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The phase boundary and Nb solubility of Zr-xNb binary alloys containing 2000 ppm oxygen were investigated by using DSC, metallography and TEM. To obtain the equilibrium phase, the alloys of Zr containing 0 to 10 wt.％ Nb were annealed for a long time at different temperature. The phase transformation temperature from alpha to alpha+beta and alpha+beta to beta in the Zr containing 2000 ppm oxygen was analyzed at 980℃ and 1025℃. The phase transformation temperature of the Zr containing 2000 ppm oxygen was increased than that of the Zr having 700 ppm oxygen. The monotectoid temperature of the Zr-Nb alloys containing 2000 ppm oxygen was revealed at 585℃. From the analysis of precipitates using TEM equipped EDS, Nb-containing precipitates were observed on the Zr-0.2Nb alloy that annealed at monotectoid temperature. This means that the maximum Nb solubility in Zr-xNb binary alloy containing 2000 ppm oxygen would be lower than 0.2 wt.％.

keyword : Sponge Zr, Zr-Nb, Alpha, Beta, Monotectoid, Microstructure, Precipitation

Thermal barrier coating(TBC) is generally applied to turbine engines in order to protect metallic components from high temperatures, and thus, to increase the combustion efficiency. Most widely used yttria stabilized zirconia YSZ;ZrO_(2)-8 wt.％Y_(2)O_(3) have the poor resistance to hot corrosion under the operating conditions of low quality fuel and sea or desert environments. In order to overcome this problem, ceria stabilized zirconia (CSZ; ZrO_(2)-25wt.％CeO_(2)-2.5wt.％Y_(2)O_(3) has been suggested as a new protective materials for the TBC. In this study, Duplex type YSZ- and CSZ-TBCs with similar microstructure were fabricated by detonation gun spraying. High temperature corrosion tests of the TBC specimens, on which NaVO_(3) salt were pasted, were performed at 900°C. Results showed that CSZ TBCs is more resistant to NaVO_(3) salt than YSZ TBCs in terms of phase stability and overall damage by salt. Studies in effects of microstructure indicate that YSZ and CSZ TBCs are degraded by different behavior of stabilizer during the tests and microstructure of TBCs, such as pores, micro cracks and splat boundaries, plays an important role in hot corrosion. From the results stepwise microscopic degradation mechanism of zirconia based TBCs by hot corrosion was discussed.

keyword : Thermal barrier coating, Hot corrosion, YSZ, CSZ

The oxidation behavior of TiN, TiCN and CrN coatings sputtered on a steel substrate was investigated at 850℃ in air. Both TiN and TiCN oxidized to Tio₂ while CrN oxidzed to Cr₂O₃. The oxidation resistance decreased in the order of CrN, TiN and TiCN. During oxidation, the outward diffusion of substrate elements such as Fe, Si occurred to a certain degree. The enrichment of substrate element of Cr at the oxide scale and the depletion of nitrogen at the lower part of the unoxidized CrN coating were observed.

keyword : Oxidations behavior, TiN, TiCN, CrN

The microstructure and the corrosion behavior of aluminized steel sheet and hot-dipped Al-Cr wteel sheet were investigated. The EPMA, GDLS and XRD measurements of the Al-Cr coated layer showed that the coating layer was characterized by three phases. The surface layer was Al, the middle layer was Al_(13)Cr_(2), and the interfacial layer between the Fe substrate and the coated layer was composed of Al_(13)Fe_(4) and Al_(5)Fe_(2). The polalization curve indicates that the corrosion behavior of Al-Cr coated layer was different from that of Al coated layer. For the Al coated layer, porosities in the Al-oxide film on the surface layer lead to preferential corrosion. After the Al coated layer was completely removed, the Al_(5)Fe_(2) layer was selectively corroded and then, the steel substrate was exposed to the corrosion medium. In case of the Al-Cr coated layer, the Al_(13)Cr_(2) layer became a barrier against further corrosion after the Al layer was removed. In this case, corrosion was not proceeded even through the same current density. The high corrosion resistance was due to the existence of the Al_(13)Cr_(2) layer.

keyword : Aluminized steel sheet, Al-Cr coated steel sheet, Corrosion resistance

The correlation between sensor output and corrosion rate of reinforcing steel was evaluated by laboratory electrochemical tests in saturated Ca(OH)_(2) solution with 3.5wt.％ NaCl. In this paper, two types of electrochemical probes were developed: galvanic cells containing of steel/copper and steel/stainless steel couples. The corrosion behavior in saturated Ca(OH)_(2) solution with or without 3.5 wt％ NaCl addition for the different electrodes was investigated by potentiodynamic test. Weight loss measurement and galvanic corrosion test were conducted to obtain the corrosion rate of reinforcing steel and the charge of sensor in saturated Ca(OH)_(2) solution with 3.5 wt.％ NaCl addition, respectively. The results of the potentiodynamic test indicated the possibility of detecting an ingress point of chlorides by measuring the galvanic current. In galvanic corrosion tests, the galvanic current of steel/copper couple was higher than that of steel-stainless steel couple, i.e., the steel/copper sensor is more suitable for high resistance environment. The steel/stainless sensor showed a better linear correlation than the steel/copper sensor. Through the relationship between the sensor system output and the weight loss of steel, real corrosion damage of the steel embedded in concrete can be detected.

