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It was evaluated that the microstructural and mechanical properties of Ta-bearing 9Cr-0.5Mo-2W ferritic/martensitic experimental steels. All the experimental steels showed the tempered martensitic microstructures, and M23C6 carbides, whose sizes were ranged from 200 to 300 nm, were easily observed at both boundaries of the prior austenite grain and the martensite lath. In addition, a relatively large Nb-rich MX carbonitrides were intermittently detected at the prior austenite grain boundaries, whereas a lot of V-rich MX carbonitrides, whose mean diameter was less than 50 nm, were observed randomly at both boundaries. Ta was mainly incorporated into the V-rich MX carbonitrides rather than the Nb-rich ones and their content was spanned from 5 to 20 at.%. Ta contents within the MX precipitates also increased as the content of Ta increased. Because the Ta addition into the steels would be attributed to the precipitation strengthening, solid solution strengthening and lath width reduction, it was shown that the mechanical properties, including hardness, tensile strength and creep rate of the 9%Cr-0.5Mo-2W steels were improved by the increase of Ta content. Especially, 9Cr-0.5Mo-2W-0.3V-0.05Nb-0.14Ta steel was revealed to be relatively excellent in the application for the SFR fuel cladding.

keyword : sodium-cooled fast reactor, SFR, fuel cladding, ferritic/martensitic steel, Ta-bearing steel, microstructure, mechanical properties

Cu as a tramp element has been reported to encourage transverse cracking upon straightening operation during continuous casting or mini-mill processing. Therefore, the hot workability of steels containing Cu should be investigated. The purpose of the present study was to examine the effect of Cu contents on the hot ductility of low carbon steels by using hot compression test. Hot compression test was carried out using a Gleeble. The specimens were heated to 1300℃ for solution treatment and then held for 300s before cooling at a rate of 1℃/s to test temperatures in the range of 650~1150℃ (50℃ intervals) with strain rate of 5×10(-3)/s. In Cu containing steels, the hot ductility was decreased with increasing Cu content at high temperature region which is to be attributed to copper enriched phase formed at scale/steel interface, and low hot ductility with increasing Cu content at low temperature region is attributable to the strengthening of matrix by the formation of ε-Cu. The width of ductility trough region was decreased with increasing Cu content.

keyword : hot ductility, copper, continuous casting, ε-Cu

The precipitates, hardness, and tensile properties of Al-6.2Si-2.9Cu AC2B alloy were investigated with respect to solution treatment time at 500℃. Al(Cu)-Al2Cu eutectic, Si, θ-(Al2Cu), and Q-(Al5Cu2Mg8Si6) phases were observed in the as-cast specimen. With increasing the solution treatment time at 500℃, the Al(Cu)-Al2Cu eutectic and θ-(Al2Cu) phases were gradually reduced and finally almost disappeared in 5 h. The mechanical properties, such as hardness, tensile strength, and elongation, were improved with solution treatment time until about 5 h due to the dissolution of the Al2Cu particles. With further holding time, the mechanical properties did not change much. The solution treated specimens for over 5 h at 500℃ exhibit almost the same tensile properties even after aging at 250℃ for 3.5 h. Accordingly, the optimal solution treatment condition of the Al-Si-Cu AC2B alloy is considered to be 5 h at 500℃.

keyword : Al-Si-Cu AC2B alloy, θ-(Al2Cu), solution treatment, precipitation, aging

It is well known that the formation of SiO2, Al2O3 and/or other oxides at the steel surface during the annealing process deteriorates the surface quality of galvanized steels. It is important to minimize oxide formation during the annealing process for the superior surface quality of galvanized steels. In order to minimize the oxide formation on the steel surface, antimony was chosen as an alloying element to the commercial steels. Then, the effect of alloying element on the oxidation behavior was investigated. A small amount of antimony was added to two types of steels, one with 0.1% C, 1.0% Si, 1.5% Mn, 0.08% P, and the other with 0.002% C, 0.001% Si, 0.104% Mn, 0.01% P. Then, the oxidation behavior was investigated from 650~900℃ in the air. The addition of antimony to the steels retarded the outward diffusion of elements during the oxidation, resulting in reduction of the oxidation rate.

