발간논문

Home > KJMM 논문 > 발간논문

The objective of this paper is to understand the anisotropy of the deformation behavior along the three major directions of the Zr-2.5％Nb pressure tube materials. Because the deformation behavior in the radial direction has never been established although the anisotropy of the DHC behavior might be attrihuted to the anisotropy of the deformation. The small tensile specimens with a gage length of about 2 mm were machined and tested from room temperature to 560℃. The validity of the results from the small tensile specimens were checked by comparison with those from the larger specimens, an were found to be comparable. The stress-strain curves in both the transverse appeared to be different. The yield and the tensile strengths were low in the transverse, longitudinal, and radial directions in the range of 100 to 500℃. These behaviors could be explained using the deformation mechanisms and the texture of the material. The mechanical properties in the radial direction were successfully obtained using the small tensile specimens.

keyword : Anisotropy, Small tensile specimen, Strength, Strain-stress curve, Zr-2.5%Nb pressure tube, Texture Inverse pole figure, Deformation mechanism, Slip, Twinning

Influence of thermal exposure on microstructural evolution and tensile properties of Ni-base superalloy B1900 has been investigated. Phase transformations during thermal exposure and their effects on tensile behavior of the alloy have been analyzed by microstructural observations. Considerable amount of acicular type M_(6)C carbide precipitated in the interdendritic regions (including γ/γ` eutectic) and in the vicinity of MC carbide as a result of thermal exposure above 871°C. Thermal exposure also assisted precipitation of small discrete M_(23)C_(6) carbide which is hard to be found in the alloy. Thermal exposure deteriorated tensile properties of the alloy. The effect of M_(6)C precipitation on the tensile properties of the alloy is not clear. However, degradation of tensile properties of the alloy is primarily attributed to the coarsening of y` phase during thermal exposure. While the localized deformation bands were observed in the as-cast tensile specimen due to shearing of γ` particle, dislocation network formed at γ/γ`interface in the thermally exposed tensile specimen.

keyword : B1900, Carbide, Thermal exposure, Tensile properties, γ`, Deformation band

Al-Ti-C grain refiners form a relatively new alternative to the existing class of Al-Ti-B type grain refiners for achieving fine equiaxed structures in aluminum alloys during casting and solidification. The present study was carried out to investigate the influence of Al-Ti-C master alloys on the grain size of high purity aluminum and hypoeutectic Al-Si alloys. It is well known that typical alloying elements affect significantly the grain refining performance in hypoeutectic Al-Si alloys. This study investigates the relationship between grain refining efficiency and concentrations of Fe and Si in aluminum alloys. It has been found that grain reflnement was successful when using small addition of Al-3 wt, ％Ti-0.15.wt.％C master alloy(0.006wt. ％ Ti addition levels) in hypoeutectic Al-Si alloys. Furthermore, several factors such as holding time, alloy composition, pouring and temperature had an influence on the grain refinement performance.

keyword : Grain refinement, Al-Si alloy, Al-Ti-C, TiC, Grain refiner

Giant magnetostrictive material Terfenol-D was grown by Czochralski method in various melt temperature under purified argon at 1.2 atm and the macrosegregation of RE along growth direction was investigated. It can be expected that concentration of RE elements follow the Burton-Prim-Slicter relation in Czochralaki system. However, the aspects of macrosegregation were changed with melt temperature: At lower melting temperature, RE elements concentration increased slong growth direction, while at higher, it decreased. This was due to the change of melt composition during growth caused by loss of RE. Therefore, we discussed the reaction of RE with crucible and evaporation through free surface, which are the main reasons of RE loss. In addition, the macrosegregation was discussed considering the loss of RE with melt temperature.

keyword : Terfenol-D, Magnetostrictive material, Czochralski, Macrosegregation

The present study aimed at investigating application possibility of the percolation concept to the microstructure control of thermoelectric materials for improving thermoelectric properties. Two kinds of PbTe powders, large particle size and small particle size, were prepared by mechanical grinding process. Effect of mixing of the these powders on theremoelectric properties of the sintered body was studied by measuring Seebeck coefficient, specific electric resistivity and thermal conductivity. The PbTe sintered body of large particle size powder has a lower specific electric resistivity and higher thermal conductivity than the values of the one of small particle size powder. The PbTe sintered body of the mixed powder showed percolation transition behavior on the specific electric resistivity at a specified powder mixing ratio. This transition behavior can be predicted by a two-dimensional particle configuration model based on the percolation concept. The PbTe sintered bodies of mixed powder based on the percolation concept have a higher figure of merit than the value of the one of unmixed powder.

