발간논문

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Phase transformation behavior and shape memory characteristics of Ti-Ni-Cu alloy ribbons fabricated by melt spinning have been investigated by means of differential scanning calorimetry, X-ray diffraction, optical microscopy and thermal cycling tests under constant load. With increasing linear velocity and decreasing ejection pressure, transformation start temperatures of ribbons decreased and stability of the B19 martensite was lowered, comparing to those of alloys fabricated by conventional casting since the cooling rate increased. Only after heat treating ribbons at temperatures higher than 873 K where recrystallization occurred, the B19 martensite was observed in Ti-40Ni-10Cu alloy ribbons. Transformation elongation and hysteresis associated with the B2-B19 transformation in Ti-Ni-Cu alloy ribbons decreased with increasing Cu-content, which was attributed to the decrease in lattice deformations. Transformation elongations of ribbons were larger than those of alloys made by casting, which was closely related to a texture developed in ribbons.

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The effect of nitrogen contents on the microstructure, tensile and corrosion behaviors of CD4MCU (Fe-25Cr-5Ni-2.8Cu-2Mo) cast duplex stainless steel was examined in the present study. The slow strain rate test was conducted at a nominal strain rate of 1×10^-6/sec in air and 3.5%NaCl+5% H_2SO_4 solution for studying the stress corrosion cracking behavior of the present alloy. It was observed that the volume fraction of austenitic phase in CD4MCU alloy increased from 49% to 65% with increasing nitrogen content from 0.13 to 0.27 wt.%. The yield strength decreased, while the ultimate tensile strength and the tensile elongation increased, with increasing nitrogen contents. The variation of tensile behavior appeared to be strongly related to the volume changes in ferritic and austenitic phases associated with different nitrogen contents. The increase in the volume fraction of austenitic phases was beneficial for both general corrosion and stress corrosion cracking in 3.5% NaCl+5% H_2SO_4 solution. It was believed that the selected corrosion of ferritic phases in high austenitic duplex stainless steel smoothened the initial stress corrosion cracks and, consequently, increased the resistance to stress corrosion cracking.

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Effects of plate thickness on high-cycle fatigue properties and fatigue crack propagation behavior of investment-cast Ti-6Al-4V alloy plates having Widmansta¨tten structure were investigated in this study. High-cycle fatigue test and fatigue crack propagation test were conducted on three cast plates having different thickness, and then the test data were analyzed in relation with microstructures, tensile properties, and fatigue fracture mode. The high-cycle fatigue results indicated that fatigue strength of the three cast plates was quite similar because of their similar tensile strength. In the case of the fatigue crack propagation, the thicker cast plate composed of thinner α platelets had the slightly faster crack propagation rate than the other plates. The effective microstructural factor determining the fatigue crack propagation rate was found to be the thickness of a platelets because it was well matched with the reversed cyclic plastic zone size calculated in the threshold ΔK regime.

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Low cycle fatigue tests were carried out using four kinds of Al-7%Si-0.4Mg cast alloys, i.e., two kinds of sand mold casts, permanent mold cast and semi-solid die cast. They were heat-treated in the condition of under aging and over aging to investigate effects of precipitates on fatigue. All tests were conducted under axial plastic strain amplitude control. Stress level of cyclic hardening curves increased sensitively with needle like eutectic Si particle, refined grain size and dendrite arm spacing(DAS). In particular, the refined grain structure of under aged matrix was more effective encourager for cyclic hardening compared with DAS and eutectic Si particle size. After rapid increase in cyclic hardening during several number of cycles, the stress amplitude kept increasing steadily until fracture in under aged alloys strengthened by shearable G.P. zone. On the other hand, over aged alloys strengthened by non-shearable β` precipitates generated more drastic initial hardening and the stress amplitude reached the saturation state in quite early stage of the fatigue.

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Assessment of residual stress is an important point to prevent the failures of thin film devices. Nanoindentation technique was proposed as a stress-measuring method of thin film in a previous research. However, the stress-analyzing model used at the previous research had the problems of a partial deformation-irrelative stress interaction concept and no consideration of the change in the contact area during continuous stress relaxation. Therefore, we modified the model by analyzing a new residual stress-induced normal load based on deformation-dependent shear stress component and by calculating the continuous relaxation procedure as an integration method. The modified model was applied to evaluate the residual stresses in two diamond-like carbon films. The evaluated residual stresses from the newly modified model were more consistent with the results from the conventional curvature method than those from the earlier model.

