발간논문

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A fixed grid, finite volume model was applied to investigate hydrogen absorption, diffusion and moving boundary α/βphase transformation that occurs during hydrogenation and homogenization annealing of cylindrical titanium specimens. Effects of parameters such as temperature, specimen radius, amount of added hydrogen on the times required for hydrogenation, for completion of α/βphase transformation and for homogenization were studied. It was found that hydrogen absorption rate is mostly influenced by surface reaction rather than solid-state diffusion. Time for hydrogenation becomes somewhat shorter with increase of temperature, due to decrease of hydrogen saturation concentration in spite of large increase of diffusivity and reaction rate constant. It increases almost linearly with increasing specimen radius or with amount of hydrogenation, showing positive deviation from linearity due to increase of diffusion resistance or surface hydrogen concentration. The completion of α/βphase transformation and homogenization are finished quickly with raising temperature due to increase of diffusivity and decrease of equilibrium hydrogen concentration in the βphase. They are nearly proportional to the square of specimen radius. With increase of the amount of added hydrogen, the completion of α/β phase transformation and homogenization are finished quickly during annealing due to the progress of α/β phase transformation during hydrogenation (Received August 23, 2003)

keyword : Thermo hydrogenation, Phase transformation, Numerical analysis, Titanium

It was studied to elucidate the high-temperature oxidation behaviors of Zirealoy-4 and A claddings in LOCA temperature ranges (700∼1200℃), using a TGA(Themogravimetric analysis) method. As the temperature increased, the weight gain of two claddings increased according to the parabolic rate law. But the discontinuity of K_(p) in Zircaloy-4 was attributed to the microstructural changes of oxide at the temperatures of 1000∼1050℃, where the oxide structures were transformed from monoclinic-ZrO_(2) to tetragonal-ZrO_(2). These results were matched to the surface color of tested specimens and the calculated parabolic rate constants. And the discontinuity temperature of K_(p) in A cladding appeared at 1100℃ higher than that in Zircaloy-4. It was thought that discrepancy of oxidation kinetics in A cladding were resulted from the stability of tetragonal-ZrO_(2) phase in A cladding due to Nb addition. Since the parabolic rate constants of A cladding were similar to that of Zircaloy-4, A cladding would maintain the mechanical integrity under the LOCA conditions. (Received May 28, 2003)

keyword : Zr, Fuel cladding, Steam oxidation, LOCA, Oxidation kinetics

This study investigated the effect of the tempering before aging on the precipitation of Laves phase and mechanical properties of 10Cr-6W and 10Cr-6W-3Co ferritic heat-resistant steels. Laves phase was precipitated by aging, which increase tensile strength but decreased elongation and impact value. Toughness was rapidly decreased by the presence of the very fine disk-like Laves phase precipitated at early aging stage. Strength and impact value were decreased by tempering treatment and that is due to the increase of the particle number per unite area and to larger particle size of Laves phase. Laves phase increased by adding Co, and consequently strength was increased and elongation and toughness were decreased. (Received February 26, 2004)

keyword : Laves phase, Ferritic heat-resistant steels, Tempering, High temperature strength, Aging

Low cycle fatigue(LCF) properties of Nb-stabilized type 347 and unstabilized type 316N austenitic stainless steels(SS) having nitrogen content of about 0.1wt% were evaluated at ambient and nuclear power plant operating temperature of 330℃. The resistance to LCF in two steels was compared in view of the underlying microstructure. Type 316N showed longer fatigue life than type 347 at both ambient temperature and 330℃. Higher test temperature resulted in shorter fatigue life in type 316N but similar fatigue life in type 347. The similar life in type 347 was attributed to the different cyclic peak stress response with temperature-cyclic hardening at ambient temperature and cyclic softening at 330℃. Dislocation microstructure showed more homogeneous behavior in type 347 than 316N, which came from fine Nb carbide dispersion and higher planar slip property. From the fractography of the tested samples, we found different crack initiation mechanisms in the two types of steels. For type 316Nm initial fatigue cracks mostly formed intergranularly at high input strain range while transgranularly at low input strain range. But type 347 showed intergranular initiation regardless of input strain range. This seemed to be attributed to the fine carbide distribution and higher planar slip property which prevent persistent slip band development especially at low input strain range. Coarse Nb(CN) particles in type 347 accelerated fatigue crack growth through forming voids under the repeated small deformations. (Received February 2, 2004)

keyword : Low-cycle fatigue, LCF, Stainless steels, Deformation, Dislocation cell, Fracture, Crack initiation

Microstructure and hardness changes of the Cu-35%Zn alloy prepared by equal-channel angular pressing (ECAP) with different feed directions were examined. Two different feeding methods such as A (parallel passes) and C (180° rotation after each pass) were employed and the relationship between the microstructure and the hardness was investigated. Shear bands with 45 degree on transverse direction and twins were observed after ECAP. Relative amount of twins to shear band on the microstructure decreases with number of ECAP passes, suggesting that dislocation banding is the primary strengthening component although twinning is contributory to strengthening. The parallel ECAP pass (route A) gave finer shear band and higher hardness increase than the 180°rotated ECAP pass (route C), which is in sharp contrast with that observed in Al with a high stacking fault energy. Reversible movement of dislocations in planar slip bands due to the low stacking fault energy is thought to be linked with the lower hardness in route C. (Received October 6, 2003)

keyword : Equal channel angular pressing, Stacking fault energy, Cu-35% Zn, Deformation band, Twin, Feed direction

