발간논문

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The objective of this study is to investigate the correlation of microstructure and mechanical properties of the AZ91 Mg and the pure Mg matrix composites fabricated by squeeze casting techinque with a variation of applied pressure. Microstructure observation, fractographic observation and in-situ fracture tests were conducted on the composites to identify the microfracture process. Detailed microstructural analyses indicated that the grain refinement and the increase in volume fraction of reinforcing short, fibers could be achieved with increasing applied pressure. It was also found clearly from in -situ observation of crack initiation and propagation that in the composite processed by the larger applied pressure, microcracks were initiated easily at short. fibers already cracked during the squeeze casting process before the matrix/fiber decohesion or the matrix deformation occurred, thereby resulting in the drop in strength. Thus, the effect of the applied pressure on microstructure and mechanical properties can be explained using two competing mechanisms; the deterious effect of fiber breakage might override the beneficial effect of grain refinement or increased reforcements as the applied pressure was increased.

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The effect of N₂ pressure on the microstructure and properties of AlN films, which were formed on Si(100) single crystal by the activated reactive evaporation(ARE) methods, was studied. According to the ESCA and FTIR analysis, as well as the transparency of the film, it could be concluded chat films of Al-N bonding were formed. At high N₂ pressure, films mainly with (002) crystal orientation were formed. It was also found that the deposition rate and the packing density decreased, while the grain size and the surface roughness of the film increased with increasing N₂pressure. It was assumed that as N₂ pressure increases, the number of Al particles and the ratio of Al to N₂ particles, as well as the kinetic energy of the incident particles, decrease, causing such changes in the microstructure and properties of the deposited AlN films.

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The superplastic deformation behavior of Ti-6Al-4V alloy with two different microstructures, a banded microstructure with continuous a phase and an equiaxed microstructure, was investigated by performing both strain-rate-change and constant-strain-rate tests in a longitudinal direction. It was found that the continuous a phase significantly affect the superplastic deformation behavior of the alloy. At 925℃ the continuous α phase remained unbroken during deformation causing higher flow stress at lower strain rate range and tow value of strain rate sensitivity, m. At lower temperature below 880℃, on the other hand, the α phase was broken up to recrystallize dynamically resulting in higher m values at 850 and 880℃. The m values of the cross rolled materials which do not have the continuous α phase steadily increased with temperatures up to 880℃ and did not show a significant decrease at 925℃ contrary to the unidirectionaliy rolled materials.

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Precipitates of goethite have been obtained by the oxidation of ferrous ion by oxygen as an oxidant in the ferrous sulfate solution which added the various concentration of sodium hydroxide. When the mole ratio, R(=2｜OH ｜/｜Fe^(2+)｜) is 0.5 in the formation of goethite, the reaction seems to begin with the dissociation of ferrous hydroxide and to proceed with the formation of Fe(OH)₂. The reaction mechanism of goethite in R=4 was very different from that in R=0.5. From this fact, the reaction can be thought of oxidation reaction in HFeO₂. As the temperature was increased, the particle length of the products was increased in spite of decreasing of reaction time. When the flow rate of oxygen as an oxidant was increased, both the particle length of product and reaction time were decreased. During the reaction of goethite formation by aging the ferric hydroxide in the condition of 50℃ and pH=2, the hydrolysis reaction of Fe(OH)₃ proceeds with aging time.

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The separation of valuable metals from copper based alloys has important implications for treatment of byproducts from the ferrous and nonferrous metallurgical processes as well as for treatment of various copper scraps. To investigate the distribution coefficients of copper, cobalt, and nickel between L₁ and L₂ phase, some fundamental experiments on the metal solvent extraction process were conducted using liquid iron-tin-lead and iron-tin-silicon alloys as solvents. The purpose of these experiments was to investigate a pyrometallurgical process to separate directly cobalt and nickel from liquid copper based alloys or to remove preferentially the copper component from the alloys. The distribution coefficients of cobalt for both solvent systems represented very small values, suggesting the suitability of these solvents for the separation of cobalt from various kinds of copper based alloys. On the other hand, the distribution coefficients for both nickel and copper were not largely different.

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In the present work, the hyperelasticity theory of rubber material behavior is presented and finite element formulation for that material is briefly discussed. The tests to get the information of rubber material behavior are carried out. Two types of strain energy models for the material behavior are introduced and how to define the form of the strain energy potential from the test results is described. As an application, the deformation characteristic of a bush type engine mount for a passenger car is analyzed using the commercial finite element method program ABAQUS. The results of analysis using each form of strain energy potential are compared with the experimental results. And the reliability of the forms of strain energy for this case is checked. Finally, the current status of rubber part design process and future works for more systematic approach are discussed.

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