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A lattice variation with ordering treatment at 400 ℃ in Alloy 600 was systematically investigated using neutron diffraction. Alloy 600 was water-quenched, air-cooled, and furnace-cooled from solution anneal- ing treatment at 1095 ℃. These were ordering-treated up to 5,500 hours at 400 ℃. The results showed that the magnitude of lattice contraction depended on the cooling rate and decreased with the cooling rate. This is due to the fact that the number of ordering bonds increases with decrease in cooling rate. The lattice contraction appeared to be anisotropic according to the crystallographic planes. The water quenched specimen showed a maximum lattice contraction up to 0.04% in the {111} and {200} planes, and this was saturated after 2,000 hours. The lattice contraction due to the ordering reaction should be considered in the assessment of integrity in components made of Alloy 600, because the ordering reaction is a spontaneous and unavoidable phenomenon below 520 ℃ in the Alloy 600.

keyword : Alloy 600, cooling rate, ordering reaction, neutron diffraction, lattice contraction, solution anneal

The effect of temperature and substrate bias on the deposition behavior of boron carbide (BC) thin film was studied. BC thin films were deposited by an unbalanced magnetron sputtering (UBM sputtering) method. The B4C target was connected to a DC power supply at 200 W and sputtered by ionized argon gas. The distance between the substrate and target was 7.5 cm and the deposition pressure was 3 mTorr with argon gas. Silicon substrates were heated by halogen lamps from room temperature to 450 ℃. DC bias voltage applied to substrates up to .100 V. The deposited films showed no diffraction peak on either X-ray diffraction and transmission electron microscopy analysis, which indicated an amorphous nature of the films irrespective of deposition temperature and substrate bias in this study. Only a 1280 cm.1 absorption peak of Fourier transform infrared spectroscopy was observed. The hardness of the BC films was about 40 GPa regardless of deposition temperature.

keyword : thin films, sputtering, crystallization, transmission electron microscopy, boron carbide

Grain refinement of cast magnesium alloys containing Al is successfully attained using MnCO3 as a grain refiner. However, the direct addition of MnCO3 in a magnesium melt causes the oxidation and ignition of the melt due to the gases that formed during the decomposition of MnCO3. To improve the effectiveness and safety of the MnCO3 addition process, inoculation was conducted by adding an Al master alloy with MnCO3. The Al master alloy was prepared using cylindrical pressed pellets composed of a mixture of MnCO3 powders and metal chips in the aluminum melt. The results revealed that the inoculation method using the Al master alloy is more effective and stable than previously used methods. The grain size of the as-cast AZ91 Mg alloy was significantly decreased from 420 to 70um by adding the Al master alloy of 3 wt%, which is mainly attributed to the formation of thermally stable Al4C3 particles as an effective grain refiner in the melt.

keyword : metals, casting, grain refinement, optical microscopy, magnesium

This study is an evaluation on the effects of pre-heating on the weldability and corrosion resistance of 690 MPa grade, quenched and tempered steel. Micro-structural examination on pre-heated specimen confirmed that pre-heating affects coarse grain microstructures in comparison to the none pre-heated specimen. The tensile strength of welded specimen without pre-heating is higher than the tensile strength of welded specimen with pre-heating. The weld of pre-heated specimen, however, exhibits higher impact absorbed energy capability. The weld fracture morphology reveals both quasi-cleavage and ductile fractures. Corrosion test results on both welds showed increasing current density with increasing concentrations of NaCl solution. The pre-heated specimen exhibited higher corrosion current. Image Quality Mapping analysis in EBSD after impact testing determined less deformation on the pre-heated specimen.

keyword : metals, welding, mechanical properties, electron backscattering diffraction(EBSD), preheating

This paper identified the effects of Nb on microstructure. Also, it has studied on uniform and pitting corrosion resistance in a ferritic stainless steel weld metal of the automobile exhaust system. We fabricated 3 flux cored wires designed with 0-1.0 wt% Nb and performed Flux Cored Arc Welding. We observed the microstructure with the SEM/EDS and EBSD. To evaluate the uniform and pitting corrosion resistance, we performed a potentiodynamic polarization test in 0.2 M H2SO4 and 0.1, 0.3, 0.5 M NaCl. As a result of the tests, we found that as the amount of addition of Nb rose, the amount of Cr-carbide fell. The microstructure became more refined. The specimen with 1.0%Nb added had the highest uniform and pitting corrosion resistance. After the pitting corrosion test, a pit was formed at the grain boundary that has no addition of Nb. In addition, in the specimen with added Nb, pits were formed at the inclusions.

