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Dynamic deformation behavior of ultra-fine-grained pure coppers fabricated by equal channel angular pressing (ECAP) was investigated in this study. Dynamic torsional tests were conducted on four copper specimens using a torsional Kolsky bar, and then the test data were analyzed by their microstructures and tensile properties. The 1-pass ECAP`ed specimen consisted of fine dislocation cell structures elongated along the ECAP direction, which were changed to very fine, equiaxed subgrains of 300~400 nm in size as the pass number increased. The dynamic torsional test results indicated that maximum shear stress increased with increasing ECAP pass number. Adiabatic shear bands were not found at the gage center of the dynamically deformed torsional specimen of the 1- or 4-pass ECAP`ed specimen, while some weak bands were observed in the 8-pass ECAP`ed specimen. These findings suggested that the grain refinement according to the ECAP was very effective in strengthening of pure coppers, and that ECAP`ed coppers could be used without serious reduction in fracture resistance under dynamic torsional loading as adiabatic shear bands were hardly formed.

keyword : Dynamic torsional test, ultra-fine-grained oxygen-free copper, equal channel angular pressing, ECAP

The distribution and prediction interval for the misorientation angle of grain boundary at which Cr2N was precipitated during heating at 900℃ for 10(4) sec were newly estimated, and followed by the estimation of mathematical and median rank methods. The probability density function of the misorientation angle can be estimated by a statistical analysis. And then the (1-α)100% prediction interval of misorientation angle obtained by the estimated probability density function. If the estimated probability density function was symmetric then a prediction interval for the misorientation angle could be derived by the estimated probability density function. In the case of non-symmetric probability density function, the prediction interval could be obtained from the cumulative distribution function of the estimated probability density function. In this paper, 95, 99 and 99.73% prediction interval obtained by probability density function method and cumulative distribution function method and compared with the former results by median rank regression or mathematical method.

keyword : Cr2N, misorientation angle, pdf and cdf methods, mathematical method, median rank method

The influence of annealing treatment on dynamic deformation behavior of ultra-fine grained aluminum alloys was investigated in this study. After equal-channel angular pressing at 200℃, most of the grains were considerably reduced to nearly equiaxed grains of 0.3 ㎛ in size. With an increment of various annealing treatments for 1 hour, resultant microstructures were found to be fairly stable at temperatures up to 200℃, suggesting that static recovery would be dominantly operative, whereas grain growth was pronounced above 250℃. The tensile test results showed that yield and ultimate tensile strengths decreased, but elongation-to-failure and strain hardening rate increased with increasing annealing temperature. The dynamic deformation behavior retrieved with a series of torsional tests was explored with respect to annealed microstructures. Such mechanical response was analyzed in relation to resultant microstructure and fracture mode.

keyword : Aluminum alloy, ultra-fine grained structure, annealing, dynamic torsional test

Elastostatic compression tests were carried out on amorphous alloys to evaluate their energy absorption capability during homogeneous deformation at room temperature. Experiments demonstrated that a compressive stress below the global yield imposed on amorphous alloys for extended periods causes homogeneous plastic strain associated with the irreversible structural disordering. During the disordering process, free volume was created, dissipating the externally applied strain energy and the rate of creation was found to converge to a saturated value. We evaluated the capability of energy absorption of amorphous alloys during homogeneous deformation using recent theories on the evolution of the structural state.

keyword : Amorphous alloy, strength, free volume, structural disordering, relaxation heat

The electrolytic reduction of spent oxide fuel involves the Liberation of oxygen in a molten LiCl electrolyte, which results in a chemically aggressive environment that is too corrosive for typical structural materials. So, it is essential to choose the optimum material for the process equipment handling molten salt. In this study, corrosion behavior of Inconel 713LC, Inconel MA 754, Nimonic 80A and Nimonic 90 in the molten salt LiCl-Li2O under an oxidizing atmosphere was investigated at 650℃ for 72~216 hrs. Inconel 713LC alloy showed the highest corrosion resistance among the examined alloys. Corrosion products of Inconel 713LC were Cr2O3, NiCr2O4 and NiO, and those of Inconel MA 754 were Cr2O3 and Li2Ni8O10 while Cr2O3, LiFeO2, (Cr, Ti)2O3 and Li2Ni8O10 were produced from Nimonic 80A. Also, corrosion products of Nimonic 90 were found to be Cr2O3, (Cr, Ti)2O3, LiAlO2 and CoCr2O4. Inconel 713LC showed local corrosion behavior and Inconel MA 754, Nimonic 80A, Nimonic 90 showed uniform corrosion behavior.

keyword : High temperature corrosion, structural material, molten salt

Flip-chip bonding using Cu-Sn mushroom bumps composed of Cu pillar and Sn cap was accomplished, and the contact resistance and the thermal cycling reliability of the Cu-Sn mushroom bump joints were compared with those of the Sn planar bump joints. With flip-chip process at a same bonding stress, both the Cu-Sn mushroom bump joints and the Sn planar bump joints exhibited an almost identical average contact resistance. With increasing a bonding stress from 32 MPa to 44MPa, the average contact resistances of the Cu-Sn mushroom bump joints and the Sn planar bump joints became reduced from 30 mΩ/bump to 25 mΩ/bump due to heavier plastic deformation of the bumps. The Cu-Sn mushroom bump joints exhibited a superior thermal cycling reliability to that of the Sn planar bump joints at a bonding stress of 32 MPa. While the contact resistance characteristics of the Cu-Sn mushroom bump joints were not deteriorated even after 1000 thermal cycles ranging between -40℃ and 80℃, the contact resistance of the Sn planar bump joints substantially increased with thermal cycling.

keyword : Flip chip, chip on glass, mushroom bump, contact resistance, thermal cycling

In this study, effects of alloying elements on the sticking occurring during hot rolling of five kinds of ferritic STS430J1L stainless steels were investigated by analyzing high-temperature hardness and oxidation behavior of the rolled steels. Hot-rolling simulation tests were conducted by a high-temperature wear tester which could simulate actual hot rolling. The simulation test results revealed that the sticking process proceeded with three stages, i.e., nucleation, growth, and saturation. Since the hardness continuously decreased as the test temperature increased, whereas the formation of Fe-Cr oxides in the rolled steel surface region increased, the sticking of five stainless steels was evaluated by considering both the high-temperature hardness and oxidation effects. The addition of Zr, Cu, or Si had a beneficial effect on the sticking resistance, while the Ni addition did not show any difference in the sticking. Particularly in the case of the Si addition, Si oxides formed first in the initial stage of high-temperature oxidation, worked as initiation sites for Fe-Cr oxides, accelerated the formation of Fe-Cr oxides, and thus raised the sticking resistance by about 10 times in comparison with the steel without Si content.

keyword : Ferritic stainless steel, sticking, high speed steel roll, hot rolling

The Layered compounds vanadium disulfide(VS2) and vanadium dioxide(VO2) intercalated with Li are investigated for using the Discrete Variational (DV)-Xα molecular orbital method. The chemical bonding properties of the atoms were examined by bond overlap population of electronic states. The plot of density of states supports the covalent bonding properties by showing the overlap between the atoms. There is a strong tendency of covalent bonding between V-S and V-O. The intensity of covalent bonding of VS2 is stronger than VO2. The net charge of LiVO2 is higher than that of LiVS2. This results of the calculation of VO2 and VS2 indicate that DV-Xα method can be widely applied in the new practical materials.

keyword : Vanadium disulfide, vanadium dioxide, discrete variational-Xα, bond overlap population, electronic states, cluster calculation