발간논문

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Joinning of 304 stainless steel to Si₃N₄, ceramic using Cu-48%Ag-3.8%Ti brazing alloy was made in vacuum furnace. Effect of Mo addition in Cu-Ag-Ti alloy, brazing temperature, and brazing time on interfacial reactions and banding strength of Si₃N₄-304 joints were investigated. Bonding strength was determined by fracture shear loading and high shear strength value of more than 180 ㎫ were obtained for joints brazed with Cu-Ag-Ti- Mo alloy at 1083 K for 1.2 ks. The strong bonding is attritutable to the formation of FeTiSi and TO_5Si₄ by redox reaction between Si₃N₄ and Ti at Si+3N₄ ceramic interface. Thickness of interfacial reaction layer was increased and bonding strength was decreased as brazing temperature and brazing time were increased. It was found that interfacial reaction was retarded by Mo addition.

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The pulverization and alloy degradation phenomena in Nd_(15)Fe_(77)B_8 and Nd_(12)Fe_(70)Co_(10)B_7Ga₁ alloys upon the hydrogen absorption and desorption cycling have been investigated by measuring hydrogen solubility changes, microstructure observation and DSC thermal analysis. The cycling was performed by varing the temperature from 25℃ to 400℃ at a constant hydrogen pressure. As the number of cycles increased upto 15 cycles, the hydrogen solubility decreased markedly implying severe alloy degradations in both alloys, whereas the pulverization progressed small. The degadation could be attributed to the segregation of Nd or the precipitation of Nd-rich phases on the alloy surface and to the oxidation of those phases.

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Different volume fraction of SiC_p/6061-T6 Al composites manufactured by powder metallurgy process and hot extrusion were investigated by characterizing fractography and microstructure. The following conclusion was obtained from the results of tensile test, static and dynamic fracture toughness tests and the distribution of residual stress with the depth direction of fracture surface. While elongation and fracture toughness were decreased, tensile strength and yield strength were increased as the volume fraction of SiC particle increased. It is assumed that higher dynamic fracture toughness is attributed to the energy absorption of damaged area at the sub-fracture surface. Fracture toughness was proportional to the plastic zone size in front of crack tip.

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An attempt has been made to investigate the effects of SiC volume fraction on the fracture toughness. Especially to estabilish the relationship between fracture toughness and microstructure in this composires, micromechanical fracture behavior in front of crack tip was examined quantitatively. It was possible to decide the average tensile strain in the process zone, ε￣ by experimental method. The microstructural distance r^* which can correlate a fracture toughness value with the microstructure was obtained by substituting εfor ε^p of HRR solution. According to the fracture criterion which was observed in this study, r^* was related to the spacing of critical particle diameter, d_(crit). From the above analytic method, following equation was obtained. J_(ic)=ασ_yε￣_fd_(crit). where α : constant(α=2I_N/ε^p(θ,N)) ε￣_f : average tensile fracture strain in a process zone d_(crit) : inter-particles spacing of a critical particle diameter

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The crystallinity of CdTe and CdZnTe grown by Bridgman method were evaluated using a double crystal diffraction technique. The FWHM of the rocking curve was compared with the etch-pit density. FWHM increased with the increase of etch-pit density, the increasing rate depending on the density and distribution of etch-pits.

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