In the present study, the unidirectionally aligned carbon nanofiber (CNF) reinforced Cu composite was fabricated by the liquid infiltration process, and the deformation behavior of the composite was evaluated. For the unidirectional alignment of the severely entangled CNFs, the mechanical drawing process was utilized. The bundle of drawn Cu tubes compacted with the unidirectionally aligned CNFs was placed on the specially designed mold, which was subsequently canned, degassed, evacuated, heated at 1100℃ for 10 minutes in the electric furnace to liquefy the Cu, and finally transferred to the press. The optical and SEM micrographic observations of the final product demonstrated that, even though the CNFs were not uniformly distributed throughout the specimen due to the molten metal flow during melting process, the local unidirectionality was found to be preserved in those areas where the CNFs were densely distributed. Tensile test results showed that approximately two-fold increase in the tensile strength was obtained with the CNF reinforced Cu composite. Based on the micrographic and fractographic observations, the strengthening mechanism of the CNF reinforced Cu composite fabricated by the liquid infiltration process was discussed. (Received March 2, 2004)

keyword : Carbon nanofiber, Cu matrix composite, Liquid infiltration process

Pretreatment of the underlying film surface is essential in molecular organic chemical vapor deposition (MOCVD) of metals. Effects of surface treatment techniques including Ar plasma, H_(2) plasma, and Pd sputtering treatments of the underlying TaSiN film surface on the Ru nucleation density in Ru-MOCVD were compared with each other. According to the scanning electron microscopy (SEM) and Auger electron emission spectrometry (AES) analysis results Pd sputtering was found to be most efficient in enhancing Ru nucleation. Thin Pd buffer layer formed on the TaSiN film by sputtering offers catalytic surface for easier nucleation of Ru. On the other hand hydrogen plasma treatment enhances Ru nucleation by removing oxygen and nitrogen atoms at the TaSiN film surface. In contrast Ar plasma treatment removes not only oxygen and nitrogen atoms but also Ta atoms. Pd sputtering seems to be the best of these three pretreatment techniques for Ru-MOCVD. (Received December 22, 2003)

keyword : Ru MOCVD, Nucleation enhancement plasma treatment, Pd sputtering

This paper addresses a theoretical approach to calculate the amount of the stored energy during a deformation using atomistic level simulation. During a deformation, only a small percent of the energy input is stored in the material, and most of input energy is converted into heat. The cause of the temperature rise within materials is traditionally credited to dislocations, vacancies and other defects. An atomistic material model for fcc such as copper is used to calculated the stored elastic energy. The potential energy is obtained by a molecular dynamics (MD) simulation. Two different states are considered for comparison: one is a perfect state and the other is a state with dislocations. The calculated potential energies in these two states are compared. The difference in the potential energies is considered the amount of the stored elastic energy of dislocations. The conversion factor, i.e., the fraction of the input energy that is stored as elastic energy within a material is then calculated. (Received December 22, 2003)

keyword : Molecular dynamics, Dislocation, Stored energy

To characterize the nano-oxide layer (NOL) in specular spin valve multilayer, the naturally oxidized CoFe layer was inserted between pinned CoFe layers. The under-, optimum, and over-oxidized samples were obtained by control of the oxidation time and the oxygen flow rate. The NOL was analyzed to be Fe oxide mainly by XPS data. As oxidation increased, the content of Fe oxide and the thickness of the NOL increased and too much thicker was obtained NOL in case of over oxidation. We concluded that the lower MR and the more slanted magnetization curve than the optimum could be attributed to the Fe oxide as an impurity in case of under-oxidation and the too thick NOL in case of over-oxidation. (Received February 16, 2004)

keyword : Specular spin valve, Nano oxide layer, Fe oxide

To evaluate the possibility of SnBi coated Sn3.5 wt%Ag solder alloy to be used as low melting temperature solder, 3 different compositions of Sn-Bi layers was electroplated on Sn3,5wt%Ag solders. SnBi coated Sn3.5 wt%Ag rolled solders were bumped on Cu coupons by reflow machine at different temperatures. The microstructures were evaluated with SEM and EPMA, and the compositions of the microstructure were analyzed with EDS. The Sn-Bi coated Sn 3.5 wt%Ag solder was possible to be bonded at low temperature such as 200-250°C. Intermetallic compounds(IMCs) along the bonded interface were well formed at the coated solder composed of high Bi percentage, and IMCs were CuSn_(5), Ag_(3)Sn on the interface. In addition, Cu_(6)Sn_(5) IMCs were formed in solder due to Cu diffusion to the solder. However Bi was not observed on the interface and in solder. (Received December 22, 2003)

keyword : Low melting solder, Sn-Bi, Sn-3.5Ag, Electroplating