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Relationship between the phase stability and the composition of L1_2-based Ti-Al-Cr alloys at 800℃ was investigated. It was found that the yield strength of Al-21Ti-23Cr and Al-21Ti-15Cr alloys with Ti-lean composition after exposure at 800℃showed a relatively small increase compared with that of alloys homogenized at 1150℃. However, yield strength of al-30Ti-15Cr alloy with Ti-rich composition after exposure at 800℃ increased very remarkably. The difference in strength increment after exposure at 800℃ was due to the partial decomposition of L1_2 phase in Al-21Ti-23Cr and Al-21Ti-15Cr alloys, and the complete decomposition of L1_2 phase in al-30Ti-15Cr alloy. Therefore, alloys with Ti-lean composition were expected more suitable for light heat resistant and oxidation resistant coating material with respect to stability of L1_2 phase. It was also revealed that the phase decomposition of L1_2 phase resulted from the shift of L1_2 phase region to Al-rich corner in Al-Ti-Cr phase diagram with decreasing temperature.

keyword : L12 phase, (Al, Cr)3yTi alloys, Phase stability, Phase decomposition

This study was investigated the effect of the Cu addition and subzero treatment in austempered ductile cast iron. With increasing austempering time, volume fraction of retained austenite increased and is decreased after showed the peak value. Specimen with more Cu contents had a less quantity of retained austenite. Retained austenite transformed to martensite by subzero treatment. With decreasing subzero treatment temperature, retained austenite of specimens with more Cu contents transformed more Cu contents transformed more to martensite. With decreasing subzero treatment temperature the value of strength and hardness increased but the value of elongation and impact decreased. In this study, we could find that in subzero treated specimens an addition of Cu had a little of effect on the strength and the hardness but had very much effect on the value of the impact.

keyword : Subzero treatment Retained austenite, Ductile cast iron, Austempering treatment, Volume fraction of martensite

The effects of growth of the Υ` and η phase on the LCF in Nb-A286 superalloy are investigated by fatigues test, transmission electron microscopy (TEM) and scanning electron microscopy (SEM). The change of precipitates morphology, amount of phase and control of size are also discussed. Under the continuous low cycle fatigue, fatigue life is increased with increasing Υ` size and amount of ηphase. In the investigation of a fracture surface after low cycle fatigue test, fracture mode is changed from intergranular fracture mode to transgranular fracture mode with increasing Υ` size and amount of η phase. It is changed from planar slip to bowing dislocation with increasing Υ` size. Slip induced cavitation by local stress concentration due to planar slip impinged on the grain boundary is retarded by a larger precipitation than that of unaged sample. Therefore, fatigue life is increased with Υ` size and amount of η phase. These results will by applied to the development of a new alloy for the improvement of fatigue life.

keyword : High temperature low cycle fatigue, γ`, h phase, Intergranular fracture, Nb-A286 superalloy

The objective of this study is to investigate the delayed hydride cracking (DHC) velocity and the incubation time for the water-quenched and furnace-cooled Zr-2.5Nb tube with the different radius of a notch tip. DHC tests were carried out at constant K_1 of 20 MPa√m and 250℃ on the cantilever beam (CB) specimens subjected to furnace cooling or water quenching after electrolytic charging with 57 or 72 ppm hydrogen, respectively. An acoustic emission sensor was attached to the CB specimens to detect the incubation time before the start of DHC. The shape of the notch tip changed from the fatigue crack to the dull crack with its radius ranging from 0.1 to 0.15 mm. The DHC incubation time increased remarkably with the increasing radius of the notch tip, which appeared more strikingly on the furnace-cooled CB specimens than on the water-quenched ones. However, both furnace-cooled and water-quenched CB specimens indicated little change in the DHC velocity with the radius of the notch tip unless their notch tip exceeded 0.125mm. These results demonstrate that the uncleation rate of hydrides at the notch tip determines the incubation time and the DHC velocity becomes constant after the concentration of hydrogen at the notch tip reaches the terminal solid solubility for dissolution (TSSD). This observation agrees well with Kim`s DHC model. A difference in the incubation time and the DHC velocity between the furnace-cooled and water-quenched specimens is discussed in terms of the uncleation rate of hydrides at the notch tip and the hysteresis of hydrogen solubility.

keyword : Zr-2.5Nb, DHC Velocity, Incubation time, Furnace-cooling, Water-quenching, Hydrogen, Solubility

A bimodal microstructure containing very fine α_2 particles was obtained by over-aging of a Ti-6Al-4V alloy, and its quasi-static and dynamic deformation behaviors were investigated in comparison with those of an unaged bimodal microstructure. Quasi-static and dynamic torsional tests were conducted on them by using a torsional Kolsky bar. The quasi-static torsional test data indicated that the over-aged specimen showed higher fracture shear strain than the unaged one, while their maximum shear stress was similar. Under dynamic torsional loading, both maximum shear stress and maximum shear strain showed higher than those of the unaged microstructures. The possibility of the adiabatic shear band formation under dynamic loading was quantitatively analyzed by critical shear strain and absorbed plastic deformation energy concepts. Fine α_2 precipitation in αphase by over-aging was effective in both the improvement of quasi-static and dynamic torsional properties and the reduction in the adiabatic shear banding, which provided a new idea to improve ballistic performance of Ti alloy armor plates.

keyword : Ti-6Al-4V alloy, Bimodal, Dynamic torsional test, α2-Ti3Al precipitate, Adiabatic shear band, Void initiation, Critical shear strain

