-50.2 binary alloy decreased largely. By repeating thermal cycling tests under constant load, the transformation temperature increased while the transformation hysteresis de-creased.

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