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The effects of volume fraction of tempered martensite on quasi-static tensile and dynamic deformation properties of Ti-6Al-4V alloy with bimodal microstructure were investigated in this study. Five microstructures with various tempered martensite volume fractions were obtained by varying heat-treatment temperatures and times. Dynamic torsional tests were conducted on them by using a torsional Kolsky bar. The test data were analyzed in relation to microstructures, tensile properties, and adiabatic shear band formation. Under a dynamic loading condition, maximum shear stress increased with increasing tempered martensite volume fraction, whereas fracture shear strain decreased. Observation of the deformed area beneath the fracture surface after the dynamic torsional test indicated that a number of voids initiated mainly at a/martensite interfaces, and that the number of voids increased with increasing martensite volume fraction. Adiabatic shear bands of 6-10 lam in width were formed in the specimens having lower martensite volume fractions, while they were not formed in those having higher martensite volume fractions. The possibility of the adiabatic shear band formation was explained by concepts of absorbed deformation energy and void initiation. (Received April 7, 2004)

keyword : Ti-6Al-4V alloy, Tempered martensite, Bimodal structure, Dynamic torsional test, Adiabatic shear band

In the present study, the mechanical and corrosion properties of the cast/forging and the low-pressure cast A356-T6 alloy were evaluated. It was observed that the tensile elongation and impact energy value were substantially higher for the cast/forged A356-T6 compared to those of the low-pressure cast counterpart, while the tensile strength values were similar with each other. The resistance to high cycle fatigue was similar with each other but slightly higher for the low-pressure cast product. The observed trends in the mechanical properties with different processing routes appeared to be largely related to the shape and size of silicon particles in the eutectic phase, as well as the shape of primary a phase. The resistance to pitting corrosion in 3.5% NaCl solution was also higher for the cast/forged product compared to the low-pressure cast counterpart. The resistance to corrosion in 3.5% NaCl solution was slightly higher for the cast/forged A356-T6 product than the low-pressure cast counterpart. The corrosion behavior of A356-T6 with different processing routes was also believed to be related to the area fraction between Si particles and the matrix. (Received March 29, 2004)

keyword : A356-T6 alloy, Cast/forging, Low-pressure casting, Mechanical properties, Corrosion properties

A blended NiCr-Cr₂O₃-Ag-CaF₂/BaF₂ feedstock was sprayed using an atmospheric plasma spraying. Physical and thermophysical properties of each constituent phase are quiet different and resulting interactions of each constituent with the plasma jet can be expected to be different. Thus, an attempt to change the plasma jet characteristics by changing the hydrogen gas flow rate in view of the thermophysical properties of the plasma jet was tried. According to the hydrogen gas flow rate, phase compositions and microstructures were evaluated. The effects of phase composition and microstructure on the Vickers microhardness and bond strength were also evaluated. For empirically tracing the each constituent particle trajectory, spot spraying bead was produced. Finally, particle temperature and velocity according to the hydrogen gas flow rate was measured using alumina-titania particle instead of the blended feedstock. (Received January 19, 2004)

keyword : Atmospheric plasma spraying, NiCr-Cr2O3-Ag-BaF2/CaF2 blended feedstock, Coating microstructure

Thermally oxidized Ti-Ag alloys, cp-Ti and Ti-A14V was investigated to find the proper heat treatment temperature and heat treatment duration for dental application. Surface microtopology by SEM showed oxide particles formed on all specimens treated at 600℃ and coarse, fully-crystallized lumps on specimens treated at 800℃. In macroscopic roughness measurement, Ra ranged from 0.083 to 0.29 }μm resulting from scratching by #600 emery paper; for non-contact method, from 0.34 to 2.84 nm resulting from the contour by oxide crystals on and between the ridges. The thicknesses of oxide layers were presumed to range from 100 to 250, 250 to 700, and over 1700 A for specimens treated at 400, 600, and 800℃, respectively. Oxide layers separated from the matrices on the cross sections of specimens atmosphere cooled after heat-treated at 800℃, which was not observed on those treated at 600℃. Oxidation behaviors of Ti-Ag alloys were observed to be similar to those of cp-Ti specimens by XRD analyses: TiO2 growth on surfaces followed by coexistence with Ti203`s as treatment temperature increased; TiO growth followed by spread of both rutiles and Ti203`s with temperature increase on specimens treated for 4 hours. Alumina was detected on Ti6A14V specimens, where microgaps between layers of alumina and titania considerably reduced the adhesive force of tinania layers on matrices. Conclusively, heat treatment temperature was more significant than treatment duration and the most desirable temperature was determined to be 600℃. (Received January 19, 2004)

keyword : Ti-Ag alloys, Thermal oxidation, Surface topography, Thickness of oxide layer, TiO2, Heat treatment temperature, Microgap

