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Dynamic deformation behavior of ultra-fine-grained aluminum alloys fabricated by equal channel angular pressing (ECAP) was investigated in this study. Dynamic torsional tests were conducted on four aluminum alloy using a torsional Kolsky bar, and then the test data were analyzed in relation to microstructures, tensile properties, and adiabatic shear band formation. The ECAP`ed specimens consisted of a considerable amount of second phase particles, which were refined and had an equiaxed shape as the ECAP pass number increased. The dynamic torsional test results indicated that maximum shear stress increased, while fracture shear strain remained constant, with increasing ECAP pass number. Observation of the deformed area beneath the fracture surface after the dynamic torsional test indicated that a number of voids initiated mainly at second phase particle/matrix interfaces, and that the number of voids increased with increasing pass number. Adiabatic shear bands of 100-150μm in width were formed in the as-extruded and 1-pass specimens having coarser second phase particles, while they were hardly formed in the 4-pass and 8-pass specimens having finer second phase particles. The possibility of the adiabatic shear band formation was explained by concepts of absorbed deformation energy and void initiation. (Received October 29, 2004)

keyword : Dynamic torsional test, Ultra-fine-grained aluminum alloy, Equal channel angular pressing, ECAP, Adiabatic shear band

In this study, cold worked (CW) 316L austenitic stainless steel was investigated by performing low-cycle fatigue test with varying temperature, strain amplitude, and strain rate. CW 316L stainless steel underwent additional hardening at room temperature and in the temperature range of 250-600℃: The hardening at room temperature came from plasticity-induced martensite transformation, and the hardening over 250-600℃ was attributed to dynamic strain aging. These hardening mechanisms competed with the softening mechanism which was induced by dynamic recovery, generally observed in cold worked materials, resulting in the cyclical non-stabilization of the material. Three fatigue parameters, such as stress amplitude, plastic strain amplitude, and plastic strain energy density, were evaluated. The results revealed that plastic strain energy density was nearly invariant through a whole life and, thus, recommended as a suitable fatigue parameter. (Received September 3, 2004)

keyword : Cyclic stress response, Non-stabilization, Dynamic strain aging, Plasticity-induced martensite transformation, Fatigue parameter, 316L stainless steel

Feedstock for plasma spraying was prepared by ball milling with A1-55 vol.%SiC and Al-75 vol.%SiC powder mixtures in different conditions. The average size of SiC particles was varied from 8 to 30 gm. Freestanding SiC_(p)/Al composites were fabricated by atmospheric plasma spraying onto a graphite substrate. Thermal conductivity of the plasma-sprayed composites was measured at room temperature of 25℃ by the laser flash method. Thermal conductivity of the sprayed composites was significantly lower than that of conventionally cast SiCp/Al composites. Thermal conductivity was varied considerably with respect to the SiC size and feedstock preparation method. The deterioration mechanism of thermal conductivity in the sprayed composites was elucidated, and numerical analyses based on theories of Maxwell and thermal boundary resistance were carried out. (Received September 30, 2004)

keyword : Thermal conductivity, Composite, Aluminum, SiC, Plasma spraying, Porosity, Thermal boundary resistance

Water drop impact erosion properties of the 12Cr steel, currently used as nuclear steam turbine blade material, surface-hardened by the flame hardening process have been characterized. For this, variations of both the maximum erosion depth de,,,,ax and volume loss V with the cumulative impacts n at the supersonic impact velocity (~350 m/s) have been investigated for the as-received and flame-hardened 12Cr steels with different hardnesses. Typically all the samples showed an erosion-time characteristic involving the incubation period initially followed by the steady state period. It is also found that the d_(e,max) increases stepwise-like with the number of impacts n, which results from a sudden formation of craters by crack propagation. Compared to those for the as-received 12Cr steel, the flame-hardened ones showed an excellent resistance to water drop impact erosion with 2.2~2.8 times higher incubation time n, and 115~1/8 times lower erosion rate a. In the incubation period the as-received 12Cr steel was damaged by ductile depression and ploughing, while the flame-hardened 12Cr steel by fatigue cracks and brittle platelet deformation. Erosion in the steady state period was developed by the cleavage fracture commonly. (Received July 22, 2004)

keyword : 12Cr steel, Flame hardening, Water drop impact, Erosion, Surface heat treatment

The correlation between sensor output and the corrosion rate of steel bar was confirmed in concrete environment. Monitoring of open-circuit potential, linear polarization resistance (LPR) measurement and electrochemical impedance spectroscopy (EIS) were used to evaluate the corrosion behavior of steel bar embedded in concrete. Also, galvanic current measurements of designed sensors were conducted to obtain the charge of sensor embedded in concrete. In this study, the results of corrosion behavior of reinforcing steel showed a consistence among the data obtained by open-circuit potential monitoring, LPR and EIS measurements. Steel/copper sensor showed a good correlation in concrete environment between sensor output and corrosion rate of steel bar. However, there was no relationship between steel/stainless steel sensor output and corrosion rate of steel bar due to the low galvanic current output. Through the relationship between the steel/copper sensor output and the corrosion rate of reinforcing steel, real corrosion damage of the reinforcing steel can reliably be detected. (Received October 29, 2004)

keyword : Galvanic sensor system, Corrosion, Concrete, Chloride, Reinforcing steel

