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The effect of the microstructure and temperature on the fatigue crack growth behavior of AISI type 347 stainless steels(SS) has been studied. The isothermal FCGR (fatigue crack growth rate) of the type 347 SS increased as the temperature increased. The FCGRs of the type 347 SS at various temperatures were slightly lower than those presented in the ASME code. Even though the effect of the microstructure on the crack growth rate was not significant in the steady-state FCGR region, it was observed that the ΔK(th) of a specimen with a high precipitate density was higher than that of a specimen with a low precipitate density. It was deduced that the difference in ΔK(th) resulted from a difference in the mean free path length of a dislocation movement. The FCGRs were higher than the isothermal FCGR when a thermal stress and a mechanical stress were imposed on the type 347 SS coincidently.

keyword : Fatigue crack growth rate, Threshold stress intensity factor, Type 347, Austenitic stainless steel, Niobium carbonitride, Thermo-mechanical Fatigue, TMF

Cu-Cr alloy is an attractive alternative material for application in metallic bipolar plates due to its good electrical conductivity, high mechanical strength and other favorable attributes. In order to improve the corrosion resistance, one barrier to its practical use, the formation of a continuous Cr-nitride on the surface of the alloy is necessary. Here, the formation behavior of Cr-nitride in Cu-Cr alloy was investigated in a nitrogen atmosphere in a temperature range of 600~1,000℃ using thermal nitridation. The optimum conditions to form the continuous Cr-nitride on the surface of Cu-Cr alloy were thereupon suggested. Nitridation of Cr started at about 600℃ and the only surface product was Cr2N. Both CrN and Cr2N phases are observed at 700℃, while only CrN phase was formed at and above 800℃. Therefore, it was concluded that Cr-nitride is formed by phase transformation in a sequence of Cr→Cr2N→CrN. The amount of Cr-nitride formed on the surface increased with an increase of Cr content, annealing time and annealing temperature. The continuous Cr-nitride was formed under the annealing for 12h at 1,000℃, but not under the annealing for 2h or below 900℃. Also it was formed in Cu-5.3(wt.%)Cr and Cu-7.6Cr, but not in Cu-2.4Cr. In Cu-Cr alloy, the continuous Cr-nitride(CrN) was formed with low Cr content in comparison with other alloys. This is attributed to the very low mutual solubility and absence of any intermetallic compound in Cu-Cr alloy.

keyword : PEMFC, bipolar plate, thermal nitridation, Cu-Cr alloy, Cr-nitride

Thicknesses of ε and γ`-iron nitride in bilayer(ε/γ`) compound were estimated using x-ray diffraction. The validity of formulation model was checked by comparing calculations with metallographic analysis of iron nitride compound layer grown on steels with gas nitriding. The thicknesses of ε and γ`-iron nitride formed on SPHC steel estimated by the model using ε(111) and γ`(200) diffraction intensity were 2μm and 1.7μm respectively. These are well matched with the results measured by microscope. The calculated thicknesses of the compound layers grown on low alloy steels well agree to the results measured by microscope. However, the estimated results for high alloy steels are thicker than metallographic analysis.

keyword : x-ray diffraction, iron-nitride, thickness analysis, bilayer compound model

In order to study the corrosion characteristics in coal-fired power plants, Cr-steels were corroded between 600℃ and 900℃ in Ar+0.2%SO2 gas for up to 300hrs, and their oxidation and sulfidation characteristics were studied. The corrosion resistance increased with increasing the Cr content. The major corrosion products were Fe3O4 and Cr5S6, indicating that both oxidation and sulfidation occurred simultaneously. The scales formed were bi-layered when the corrosion resistance were poor, while those formed were single-layered when the corrosion resistance were good.

keyword : SO2-gas corrosion, chromium steel, sulfidation, oxidation

A Ta metal has been produced by the carbon-reduction of Ta2O5 by Ar plasma melting. Concentration change of oxygen and carbon impurities in the Ta metal produced by the carbon-reduction has been also investigated. It was found that a pure Ta metal without Ta2O5 and TaC could be obtained when the ratio of carbon and Ta2O5(C/Ta2O5) was 5.0. Considering only total oxygen and carbon concentrations, the Ta metal with the purity of 99.85% could be obtained at the C/Ta2O5=5.0. The oxygen concentration in the Ta metal considerably decreased with increasing the C/Ta2O5, and the Vickers hardness of the Ta metal decreased down to about 230~240 Hv with the remarkable decrease in the oxygen concentration, which means that the hardness of Ta metal strongly depends on the oxygen concentration.

keyword : tantalum, plasma arc melting, carbon-reduction, refining, Vickers hardness, impurity

Recently, requirerr ent for the ultra thin copper foil increase with smaller and miniaturized electronic components. A study has been made to determine the effect of an electrochemically produced titanium-oxide film on the surface morphology and crystal structure of copper. The oxide films were made current step and potential step at 20℃ in 1.5M sulfuric acid on pure titanium metal. The formation of a microporous oxide layer enhances the surface morphology of copper foil. When a copper foil was electroplated on the polished titanium surface, large particles were existed on the surface of the copper film deposited. However, a smooth surface was obtained when the 130V anodized surface was used. The minimum value of surface roughness(Ra) was 0.254μm.

keyword : electrodeposition, copper foil, titanium anodizing, TiO2

Each layer of metals plated on Ball grid array package (BGA) substrates has its own specific functions. The compound of Sb is used to prevent from diffusing into solder balls. Reliability tests were conducted to investigate the effect of Lead free solder composition on ball fatigue strength. Samples were made using SnAg(3.5)Cu(0.5), SnAg4Cu(0.5) and SnAg(3.5)Cu(0.5)Sb2 solder compositions and were pre-conditioned at moisture resistance test (MRT) level 3. Temperature cycle tests (T/C) were done after pre-conditioning. It was found that two intermetallic layers formed formed when SnAg(3.5)Cu(0.5), SnAg4Cu(0.5) solders were used while only a single intermetallic layer was observed when SnAg(3.5)Cu(0.5)Sb2 solder was used. The effect of the weight ratio of Sb on the ball shear strength under reliability tests becomes very interested. The degradation of shear strength of solder balls after reliability test is discussed.

keyword : Intermetallic Layer, temperature cycle tests (T/C), Lead free solder

Nano-sized TiO2-CeO2 porous powders were synthesized by a sol-gel process using titanium iso-propoxide and Ce(NO3)3·6H20 as precursors. The particle size of as-received TiO2-CeO2 porous powders were measured range from 50 nm to 1.5μm by TEM. The as-received TiO2-CeO2 porous powders in an amorphous phase were crystallized with CeO2 at around 750℃. As the calcination temperatures increased, the amorphous TiO2 phase was transformed to anatase TiO2 at about 900℃ and the small amount of rutile TiO2 phase was also detected. The peaks obtained after calcining at 1,050℃ were mainly exhibited the rutile TiO2 and CeO2 phases. For the comparison of photocatalytic activity, TiO2-80wt.%CeO2 porous powder calcined at 900℃ showed an excellent efficiency for the removal phenol under UV light irradiation. However, TiO2-80wt.%Ce02 porous powder calcined at 600℃ showed a relatively higher visible absorption ability than other powders. TiO2-80wt.%CeO2 porous powders showed a relatively lower photoactivity efficiency under visible light irradiation compared with UV light irradiation.

keyword : nanoparticle, photoactivity, porous powder, sol-gel method