발간논문

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Ribbon-shaped amorphous Fe_(73.5-X),Co_XNb₃Cu₁Si_(13.5)B9(0≤X≤25, ΔX=5 at%) alloys prepared by rapidly quenching technique were annealed above their crystallization temperatures. The annealed Fe_(73.5-X)Co_XNb_₃Cu_₁,Si_(13.5)B_9 alloys tranformed into nanocrystalline in all range of X. Nanocrystalline Fe_(73.5-X)Co_XNb₃Cu_₁Si_(13.5)B_9 alloys annealed under optimum condition show the highest saturation magnetization and the lowest AC coercive force(f=1 ㎑, Ha=5 Oe) and core loss(f=100 ㎑, B_m0.5T) at X=5, But the initial permeability(f=1 ㎑, Ha=1 mOe) shows maximum value at X=0. An average grain size of α-Fe(Si) showing excellent soft magnetic properties is around 10 nm and increases with Co content. The degradation of soft magnetic properties during the intial crystallization process is due to the Fe-rich amorphous phase retained at grain boundary.

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Effects of coating treatment of metallic film on SiC particles on wetting behavior of liquid Al on SiC were studied. Experimental variables are included types of coated metallic films such as Ni, Cu, and Ag, and temperatures of heat-treatment under vacuum. Model experiments were performed using coarse SiC particles to observe the wetting phenomenon, and Al-SiC_P composites are prepared by powder metallurgical techniques to test mechanical properties. Analysing the experimental results concerning wetting phenomena of liquid Al on SiC, it was found that coating treatment of metallic film on SiC particles seriously improves the wetting behavior of liquid Al on SiC, especially in case of Ni coating. The tensile strength of Al-SiC_P composites made of coated SiC particles showed also higher value than the specimens made of non-coated SiC particles even though the coating treatment was not perfect.

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The influence of hydrogen on the plastic flow and fracture was studied using smooth and notched tensile specimens. Cathodic charging was performed at current densities varying from 0.04 mA/㎠ to 2 mA/㎠ under controlled galvanostatic conditions. The introduction of high-fugacity hydrogen into the matrix by electrocharging caused irreversible internal damage in the absence of stress at current densities above 0.2 mA/㎠. Hydrogen damage was closely related to softening behavior such as the suppression of yield-point phenomenon and the degradation of mechanical strength observed in the hydrogen-charged case. During tensile testing a number of surface cracks were developed from subsurface damages, resulting in a reduction of work-hardening rates compared to the uncharged case. To effectively analyze these changes in plastic properties after hydrogen charging, hydrogen-induced softening phenomenon was discussed in terms of microscopic dislocation behavior around local particle interfaces.

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To prepare the high purity vanadium metals, the purification procedures of commercial grade vanadium pentaoxide by cation ion exchange and aluminothermic reduction have been studied. After fusion of oxide with NH₄F·HF, hot water leaching and filtration have been carried out. The leaching solution is filtered and pumped onto the cation ion exchange columns to investigate the removal pattern of impurity ions. After three stages of ion exchange operation, total amounts of impurity have been reduced to $lt;150 ppm level. The purified oxide is then exothermically reduced with aluminum powder. The resulting V-Al nugget is arc melted in an argon atmosphere to remove the slag inclusions and residual volatile impurities. Vanadium alloy(V-5Al) containing $lt;75 ppmw Fe, $lt;10 ppmw Si and Cu, has been prepared by this investigation.

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The effects of Mg, Zr addition to Cu-Ga allloy on the crystal growth and the critical current density have been investigated for the V₃Ga composite sheet superconductor prepared by bronze method. The crystal growth of V₃Ga compound was found to be enhanced by Mg addition and overall growth of compound layer was confirmed to be controlled by bulk diffusion of Ga in compound layer. Critical current density was improved by Mg addition because of increase in elementary pinning force and upper critical magnetic field(B_(c2)). The degradation ratio of critical current density in high magnetic fields became small for long time annealed specimen. In low magnetic fields, it was confirmed that the global pinning force density increases with decreasing the grain size.

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The intermetallic compound TiAl alloys were investigated to characterize microstructures and to define relationships between the microstructure and mechanical property by microstructural control method. The compression and hardness tests to study mechanical properties of two TiAl alloys were experimented. Also, the study to confirm the process of the formation and growth of lamellar structure were done by TEM and SEM. The resulting lamellar structures that were preferentially formed at grain boundary could be classified into two groups. Namely, α₂plates were formed in γ matrix according to heat treatment temperature, and vice versa. Ti 48 at.% Al specimen is superior to Ti 45 at.% Al in ductility. Axis ratio and lattice volume increased with Al content and hardness decreased with heat treatment temperature.

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