Aqueous corrosion characteristics of low carbon steel with small amounts of Cr, Co, Cu, Ni and Ca, in synthetic groundwater was studied by electrochemical corrosion tests (potentiodynamic test and electrochemical impedance spectroscopy (EIS) measurements) and analytical techniques. Neither carbon steel nor newly alloyed steels showed passive behavior in this synthetic groundwater. Newly alloyed steels, containing Cr-Co, Cr-Cu-Ni and Cr-Cu-Ni-Ca showed higher corrosion resistance than carbon steel in the potentiodynamic tests. EIS measurements showed that the Nyquist plot presented two time constants. The high frequency resistance component (R_(rsut)) and low frequency resistance component (R_(et)) were affected by the alloying elements. The polarization resistance (RP= R_(et) +Ra) of steels could be clearly ranked as Cr-Cu-Ni-Ca steel >> Cr-Cu-Ni steel > Cr-Co steel > carbon steel. Results of surface analyses (XPS and EPMA) showed that Cr and Cu were concentrated in the inner region of rust. However, Co and Ni were uniformly distributed all over the rust layer. These alloying elements improved corrosion resistance of low alloy steel in synthetic groundwater. Especially, Cr-Cu-Ni-Ca steel had the lowest corrosion rate due to the inner rust film formation during the initial stage of corrosion, which suggested a potential for substituting carbon steel in soil environment (Received October 29, 2004)

keyword : Aqueous environments, Carbon steel, Alloying effect, Rust Layer, EIS

High purity model alloys with major composition Ni-15Cr-9Fe-0.03C (600CEO) and Ni-15Cr-9Fe-0.03C-0.04Ce (600CE4) were produced. Using these model alloys the effect of alloying element Ce on the SCC behavior of Alloy 600 was evaluated in a caustic solution. To obtain carbides precipitated on grain boundaries, the thermal treatment was performed on both the solution annealed model alloys. Microstructural examinations using SEM and TEM EDX showed that the same structural carbides, Cr_(7)Cc₃, were precipitated on both the alloys and no significant difference in the amount of Cr depletion along grain boundaries was observed between the two model alloys. However, it was shown that the coverage of grain boundary carbides was higher in the Ce-bearing alloy (600CE4). The SCC susceptibility of the alloys was investigated in 40%NaOH solution at 315℃. Being evaluated in terms of the maximum crack depth, the SCC susceptibility of the alloy turned out to be reduced by the addition of Ce. The increased resistance to the SCC in the alloy 600CE4 was considered to be attributable to the increased coverage of grain boundary carbides. (Received September 30, 2004)

keyword : Stress corrosion cracking, Cerium, Alloy 600, Intergranular carbide, Coverage

CASS(copper accelerated acetic salt spray) and EC(electrolytic corrosion test) tests were performed in order to evaluate the resistance to corrosion of plated Ni-Cr and Ni-microporous Cr layers on Fe substrate. Compared with the conventional methods such as CASS, the electrochemical method, i.e. EC test showed the superiority in terms of test-time span and quantitative accuracy. Furthermore, EC test also offered the alternative method to be able to evaluate the resistance to corrosion of coatings by measuring the rest potentials of the coated layers on the substrate in the electrolyte during the off-time of EC cycles, comparing them with those of pure iron, nickel, and chromium. Results showed that although there is not much difference between CASS and EC tests in case of Ni-microporous Cr specimens during the short testing time span, EC test reflected the minute variation of currents and potentials, providing the faster and quantitative results than CASS test. Therefore, EC test method offers the much better results than the conventional method like CASS, in terms of evaluating the resistance to corrosion of the coated layers. (Received July 12, 2004)

keyword : CASS, EC, Rest potential, Bright Ni-microporous Cr plating

As portable electronic products become more prevalent, their susceptibility to mechanical damage increases. In the present work, it is studied how the fracture strength of bare silicon chips is influenced by manipulating grinding mark geometry or chip dimension. It was experimentally investigated how a decrease in chip thickness from 360 μm to 240 μm influences the flexural strength of bare chips. The experimental results showed that when chip thickness was decreased to 30%, the flexural strength of the bare chips could be decreased by 2-fold. However, it was found that reducing the depth of scratches (or grooves) on the chip back surface from 1 pm to 0.1 pm, which result from the wafer back-grinding process, could compensate for a decrease in flexural strength due to chip thickness reduction. (Received October 21, 2004)

keyword : Silicon chip, Stress, Grinding mark, Dimension, Fracture