keyword : alloys, welding, precipitation, electron backscattering diffraction(EBSD), stainless steel

12-nm-thick Au films on silicon nitride coated Si substrates were annealed at elevated temperatures in a high vacuum or forming gas, and the changes in film morphology and microstructure were investigated. The microstructure was observed using SEM and in-situ TEM, and the crystal structure was analyzed using an automatic crystal orientation and phase mapping system (ASTAR) in TEM. In situ observation of the Au films showed rapid growth of fine grains from the very early stage of annealing, followed by holes formation at grain boundary junctions, and the holes grew and coalescenced. Annealing at a lower temperature of 400 ℃ showed that the holes grew with faceted edges that were parallel to the low surface energy crystal planes, while holes at a higher temperature occurred with rounded edges. These morphological and crystallographic evolutions of Au film during dewetting are discussed in detail in view of the diffusion mechanism and surface energy.

keyword : thin films, annealing, microstructure, transmission electron microscopy (TEM), dewetting, crystal orientation image mapping (OIM)

Thin film transistors (TFTs) with Mg-doped ZnSnO (MZTO) channel layers were fabricated by a sol-gel process. The effect of annealing temperature on the electrical characteristics of bottom-gate MZTO TFTs was investigated. For the sample annealed at 400 ℃, an amorphous phase was formed, while for the sample annealed at 600 ℃, a nanocrystalline MZTO film was obtained. The electrical properties of MZTO TFTs were very sensitive to the annealing temperature. High temperature annealing caused the threshold voltage to shift toward a negative direction and decreased the field-effect mobility and on/off current ratio. The MZTO TFT annealed at 400 ℃ showed an appropriate threshold voltage, on/off current ratio, and field-effect mobility.

keyword : oxides, sol-gel, electrical properties, microstructure, transmission electron microscopy (TEM)

The defects and origin of the good varistor properties in the ZnO-Co3O4-Cr2O3 system were investigated by admittance spectroscopy, I-V characteristics, and impedance and modulus spectroscopy. Two kinds of defects were detected, but (0.27 eV) was identified as a major donor level by admittance spectroscopy. The ZnO grain resistivity of ~0.4 Ωcm was calculated but somewhat increased with sintering temperature. J-E characteristics with varistor behavior was seen in this system while the nonlinear coefficient α changed from 9 to 92 with sintering temperature. The single potential barrier of 0.64-1.01 eV at the grain boundary region was confirmed by impedance and modulus spectroscopy. The origin of a good varistor behavior in ZnO-Co3O4-Cr2O3 would be due to the formation and stabilization of a double Schottky barrier by the redox reaction of Co ions and the existence of small Cr ions in the grain boundaries.

keyword : electronic materials, ZnO varistor, sintering, defects, grain boundary

Electronic structures and X-ray absorption spectra of rutile TiO2 were obtained with the first- principles calculation. Density of states, charge density and differential charge density were obtained, from which charge transfer characteristics between Ti and O was well confirmed. Direct and indirect band gap of 1.99 and 2.03 eV were evaluated, respectively. X-ray absorption spectra of rutile TiO2 was also calculated, in which the gap between main peaks was comparable with experiment. From band structure calculations, anisotropic electrical transport properties were expected, which was in good agreement with experiment.

keyword : oxides, powder processing, electrical properties, computer simulation, TiO2

Ni and LiBH4 with a high hydrogen-storage capacity of 18.4 wt% were added to MgH2. A sample with a composition of 86 wt% MgH2-10 wt% Ni-4 wt% LiBH4 (named MgH2-10Ni-4LiBH4) was prepared by milling under H2 (reaction-involved milling, RIM), and its hydriding and dehydriding properties were then examined. The activation of MgH2-10Ni-4LiBH4 for hydriding and dehydriding reactions was not required. The as-milled sample absorbed 2.54 wt% H for 5 min, 3.72 wt% H for 10 min, 4.90 wt% H for 20 min, and 4.90 wt% H for 60 min at 623 K under 12 bar H2, absorbing nearly 5 wt% H for 60 min. The as-milled sample desorbed 2.86 wt% H for 5 min, 4.78 wt% H for 15 min, and 4.94 wt% H for 60 min at 623 K under 1.0 bar H2. The inverse dependence of the hydriding rate on temperature is due to a decrease in the driving force for the hydriding reaction (the difference between applied hydrogen pressure and equilibrium plateau pressure) as the temperature increases.

keyword : hydrogen absorbing materials, mechanical alloying/milling, microstructure, X-ray diffraction, Mg-x wt% TaF5