keyword : galvanized steels, antimony, oxidation, Pt-marker

The STS304 stainless steel was oxidized isothermally and cyclically at temperatures between 1050 and 1200℃ for 1 hr in air. During isothermal oxidation, it displayed good oxidation resistance at 1050℃. However, it suffered from breakaway oxidation above 1100℃, being accompanied with internal oxidation. During cyclic oxidation, it also displayed good oxidation resistance at 1050℃, but it suffered from massive weight loss above 1125℃. The oxide scales formed consisted primarily of Fe2O3, Fe3O4 with and without Cr2O3. They were generally non-adherent.

keyword : Stainless steel, STS 304, oxidation, chromium

On the Transient Liquid Phase Bonding (TLPB) phenomenon with the MBF-50 insert metal at narrow gap (under 100), it takes long time for the bonding and the homogenizing. Typically, isothermal solidification is controlled by the diffusion of depressed element of B and Si. However, the amount of B and Si in the MBF-50 filler metal is large. This is reason of the long bonding time. Also, the MBF-50 filler metal did not contained Al and Ti which are γ` phases former. This is reason of the long homogenizing time. From the bonding phenomenon with the MBF-50 insert metal, we search main factors on the bonding mechanism and select several insert-metals for using the wide-gap TLPB. New insert-metals contained Al and Ti which are γ` phases former and decrease the B then the MBF-50. When the new insert-metal was used on the TLPB, the bonding time was decreased about 1/10 times and homogenizing heat treatment was no needed. In spite of the without homogenizing, the volume fraction of γ` phases in the boned interlayer was equal to homogenizing heat treated specimen which was TLPB with the MBF-50. Finally, the new insert metal named WG1 for the wide-gap TLPB is more efficient then the MBF-50 filler metal without decreasing the bonding characteristic.

keyword : transient Liquid phase bonding, directionally solidified Ni base superalloy, GTD111, filler metal, wide-gap brazing, powder, microstructure

A micro thermoelectric device was processed by electroplating the n-type Bi-Te nanowires and p-type Sb-Te nanowires into an alumina template with 200 nm pores. Power generation characteristics of the micro devices composed of the Bi-Te nanowires, the Sb-Te nanowires, and both the Bi-Te and the Sb-Te nanowires were analyzed with applying a temperature difference of 40℃ across the devices along the thickness direction. The n-type Bi-Te and the p-type Sb-Te nanowire devices exhibited thermoelectric power outputs of 3.8×10(-10) W and 4.8×10(-10) W, respectively. The output power of the device composed of both the Bi-Te and the Sb-Te nanowires decreased to 1.4×10(-10) W due to a large electrical resistance of the Cu electrode connecting the Bi-Te nanowire array with the Sb-Te nanowire array.

keyword : thermoelectrics, nanowire, bismuth telluride, electrodeposition, thermoelectric generation

This study examined the effects of plated temperature on the surface morphology and property of an electrodeposited copper foil. The morphology, crystal structure and electric characteristics of the electrodeposited copper foil were examined by scanning electron microscopy, X-ray diffraction, and a four-point probe, respectively. The surface roughness, crystal growth orientation and resistivity could be controlled using various temperature of electrolyte. Large particles were observed on the surface of the copper layer electroplated onto the 30℃. However, a uniform surface, lower resistivity and high flexibility were obtained when a 50℃ electrolyte was used.

keyword : FCCL, temperature, flexibility, resistivity

The mechanical and electrical properties of ball grid array (BGA) solder joints were measured, consisting of Sn-3.5Ag, with organic solderability preservative (OSP)-finished Cu pads and Electroless Nickel/Immersion Gold (ENIG) surface finishes. The mechanical properties were measured by die shear test. When ENIG PCB was upper joint and OSP PCB was lower joint, the highest shear force showed at the third reflow. When OSP PCB was upper joint and ENIG PCB was lower joint, the highest shear force showed at the forth reflow. For both joints, after the die shear results reached the highest shear force, shear force decreased as a function of increasing reflow number. Electrical property of the solder joint decreased with the function of increasing reflow number. The scanning electron microscope results show that the IMC thickness at the bonding interface gets thicker while the number of reflow increases.

keyword : BGA, Solder joint, Pb-free solder, OSP, ENIG