발간논문

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The high speed deformation behavior of TRIP steels and HSLA steels has been investigated at strain rates ranging from 10^(-2) to 10³/sec. The effects of metallurgical factors such as prestrain (2%) and bake hardening condition (170℃, 20 min) on tensile properties at various strain rates are being evaluated. Ultimate tensile strength (UTS), total elongation, strain rate sensitivity, and absorbed energy are reported. In general, UTS increases with increasing strain rate, that is, it shows positive strain rate sensitivity. And strain rate sensitivity increases sharply over 100/sec. Strength increases with increasing the amount of prestrain. The bake hardening treatment slightly influences UTS values. The absorbed energy decreases with increasing prestrain but it is not affected by bake hardening treatment. Received February 14, 2005)

keyword : TRIP steels, HSLA steels, Retained austenite, High strain rate equipment, Prestrain, Bake hardening treatment

Thermal creep properties of the zirconium tube which was developed for high burnup application were evaluated. The creep test of cladding tubes was carried out by the internal pressurization method in the temperature range from 350℃ to 400℃ and from 100 MPa to 150 MPa in the hoop stress. Creep tests were lasted up to 900 days, which showed the steady-state secondary creep rate. The creep resistance of zirconium claddings was higher than that of Zircaloy-4. Factors that affect creep resistance, such as alloying element, applied stress, activation energy, and solute impurities were discussed. Tin as an alloying element was more effective than niobium due to solute hardening. Viscous glide where mobile dislocation was interacted and hindered by the diffusion of solute atoms was the most dominant deformation mechanisms in advanced cladding tube. (Received December 2, 2004)

keyword : Creep, Zirconium, Cladding, Niobium, Tin

Deformation behavior of a Zr_(41.2)Ti_(13.8)C_(u12),Ni_(10)Be_(22.5) bulk metallic glass was investigated. Three different types of true stress-strain curves were identified under compressive loading within undercooled liquid region depending on the strain rates and test temperatures. The two theoretical relations, based on a Newtonian viscous flow equation and a transition state theory, have been applied to analyze the transition behavior of deformation modes in homogeneous flow regime in terms of steady-state flow stress. In addition, the change of glass transition peak height by Differential Scanning Calorimetry (DSC) and qualitative RDF by Extended X-ray Absorption Fine Structure (EXAFS) have been used to account for structural states of Zr_(41.2)Ti_(13.8)C_(u12),Ni_(10)Be_(22.5) bulk amorphous alloys after annealing and disrupted compression test at =10^(-3)/s. At 370℃, only a small structural relaxation was observed to proceed in a period of 680 s by annealing. A deformation induced structural disordering (DISD) was, on the other hand, observed at the initial state of a non-Newtonian flow (e=0.03 and 0.076) around the stress overshoot. (Received November 26, 2004)

keyword : Bulk amorphous alloy, Deformation behavior, Homogeneous flow, Newtonian viscous flow, Non-Newtonian flow, Deformation induced structural disordering

Fe/Ag multilayered composites which have the layer thickness below 50 nm have been fabricated by the repeated bonding and rolling method. The effects of processing variables on the microstructures of Fe/Ag multilayered composites were investigated. Multiple necking and multiple rupture occur during the rolling process. The initial thickness ratio of the constituent phase of the multilayers affects the occurrence of the multiple necking and rupture. The smoothness of the layer is related with the total thickness reduction in the bonding/rolling cycle. In the initial stage of the process, large strain partition occurs in the soft Ag layer, and due to hardening of the Ag layer during the rolling process, the strain partitioning of the Fe and Ag layer becomes similar. Accordingly the layer thickness of the two constituent phase becomes similar in the final stage of the process. (Received January 19, 2005)

keyword : Multilayer, Multiple necking, Repeated rolling, Fe/Ag

In this study various types of Al foam filled tubes were fabricated using foam of A1-12wt%Si by powder metallurgical process. Their energy absorbing efficiencies were investigated with compression test, focusing on the structure and bonding effects between tube and foam. The results show that the energy absorption is affected by structure of Al foam filled tube. Among fully foam filled tube, partially foam filled tube and Al hollow tube with the same weight, fully foam filled tube seems to have superior potential for industrial application to energy absorption parts. Also energy absorption efficiency is increased by interaction between tube and foam and especially, bonding between tube and metallic foam gives rise to increase the energy absorption efficiency up to above 1.2 times. This results from the arrest of folding of tube by metallic foam and the change of stress mode from uni-axial to multi-axial during compression. (Received September 30, 2004)