keyword : Galvanic sensor system, Potentiodynamic test, Immersion test, Galvanic corrosion test, 3.5 wt% sodium chloride, NaCl, Calcuim hydroxide, Ca(OH)2

The effect of equal channel angular pressing (ECAP) pass number on the electrochemical properties of AA 1050 (UNS A91050) in 0.1M Na₂SO₄ solution containing 100 ppm chloride were investigated by electrochemical techniques (potentiodynamic test, potentiostatic test, electrochemical impedance spectroscopy (EIS) and surface analyses (OM, SEM, EDS). The ECAPed specimens that have a various numbers of passes from N=l to N=5 were selected for electrochemical experiments in this study. The results of potentiodynamic test indicated that the breakdown potential increased with increasing the ECAP pass number. EIS measurements also showed that the corrosion resistance of the AA 1050 increased with increasing the ECAP pass number at the passivation and breakdown states. The results of surface analyses (SEM, EDS) showed that silicon-containing impurities were observed on the surface of the alloy. It was found that the size of these Si-containing impurities decreased with increasing the ECAP pass number. The Si-containing impurities induced the microgalvanic reaction(i) between the Al matrix and the Si-containing mixed oxide and (ii) between the Al matrix and the Si-containing impurities. Due to its ultrafine-grained microstructure, ECAPed AA 1050 had smaller Si-containing impurities, and reduced microgalvanic currents. Thus, in the case of ECAPed AA 1050, it has increasing corrosion resistance with increasing ECAP pass number because of its small cathodic impurities.

keyword : ECAP, AA 1050, Cathodic impurity, Local microgalvanic cell

Both fluid flow in the plasma region and heat/momentum transfers from the plasma to the molten steel surface in a DC arc furnace were numerically analyzed by considering the shape change of a molten steel surface due to the high velocity plasma jet. Calculated plasma characteristics and heat and momentum transfers from the plasma to the molten steel were compared and discussed with previous results which had been calculated under the assumption of the flat surface of the molten steel. It was found that the formation of a groove enlarged the current flowing area on the melt surface. Although the heat transferred on the melt surface and current flowing area increased by the groove formation, the shear surface acting on the melt surface decreased. By increasing arc current, the radius and depth of grooves, the joule heat, the radiation loss, the heat and shear force transferred to the melt surface and groove area increased. By increasing arc length, the groove radius increased and the groove depth decreased, respectively. And the joule heat generated, the radiation loss and the heat transferred to the melt surface and the shear force acting on the melt surface decreased. But the heat transferred to the groove area and the shear force acting on the groove area decreased.

keyword : DC arc furnace, Numerical analysis, Molten steel, Electromagnetic force

Heat- and fluid-flow phenomena in the molten steel region in a DC arc furnace were investigated by simultaneous analysis of electromagnetic, flow and temperature fields. It was found that the flow pattern in molten steel was determined by electromagnetic and buoyancy forces rather than the shear force at the melt surface due to the high velocity plasma arc. While the short arc with high current made the hot region beneath the arc expanded, the long arc with high current drove the melt temperature high. And the low arc current led to the non-homogeneous temperature distribution of the melt because the bottom region of the melt was not stirred sufficiently.

keyword : DC arc furnace, Numerical analysis, Molten steel, Electromagnetic force, Buoyancy force

Solvent extraction equilibria of FeCl_(3) from chloride solutions with TBP were analyzed by considering chemical reaction, mass balance, charge balance and extraction reaction. The activity coefficients of solutes in the aqueous phase were calculated by Bromley equation. Extraction reaction of FeCl_(3) with TBP in the experimental range of this study was determined from the dependence of the distribution coefficient of FeCl_(3) on the chloride ion concentration. By applying ionic equilibria to the results of solvent extraction, equilbrium constant of the extraction reaction was estimated to be 3.43×10^(2). The equilibrium concentrations of solutes in both phases after extraction could be calculated with the ionic equilibria developed in this study. The predicted concentrations of Fe in the aqueous phase after extraction agreed well with those experimentally measured.

keyword : FeCl3, TBP, Ionic equilibria, Equilibrium constant, Bromley

Cu thin films have been deposited on Si (100) substrate by using a non-mass separated ion beam deposition system. The effect of the substrate bias voltage on the properties of the deposited films was investigated using X-ray diffraction, resistivity measurement and scanning electron microscopy. In the case of Cu thin films deposited without bias voltage, a columnar structure and small grains were observed distinctly, and the electrical resistivity of the deposited Cu films was very high. By applying the substrate bias voltage, no clear columnar structure and grain boundary were observed. The resistivity of Cu films decreased remarkably and reached a minimum value of 1.8±0.1μΩcm at a bias voltage of -50V, which is close to that of Cu bulk (1.67 μΩcm).

keyword : Copper, Ion beam deposition, Resistivity, Substrate bias voltage