발간논문

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A model for densification of metallic powders is proposed. It involves viscoplastic constitutive equations based on dislocation density evolution and also accounts for an effect of porosity. The model was applied to the case of cold powder compaction under uniaxial compression conditions. Densification behaviour during powder compaction was simulated using an explicit integration method as applied to the dislocation density evolution and the variation of the relative density of the compact. The model was gauged by comparing the experimental data generated by cylindrical die compaction tests on Cu powder with the simulation results. The model accounts for the grain size and the deformation rate dependence of on the densification process.

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The characteristics of the surface layers of the plasma oxidation treated ductile cast iron was investigated. The non-intrusive technique of optical emission spectroscopy was used to identify the oxygen plasma species. The etching behaviors with the substrate temperature of graphite in the plasma treatment were investigated using atomic force microscopy. The etching depth increased in the temperature range of 500℃-700℃. The chemical reaction at the surface layer is analyzed by X-ray photoelectron spectroscopy(XPS). The C 1s peak of the XPS spectra shows that oxygen is implanted in the graphite and forms single and double bonds.

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The present study is concerned with fabrication and microstructural analysis of boride/Ti-6Al-4V surface-alloyed materials using irradiation of high-energy electron beam. Mixtures of TiB₂ or MoB boride powders and CaF₂ flux were placed on a Ti-6AI-4V alloy substrate, and then electron beam was irradiated on these mixtures using an electron beam accelerator. In the specimens processed with a flux mixing ratio of 40 wt.%, the melted region of 1.1∼1.5 ㎜ in thickness was homogeneously formed without defects, and contained a large amount of TiB borides. This microstructural modification including the formation of TiB borides and β-Ti matrix in the surface-alloyed layers greatly improved hardness, about twice higher than hardness of the Ti alloy substrate. These findings suggested that the surface-alloying using high-energy electron beam irradiation was economical and useful for the development of titanium-base surface-alloyed materials with improved hardness.

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Discrete-variational(DV)-Xα method has been applied to investigate the electronic structures of layered compound TiS₂ intercalated with Li or Fe. The chemical bonding properties of the atoms were examined by plotting bond overlap population of electron states. The plot of density of status supported the covalent bonding properties by showing the overlap between the electronic states. There is a strong tendency of covalent bonding between Ti and S. In the presence of Li and Fe, the bond overlap population of Ti-S bonding decreases sharing the covalency with Li-S or Fe-S. The ionic character of the intercalating atom was negligible. The results of the calculation of TiS₂ indicate that DV-Xα method can be widely applied in the research area of electronic structures. It could also be used to develop a new practical material.

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CGHAZ(Coarsened Grain HAZ) which is the sub-zone closest to the weld metal is the region where many weld difficulties appear such as cracks, low toughness due to grain growth, etc. So, prediction of grain growth and microstructure under severe welding thermal cycle is very useful research to remove the defects forming in the HAZ. Therefore, in this study, welding thermal cycle was predicted by FDM mathematical analysis for three dimensional bead on plate model, and to predict the grain growth, the metallurgical modeling for the grain growth behavior was developed on the basis of phase transformation and grain growth theory. Phase transformation phenomena which appears between A₁ and A₃ temperature was analyzed by Ham`s theory and classical transformation theory. The grain growth phenomena was analyzed by the general grain growth equation. The general grain growth equation established by many researchers contains the relationship between the grain size and grain boundary mobility and grain growth driving force. In this study, such relationship was examined more precisely, and the grain size variation was predicted by the computer simulation based on this equation. For more precise analysis, the variation of the phase transformation temperature range with the heating rate was considered by the dilation experimental data. And then, through these mathematical approach and experimental data, phase transformation model and grain growth model were constructed. The prediction results were very good coincident with the HAZ microstructure experiment results. From the results of simulation, the HAZ grain size diagram was drawn up, in which we can acquire very many informations about welding condition, material selection for welding and defect forming part, etc. It is concluded that the grain growth modeling of this study is proper for the prediction of prior austenite grain size variation in the HAZ and general enough to apply for other heat treatment processes.

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The extraction characteristic and stoichiometric relation for the solvent extraction of indium from acid solution was studied with D2EHPA(di-2-ethylhexyl phosphoric acid). The results showed that the extraction efficiency of indium increased with increasing pH of solution and D2EHPA concentration at the same pH condition. Also the extraction aspect and efficiency of indium in the HCI solution were similar to that in the sulfuric acid solution. The presence of SO^(2-)₄ and NO₃ would not affect the extraction of indium while Cl^- caused considerably the lowering of the indium extraction resulting from the formation of ligand complexes between Cl^- and indium in the solution. Generally, the optimum conditions for the extraction of indium appeared to be above 0.36M/L [R₂H₂] of D2EHPA and pH 0. On the other hand, the indium was extracted by non-solvated form like [InR₃] with D2EHPA and the extracted species were not associated with each other. The extraction relation would be as follows : [In^(3+)]_(aq) + 3/2[R₂H₂]_(org) = [InR₃]_(org) + 3[H^+]_(aq). The equilibrium constant for the reaction was found to be 12.531.

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For a development purpose of thick metal/metal graded-boundary materials, a preliminary research on the fabrication of graded-boundary Ni-Cr/steel material was carried out by an electron beam and its mechanical properties and thermal characteristics were investigated. In order to produce a compositionally graded boundary region between substrate steel and added Ni-Cr alloy, a series of surface alloying treatments was performed with an electron beam. Ni-Cr sheet was placed on a low carbon steel plate(0.18%C), and then an electron beam was irradiated on the surface to produce a homogeneous alloyed layer. On this first surface-alloyed layer another Ni-Cr sheet was placed and then the electron beam was irradiated again to produce second surface-alloyed layer. Sequential repetitions of electron beam surface alloying treatment 4 times resulted in a graded-boundary region with the thickness of about 2.2 ㎜. Simultaneous concentration gradients of different kinds of alloying elements(Ni and Cr) showed from 28%Ni, 29%Cr and 43%Fe on surface region to 0%Ni 0%Cr and 99%Fe in substrate region. A thermal conductivity gradient resulted in graded-region and its value changed from 0.028 ㎈/㎝ s℃ in surface region to 0.1 ㎈/㎝ s℃ in substrate region. Microstructural observation showed that any visible root porosities and solidification shrinkage cracks were not formed in graded region between alloyed layers and substrate steel during rapid cooling.

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In thin slab continuous casting process, the submerged entry nozzle (SEN) design is the prime controlling factor of the flow patterns of liquid steel which has an important effect on the surface and internal quality of steel produced. Four SENs were designed with different port angles and sleeve lengths. Solving three-dimensional thermo-fluid-dynamics by the control volume method with body-fitted coordinates system, horizontal velocity on the meniscus and temperature distribution were compared for each SEN. The solidified shell thickness was also investigated at each case. And also the inclusion path was traced to see the tendency of inclusion removal.

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