keyword : Aluminum foam filled tube, Energy absorption efficiency

Surface properties and high temperature oxidation behavior were investigated for FC-25 Gray Cast Iron(GCI) and the ion nitrided GCI(N-GCI). The GCI was pre-cleaned to improve hardness to the optimum pre-sputtering parameters with an Ar/H₂ ratio of 1/2, working pressure of 3 torr, working temperature of 550℃ and working time of 1hour. The optimum nitriding conditions for the maximum hardness of 560575 Hv were an N₂/H₂ ratio of 3/1, working pressure of 3 torr, and working temperature of 575℃. The thickness of graphite in the GCI was increased by increasing the working temperature from 525℃ to 595℃ for the nitriding time of 6-18hrs. XRD patterns showed FeO and Fe₂O₃ peaks for both the oxidized N-GCI and GCI at temperatures of 600℃ and 800℃ under atmospheric environment for both 24 and 60hours. At 800℃, above the Fe4N decomposition temperature of 680℃, the oxidation rate of N-GCI was greater than that of the GCI. The most abundant nitride, Fe4N, was decomposed and the nitrogen gas given off by the decomposition made the protective film porous by degassing through the film. But at 600℃, below the decomposition temperature, the degree of oxidation of N-GCI was lower than that of the GCI because the nitride film worked as protective barrier for oxidation. (Received October 29, 2004)

keyword : FC-25 gray cast iron, Ion nitriding, Surface property, High temperature oxidation

In order to develop a new corrosion sensor for detecting and monitoring the corrosion of buried pipeline, the electrochemical property of sensors and the correlation of its output to corrosion rate of steel pipe, were evaluated by electrochemical methods in synthetic groundwater, two soils of varying resistivity (5,000 ohm-cm, 10,000 ohm-cm), and synthetic tap water. In this paper, two types of electrochemical probes were used: galvanic cells containing of pipeline steel-copper and pipeline steel-stainless steel (Type 304). The results of EIS measurement indicated that the sensor current was inversely related to sensor resistance, which was governed by the corrosion behavior of cathode. In galvanic corrosion tests, the galvanic current of Cu-CS probe was higher than that of SS-CS probe. The comparison of the sensor output and corrosion rates revealed that a linear relationship was found between the probe current and the corrosion rates. A good linear quantitative relationship was found between the Cu-CS probe current and the corrosion rate of pipeline steel coupons in the soil resistivity of 5,000 ohm-cm, and synthetic tap water. In the case of the soil resistivity of 10,000 ohm-cm, although the SS-CS probe showed a better linear correlation than that of Cu-CS probe, the Cu-CS probe is more suitable than SS-CS probe, due to the high current output. (Received February 1, 2005)

keyword : Galvanic couples, Corrosion sensor, Pipeline, Galvanic current, Stainless steel, Copper

Brasses are the most widely used copper-based alloys because of the good corrosion resistance, formability, high strength and excellent hot-working properties. However, dezincification occurs when one or more components of an alloy are more susceptible to corrosion than the rest and, as a result, are preferentially dissolved. In order to enhance the corrosion resistance, the oxide film was deposited on the 65/35 brass by pulsed dc plasma oxidation with various pressure. The corrosion behavior of plasma oxidized brasses with various oxygen pressure was investigated by electrochemical methods (potentiodynamic polarization test, potentiostatic test, electrochemical impedance spectroscopy) and surface analyses (x-ray diffraction, scanning electron microscopy, auger electron spectroscopy, electron probe micro analyzer). Particular attention was paid to the effects of oxygen pressure and oxide thickness on the coating properties related to the corrosion behavior. From the results of electrochemical test, it was found that the oxidized brass under condition of 1.5 Torr oxygen pressure was effective in improving corrosion resistance. It was attributed to the protective oxidized layer (ZnO). Finally, an optimized oxidation condition for corrosion protection was found to be 1.5 Torr oxygen pressure and 1 pun thickness. (Received September 3, 2004)

keyword : Plasma oxidized brass, XRD, SEM, EPMA, Corrosion test, Dezincification

Ta films were deposited on Si (100) substrates at substrate bias voltages of 0 V, -50 V, and -125 V by a non-mass separated ion beam deposition. Secondary ion mass spectrometry (SIMS) and glow discharge mass spectrometry (GDMS) were used to determine the concentrations of dominant impurity elements, i.e., H, C, N, and 0, which are likely to contaminate Ta films during the deposition. The GDMS results revealed that dominant impurities during the deposition were C, N, and 0 elements and many unknown peaks observed in the SIMS spectra of the Ta film were assigned to cluster states such as C_(x)H_(x), O_(x)H_(x), C_(x)O_(x)H_(x). Furthermore, it was found that the concentrations of these impurities in the Ta films can be controlled by the negative substrate bias voltage. (Received September 7, 2005)

keyword : Ion beam deposition, Tantalum, Impurity, Substrate bias voltage, Secondary ion mass spectrometry, Glow discharge mass spectrometry