A coupled analysis model based on both growth model using vector calculus and heat transfer model using boundary fitted coordinate is presented, where calculation grids are generated along the preform boundary to consider the shape changes during spray forming. The effects of spray forming process parameters, such as substrate withdrawal velocity, materials of substrate, super heating of substrate, heat transfer coefficient of the preform surface, droplet temperature, droplet flow rate, on temperature profile of rod preform were investigated using the heat transfer model. As a result of numerical investigation, substrate withdrawal velocity, which changes preform shape and size, was shown to be the most effective parameter and both pouring temperature and heat transfer coefficient at the preform surface, which do not change the preform shape, were also calculated to be effective parameter to control the preform temperature. (Received february 11, 2004)

keyword : Spray forming, A coupled analysis model, Growth model, Heat transfer model, Boundary fitted coordinate

A numerical simulation based on the finite element method is used to investigate the effect of auxiliary electrode on the uniformity of deposition rate at a patterned electrode. The calculated deposition rates agreed well with the measured ones. Influences of the width of auxiliary electrode, gap between cathode and auxiliary electrode, distance between the inner wall of bath and auxiliary electrode and shape of auxiliary electrode on uniformity of electrodeposit have been studied. The optimum geometric factor of auxiliary electrode was obtained. (Received January 2, 2004)

keyword : Electrodeposition, Uniformity of current distribution, Auxiliary electrode

The melt-spun Fe_(73.5)Cu₁Nb₃Si_(13.5)B_(9) ribbons so called Finemet alloy were annealed in the temperature range of 490 to 610℃ for 1 h and milled by using a planetary ball mill into the powders with the size of 250~850 sm. Then the powder cores were produced through the blending of the magnetic powders with the 5 wt% of ceramic insulator and cold compaction. Further the annealing responses of magnetic, electrical properties and microstructure were investigated. At the annealing temperature of 550℃, the microstructure was transformed from amorphous to crystalline a-Fe phase with the grain size of 10.7 nm, and the electrical resistivity of 117 μΩ-㎝ was obtained. At the same time, the effective permeability and quality factor reached 120 and 50, respectively, and the permeability persisted up to about 500 kHz. Moreover the core loss at the frequency of 50 kHz and the magnetic induction of 0.1 T was about 300 mW/cm³. These magnetic properties were comparable to the conventional powder core materials such as MPP(molybdenum permalloy powder), Sendust etc. However the dc bias characteristics represented by the permeability stability upon dc magnetic field superposition were found to be unsatisfactory. (Received February 16, 2004)

keyword : Ultrafine grain structure, Ball milling, Annealing temperature, Powder core, Dc bias characteristics

Carbon nanotube cathodes with a high current density were investigated for use in flat panel displays. The field emission properties of cathodes fabricated by a screen-printing process were characterized in a high-vacuum chamber in the diode mode. The change in composition of the bonding materials comprise of a glass-frit and an Agpaste greatly affected the sheet resistance of the films after a heat treatment at 350℃. A higher volume fraction of insulating glass-frit in the bonding materials resulted in a larger value for the sheet resistance. However, the electric field enhancement factor(β) in the Fowler-Nordheim equation was not greatly affected by the composition changes of the bonding materials. Compared with the field enhancement factor, the current density was increased much more. Based on these findings, we conclude that the higher value of the current density was due to both an increased field enhancement factor and an increased number of carbon nanotubes protruding from the film surface. (Received May 10, 2004)

keyword : Carbon nanotube, Field enhancement, Field emission, Sheet resistance

Desulfurization behavior of hot metal was studied by bubbling Ar+CH₄mixture gas through submerged lance. The effects of melt temperature, CH₄partial pressure and gas flow rate on the desulfurization rate of hot metal are discussed. Desulfurization rate was increased by increasing temperature and partial pressure of CH₄. Desulfurization rate of hot metal was described by mixed control of the mass transfer of H₂ gas and [S] through gas and liquid film around the rising gas bubbles and chemical reaction at the interface. It was concluded that the desulfurization of hot metal by Ar+CH₄mixture gas bubbling is controlled by chemical reaction of H₂ gas and [S] at the gas bubble and liquid interface under the present experimental conditions. (Received February 2, 2004)

keyword : Desulfurization, Methane, Hot metal, Bubbling, Chemical reaction, Kinetics, Mass transfer

The effects of homogenization on hot rolling properties of gravity cast AZ31-xCa (x=0-2.0 wt.%) alloys were evaluated by examination of surfaces and microstructures of sheets. The side cracking during hot rolling was decreased by homogenization through the elimination of local inhomogeneity and solution of brittle precipitate into matrix. The coarse grains in raw materials were changed to fine grains with the average particle size of 5-10 μm by repeated hot rolling through the dynamic recrystallization. A deformation twinning was more favorably formed in the hot-rolled sheets after homogenization. It is considered that the grains grow and the precipitates are solved into the matrix during homogenization heat treatment, which results in the increased possibility of formation of deformation twinning during hot rolling. (Received March 29, 2004)

keyword : Magnesium alloy, Hot rolling Sheet, Homogenization