발간논문

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In this study, STS304-continuous-fiber-reinforced Zr-based amorphous matrix composites were fabricated by liquid pressing process, and their microstructures and mechanical properties were investigated. The fabricated composites homogeneously contained about 60 vol.% of continuous fibers in the amorphous matrix, in which considerable amounts of polygonal crystalline particles and dendritic crystalline phases were formed by the diffusion of metallic elements from fibers. The ductility of the composites under compressive or tensile loading was drastically improved over that of the monolithic amorphous alloy, while their compressive and tensile strengths satisfied the rule of mixtures. Particularly in tensile loading, the deformation and fracture proceeded by crack formation and opening at matrices, necking of fibers, fiber/matrix interfacial separation, and cup-and-cone-type fracture of fibers, thereby resulting in very high tensile elongation of 6.6%. These findings suggested that the liquid pressing process was useful for the development of amorphous matrix composites with improved ductility and fracture toughness.

keyword : composite, amorphous alloy, Liquid pressing process, shear band

This study is concerned with the analysis of fracture toughness in the transition temperature region of three API X70 and X80 linepipe steels fabricated by varying hot-rolling conditions in accordance with the ASTM E1921-05 standard test method. Elastic-plastic cleavage fracture toughness (K(Jc)) was determined by 3-point bend tests, using precracked Charpy V-notch specimens, and then the measured K(Jc) values were interpreted by the 3-parameter Weibull distribution. The fracture toughness test results indicated that the master curve and the 98% confidence curves explained the variation in the measured fracture toughness values well. Reference temperatures obtained from the fracture toughness tests as well as index temperatures obtained from the Charpy impact tests were lowest in the X70 steel rolled in the two phase region because it had smaller effective grain size and lowest volume fraction of hard secondary phases. In this steel, few hard phases led to the higher resistance to cleavage crack initiation, and the smaller effective grain size led to the higher possibility of crack arrest, thereby resulting in the best overall fracture properties. Measured reference temperatures were then comparatively analyzed with index temperatures obtained from the Charpy impact tests, and the effects of microstructures on these temperatures were discussed.

keyword : API X70 and X80 Linepipe steel, ASTM El 921-05 standard test method, elastic-plastic cleavage fracture toughness, reference temperature

Thermal reduction of zinc oxide contained in the electric arc furnace (EAF) dust by solid carbon is very important in the rotary kiln process for recovering zinc. In the present study, the zinc oxide in EAF dust was reduced by solid carbon at elevated temperatures using pellets of the diameters varying from 0.3 to 1.4 cm. Experiments was carried out in the temperature range of 800~1,200℃ with various heating rates of 1-9℃/min. The amount of solid carbon mixed in the pellets was 1.0~2.5 times the stoichiometric amount for the reduction of zinc oxide. Dry air of 100ml/min was supplied in the reaction chamber during the reduction process. The results indicated that the reduction volatilization ratio of zinc oxide depended significantly on the amount of solid carbon mixed in the pellets, but not so much on the size of pellets. For pellets of 0.8-1.0 cm in diameter containing the carbon of 2 times the stoichiometric amount, the reduction volatilization ratio of zinc oxide in the pellets was over 95% at the temperature range of 950~1200℃ for all heating rates. On the other hand, the reduction ratio was below 70% for pellets with a heating rate of 1℃/min at the temperature range of 800~1,200℃.

keyword : rotary kiln, EAF dust, zinc oxide, thermal reduction, carbon

This study is aimed at investigating the grain refining mechanism of Mg-Al base alloys by carbon addition. In AZ91 alloys, the morphology of Al-Mn particles were modified from rod to globular-type by inoculating C2Cl6 to AZ91 alloy melt. These particles were identified as Al8(Mn,Fe)5 phase and formed in the melt before the formation of α-Mg, resulting in grain refinement by heterogeneous nucleation. However, no grain refinement was found in Mn-free Mg-Al binary alloys with an addition of C2Cl6, because most of Al carbides and Al-Fe compounds existed in eutectic region, indicating that they are not effective nucleants for Mg grains. Therefore, Al8(Mn, Fe)5 is considered as the most effective nucleant in Mg-Al base alloys. In this study, it is suggested that the grain refinement in Mg-Al base alloys is achieved by the sequences that Al carbides formed by carbon addition are surrounded by Al8(Mn,Fe)5, which in turn are surrounded by α-Mg.

keyword : Mg-Al alloy, grain refinement, carbon addition, heterogeneous nucleation, duplex nucleation

Equal channel angular pressing (ECAP) is a promising severe plastic deformation method for grain refinement of metallic materials. The properties of the materials are strongly dependent on the shear plastic deformation behavior during ECAP, which is controlled mainly by die geometry, material properties, and process conditions. In the ECAP, it is important to obtain uniform strain distribution for fabricating the workpiece without processing defects. The recently developed front pressure ECAP can impose higher hydrostatic pressure and increase the strain distribution level the material, preventing surface defects on workpiece by the tensile stress. In this study, the plastic deformation behaviour of the materials during ECAP with front pressure was investigated using the commercial two-dimensional finite element code (ABAQUS). The homogeneity within the workpiece was analyzed in terms of strain contour, path plot and statistics of strain distribution.

keyword : equal channel angular pressing, sever plastic deformation, front pressure equal channel angular pressing, FP-ECAP, finite element method, strain homogeneity, statistics of stain distribution

This study aimed to investigate the effects of thickness and heating temperature on the surface characteristics and electric characteristic of copper foil. The copper foils with 5, 10, 20, and 25μm thick were formed by electrodeposition in the electrolyte composing of Cu 100g/L and H2SO4 100g/L at 200mA/cm2. In addition, the RTA(rapid thermal annealing) was executed by using 10μm thick copper foil. SEM, XRD, AFM and four-point probe were performed to characterize the morphology and electric characteristics of copper foil. The crystal size and surface roughness increased with the increasing thickness of copper foil. The minimum specific resistance was observed in the electrodeposited copper foil with 15μm thick. After RTA treatment of 10μm thick copper foil, the lowest value of specific resistance (2.03 μΩ-cm) and surface roughness (176.3 nm) was measured in the group treated 300℃, respectively. Especially, it is expected that the RTA treatment at 300℃ is good for increasing resistance of electromigration.

keyword : copper thickness, heating treatment, surface characteristics, resistivity

Nano-sized and homogeneously distributed TiO2-20, -40, -60wt% SrO composite powders were successfully synthesized by a sol-gel method. The as-received amorphous TiO2-20wt% SrO composite powders were crystallized with anatase TiO2 at around 750℃. As calcination temperatures increased, the anatase TiO2 crystalline phase was transformed to rutile TiO2 at about 900℃, whereas nano-sized, squarish SrTiO3 phase was detected. The peaks obtained after calcining at 1,050℃ mainly exhibited the rutile TiO2 and SrTiO3 phases. However, a small number of SrO2 peaks were also detected. For the comparison of photocatalytic activity depending on light sources, TiO2-SrO composite powders were tested in phenol degradation. TiO2-60wt%SrO composite powder showed good visible light photoactivity for the photo-oxidation of phenol.

keyword : nanoparticle, photoactivity, sol-gel method