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The objective of this study was to investigate the influence of bimodal reinforcement on the fluidity and mechanical properties of SiC particle reinforced AZ91D Mg composites. The fraction of bimodal SiC particles was fixed at 20 wt% and the mixing ratios of 5 ㎛ to 50 ㎛ SiC particles were 0:1, 1:2, 1:1, 2:1 and 1:0. The results clearly showed that the fluidity and mechanical properties of the composites with bimodal reinforcement were dependent on mixing ratio. In terms of molten state fabrication and tailored mechanical properties, it was obtained that the proper mixing ratio seems to be 1:2.

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The aims of this study were to develop a fabrication process of A356 aluminum alloy composites reinforced with SiC particulates using stir-casting method and to establish processing variables to improve their microstructures and mechanical properties. The water-pool modeling was conducted to simulate and establish the proper stir-casting conditions. Good quality Al-SiC_p composites having relatively homogeneous microstructure and sound Al/SiC interfacial bonding were obtained from the conditions of the stirring temperature of 710℃, the stirring speed of 220∼700 rpm, and the injection of Ar gas+SiC_p mixture, although they contained a small amount of micropores. The microstructures and mechanical properties of the composites were found to be comparable with those of the commercial composites processed by Duralcan because of the homogeneous distribution of SiC particulates. In order to improve the mechanical properties of the A356 Al-SiC_p composites, minimization of micropores, fast cooling rate, homogeneous distribution of SiC and eutectic Si particles, and development of the subsequent processes such as die-casting or squeeze-casting were suggested.

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The problem of organic contamination is still there due to the outgassing from the plastic materials in the storage boxes. Such organic contaminants have deleterious effects not only on the gate oxide integrity, but also on the chemical vapor deposition steps. In this paper, we report the experimental results for the removal of the organic contaminants existing on wafer surfaces by UV/O_3 cleaning, ECR H_2 plasma and ECR O_2 plasma cleaning. After cleaning, Si wafers were analyzed by Attenuated Total Reflection Fourier Transform Infrared Spectroscopy (ATR-FTIR) and Atomic Force Microscrope(AFM). The ECR oxygen plasma cleaning technique seems to be more effective than the ECR hydrogen plasma or the UV/O_3 cleaning technique for the removal of organic contaminants. Also, organic contaminants removal mechanisms of UV/O_3 cleaning, ECR H_2 plasma and ECR O_2 plasma cleaning are discussed.

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Four different brasses i.e., Cu-39.49%Zn (6:4 brass), Cu-39.11%Zn-0.80%Sn, Cu-38.85%Zn-0.45%Sn-0.47%Al, and Cu-37.68%Zn-0.46%Sn-1.82%Al were oxidized in air at 873, 973, and 1043 K to investigate the effects of Sn and Al on the oxidation behavior of 6:4 brass. The oxidation resistance of 6:4 brass increased with the addition of Sn and Al, The good oxidation resistance obtained in Cu-37.68%Zn-0.46%Sn-1.82%oAl was mainly attributed to the formation of Al_2CuO_4 that formed in the ZnO-rich oxide scale.

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Slow Strain Rate Tests(SSRT) were carried out to investigate the effect of dissolved oxygen on stress corrosion cracking(SCC) susceptibility of 3.5NiCrMoV steels used in discs of low-pressure(LP) steam turbines in electric power generating plants. The influence of dissolved oxygen on cracking in water was studied; for this purpose, specimens were strained to fracture at 150℃ in water environments with various amounts of dissolved oxygen. Also Tests were conducted in aerated water at temperature of 50∼200℃ and at various strain rates (5×10^-8∼1×10^-5 s-1). The maximum elongation of the turbine steel decreased with decreasing strain rate, and with increasing temperature. Dissolved oxygen significantly affected the SCC susceptibility of turbine steel in water. The increase of the SCC susceptibility of the turbine steel in a higher dissolved oxygen environment is due to the non protectiveness of the oxide layer of the turbine steel surface and the increase of corrosion current.

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