In this study, Widmansta¨tten and equiaxed microstructures containing very fine α_2 particles were obtained by overaging a Ti-6Al-4V alloy, and their dynamic deformation behavior was investigated. Dynamic torsional thests were conducted by using a torsional Kolsky bar, and deformed microstructures and torsionally deformed areas beneath fracture surfaces were observed to investigate various factors affecting dynamic deformation properties of α, β and their interfaces. The dynamic torsional test results indicated that maximum shear stress and fracture shear strain of the over-aged Winmansta¨tten and equiaxed microstructures were higher than those of the unaged microstructures. The possibility of the adiabatic shear band formation under dynamic loading was quantitatively analyzed, based on the observation how plastic deformation energy was distributed to void initiation and adiabatic shear banding. Since the number of voids formed in the over-aged microstructures drastically increased because of the α_2 precipitation, driving force for the adiabatic shear band formation decreased as much as deformation energy used for the void formation, thereby reducing the possibility of the adiabatic shear band formation. These findings provided a new idea how to improve ballistic performance of Ti alloy armor plates because the over-aging of the Ti-6Al-4V alloy was effective on both the improvement of tensile and dynamic torsional properties and the reduction in the possibility of the crack initiation due to adiabatic shear banding.

keyword : Dynamic deformation behavior, Ti-6Al-4V alloy, α2-Ti3Al precipitate, Widmanstatten microstructure, Equiaxed microstructure, Dynamic torsional test, Adiabatic shear band

The welding characteristics and grooving corrosion resistance of electrical resistance welded steel pipes were studied. The macrostructures revealed that optimal width of bond layer for welding was 80-90 μm, while those formed with low heat input showed negligible bond layer. However, metal flow and heat affected zone did not show any appreaciable differences as long as welding bead were formed. The grooving corrosion, which is typical corrosion form in the steel pipes, is caused mainly by the different microstructures between the matrix and weld those are formed during the rapid heating and cooling cycle in welding. Even though the diminishing of sulfur content is most effective to decrease the susceptibility of grooving corrosion, it requires costly process. In this study, improvement of grooving corrosion resistance was pursued by post weld heat treatment in the temperature range between 650℃ and 950℃. By employing chromno-amperometry and potentiodynamic experiments, the corrosion rate and grooving corrosion index(α) were obtained. It was found that heat treatment could improve the grooving corrosion resistance. Among them, the heat treatment at 900℃ and 950℃ endowed excellent grooving corrosion resistance. The index of heat treated specimen at 900℃ and 950℃ were average 1.2, which are almost immune to the grooving corrosion according to KS. Potential differences between base and weld metal were decreased considerably after the heat treatment. While the as-welded one measured 61∼71mV, that of the 900℃ heat treated steel pipe measured only 10 mV.

keyword : Grooving corrosion, Electric resistance welding, Galvanic corrosion, Post-weld heat treatment

The tunneling barrier is crucial to the overall performance in magnetic tunnel junction. We have suggested a new formation method for the tunnel barrier, which has utilized pseudo atomic layer deposition with sputtering (PALDS). As is well known, all metallic thin films oxidize more or less under atmospheric conditions. With using this property of metallic thin films, we have prepared tunnel barrier by sputtering metallic thin film and exposing it to the oxygen ambience. From transmission electron microscopy (TEM) observation, the formed tunnel barrier has been confirmed to have clear and good interface between magnetic layers and tunnel barrier. From atomic force microscopy (AFM) observation, it has been also confirmed to have low surface roughness. The TMR MRAM cell manufactured using PALDS method has been shown to exhibit typical tunnel resistance of 86 KΩ·μm＾2 and TMR ratio of about 40%. The 40% of MR is quite large. It is considered that this value is geometrically enhanced magnetoresistance due to the low junction resistance of MTJ.

keyword : Magnetoresistive RAM (MRAM), Pseudo Atomic Layer Deposition with Sputtering (PALDS), Magnetic tunnel junction (MTJ), Tunnel magnetoresistance (TMR)

Uniform droplet of lead-free solder balls was produced by using the Droplet-Based Manufacturing(DBM) process. Compositions of solder were Sn, Sn-3.6 wt.% Ag, and Sn-3.8 wt.%Ag-0.6 wt.% Zn. Effect of parameters such as orifice size, pressure difference, and frequency on the solder ball production was also investigated. Mean size, size distribution, and surface defect of solder balls were analyzed with variations of the parameters. Droplet spacing decreased with increasing frequency and decreasing pressure difference. Thus the size of solder ball decreased with increasing frequency and decreasing pressure difference. Size distribution fell within the narrow range of ±5% from the mean diameter of solder balls. The experimentally measured diameter of the solder balls fabricated by DBM process coincided quite well with the theoretically anticipated value of d_d=A d_o^2/3.

keyword : DBM(Droplet-Based Manufacturing), BGA(Ball Grid Array), Lead-free solder ball, Sn-Ag, Sn-Ag-Zn, Orifice size, Pressure difference, Frequency, Size and size distribution

Copper matte with 59% Cu and 22.6% S in weight ratio was oxidized at 1523K by bubbling of Ar+O_2 gas through a submerged nozzle. The effects of oxygen partial pressure and flow rate of gas on the oxidation rate of matte were discussed. The desulfurization rate and evolution rate of SO_2 gas of copper matte were significantly influenced by the oxygen partial pressure. However, the dependence of gas flow rate on the desulfurization rate and evolution rate of SO_2 gas were confounded. The desulfurization rate and evolution rate of SO_2 gas of copper matte were described by the mass transfer rate through the gas film around the rising gas bubbles. The calculated results reasonably reflect the observed values of the desulfurization rate at the initial stage of blowing.

keyword : Copper, Matte, Oxidation, Desulfurization, Mass transfer, Bubbling