keyword : Thermoelectric material, Microstructure, Powder, Percolation, Figure of merit

To investigate the microstructural change with aging treatment and the effect of microstructure on the oxidation characteristics of Zr-xNb alloys, the TEM study. the oxidation test, and oxide characterization were performed, The specimens having different Nb content in the range from 0.1 to 2 wt.％were heat-treated at 570℃ from 0 to 1000 hours to observe the process of phase transformation after β-quenching. The oxidation test was performed in water at 360℃ and the oxide characteristics of the oxidized specimens were investigated by using low-angle XRD. The oxidation behavior of Zr-xNb alloys was affected hy the soluble Nb and the second phases in the matrix. The oxidation rate was lower when the Nb content was in the equilibrium soluble state in matrix when the β_(Nb) phase was formed by aging treatment. From the oxide analysis, the soluble Nb and the β_(Nb) phase appeared to stabilize the tetra-ZrO_(2) while the supersaturated Nb in the matrix may accelerate the transformation of oxide crystal structure from the tetra-ZrO_2) to mono-ZrO_(2).

keyword : Zr, Zr-Nb, Aging, Microstructure, βNb phase, Corrosion, Oxide, Tetra-ZrO2, Mono-ZrO2

Nine nano-sized Fe-Co particle samples synthesized by Chemical Vapor Condensation(CVC) were analyzed using Mo¨ssbauer spectroscopy, XRD, BET and HRTEM. The synthesized nanoparticles consisted of metallic core and oxide shell structures, and their surfaces were covered with a-FeOOH, γ-FeOOH and Fe_(3)O_(4). The increase of cobalt content enhanced magnetic field. However, when the decomposition temperature and the oxygen content in the carrier gas (Ar) increased, the magnetic field of nanoparticles reduced with decreasing the average particle size. with increasing the vacuum pressure, the average particle size and the magnetic field increased.

keyword : Nano-sized Fe-Co particle, Chemical Vapor Condensation, Mossbauer spectroscopy, Average particle size, Magnetic property

The process and size effects on the fracture strength of single-crystal Si were evaluated using a microtensile testing method. The microtensile specimens of single-crystal Si films were fabricated by CMP(chemical-mechanical polishing) and micromachining processes, The microtensile test setup was built to accommodate requirements such as specimen handling, jig, alignment and very small loads and displacements. The variations in the fracture strength with thickness and effective volume of the Si film were evaluated. In addition, the results were discussed statistically. The effects of dry-etched surface and CMP surface on the mean fracture strength were compared. Through AFM (atomic force microscopy) observation, it could be found that the larger flaws of the dry-etched surface affected significantly on the decreasing of the mean fracture strength.

keyword : Fracture Strength, Microtensile test, Single-crystal Si, Size effect, Process effect

In this study, we have investigated the effect of electrode fabrication methods, catalyst slurry composition and electrode and membrane-electrode-assemble(MEA) preparation conditions on the performance of electrode in polymer electrolyte membrane fuel cells(PEMFCs). We have changed the amount of platinum loading, electrode fabrication method(spraying and brushing) and the composition of electrode catalyst slurry. Better electrode performance was obtained with the increase in the platinum loading. The performance of the fabricated electrode by spraying method was higher than that by brushing method. The electrode catalyst slurry using 1:1, IPA and NBA solvents gave best performance. By optimising electrode preparation conditions, a current density of 700 mA/㎠ at 0.6 V was obtained, at 80℃, with H_(2) O_(2) reactants at 1 atm.

keyword : Polymer electrolyte membrane fuel cells, Pt loading, Solvent composition, MEA

High silicon Al-Si alloys with nanocrystalline structures have been produced by mechanical alloying process. Microstructural changes of the Al-Si alloys were investigated by scanning electron microscopy and the transmission electron microscopy. X-ray diffraction analysis was also made to characterize the lattice constant, crystallite size and misfit strain. It was found that the effective milling time by attrition milling was about 12hours for Al-70 wt％Si alloy system. The Al and Si crystallites were reduced to about 30 nm and 70 nm, respectively, by the mechanical alloying for more than 12hours. The misfit strains increased with milling time UD to 240 hours, and saturated to 5.73×10^(-3) for Al and Si crystallites, respectively.

keyword : High silicon Al-Si alloy, XRD, Crystallite size, Misfit strain