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The detailed microstructural features of WC-Co nanocomposite coating, manufactured by high velocity oxy-fuel spraying, have been characterized by high resolution transmission electron microscopy and computer-aided image processing. The coating was built up by depositing many individual droplets that experience different thermal histories during flight, developing an inhomogeneous microstructure. The nanocomposite coating contained very high amounts of non-WC phases including W₂C, W, i-carbide and amorphous Co-rich phase. The unreacted WC particles were also retained in the coating, with similar size and morphology in the feedstock powder, and had stacking faults with 116<1213> atomic displacement vectors. Two different types of near-spherical W₂C particles were formed by either direct decomposition of WC into W₂C and C or precipitation from liquid Co-W-C, which was attributed to the extent of feedstock melting and decarburization in specific droplets. The different formation mechanism yielded a significant difference in W₂C particle size in the coating. Metallic W particle and fl-carbide were also formed in specific locations in the coating. Rapid solidification promoted the formation of amorphous binder phase in which large amounts of W and C elements were dissolved in Co-rich matrix. (Received September 30, 2004)

keyword : Nanocomposite, WC-Co coating, HVOF spraying, Decarburization, Phase transformation, TEM, HREM

The present study is concerned with the improvement of hardness and wear resistance in (CrB, MoB)/ carbon steel surface composites fabricated by high-energy electron beam irradiation. Three kinds of powder mixtures, i.e., 5OCrB-50MgF₂(flux), 5OMoB-50MgF₂, 25CrB-25MoB-50MgF₂ (wt.%), were placed on a plain carbon steel substrate, which was then irradiated with electron beam. In the specimens fabricated with flux powders, the surface composite layer of 0.8-1.3 mm in thickness was successfully formed without defects, and contained a large amount (up to 48 vol.%) of Cr_(1.65)Fe_(0.35)B_(0.9) or Mo₂FeB₂ in the martensitic matrix. The hardness and wear resistance of the surface composite layer were directly influenced by hard borides, and thus were about 3 to 7 times greater than those of the steel substrate. Particularly in the surface composite fabricated with CrB and MoB powders, the wear resistance was most excellent because the hardness of the cell boundary region and martensitic matrix was very high and because borides were connected to form a network structure along cell boundaries. (Received January 19, 2005)

keyword : Surface composite, High-energy electron beam irradiation, Plain carbon steel, Boride, Hardness, Flux

Cu-based bulk amorphous powders were consolidated by a spark plasma sintering (SPS) process. Consolidation conditions such as consolidation temperature, consolidation pressure, and holding time were established to obtain good microstructures and compressive properties. Cu-based amorphous powders were produced by a gas atomization process. Amorphous alloys having a few micropores and nanocrystalline phases could be obtained with a consolidation pressure of 80 MPa, and were fractured in an intergranular mode along prior powder boundaries. Microstructures and compressive properties of the amorphous alloys consolidated with a pressure of 300 MPa were significantly modified as prior amorphous powders were sufficiently bonded without micropores. Fractographic investigation of these alloys indicated that vein patterns appeared on fracture surfaces as amorphous powders were strongly bonded during the high-pressure consolidation. These findings suggested that the high-pressure consolidation was effective in suppressing the crystallization of amorphous phases and in strongly bonding amorphous powders to consolidate good quality amorphous alloys. (Received December 6, 2004)

keyword : Bulk amorphous alloy, Spark plasma sintering, Consolidation, Crystallization

The effect of strain rate and remelting on hot ductility of low carbon steel were investigated. The specimen was prepared from as cast continuous casting slab, and also the slab was remelted to assess the effect of remelting by which cast structure would be modified. High temperature tensile test was adopted to get the hot ductility data. In case of 0.18 wt.% carbon steel, hot ductility was markedly improved with increasing strain rate in both specimen obtained from as cast slab and remelted slab. Comparing the results obtained from as cast specimen and remelted specimen, it showed similar value of hot ductility in low temperature range, and in high temperature region, showed higher R/A(reduction of area) value in the specimen obtained from remelted slab. The decreased RA value of as cast specimen in high temperature region could be explained by the increase in initial grain size due to slow cooling of large slab during continuous casting. It means that we should presume lower value of hot ductility than the value obtained from remelted specimen in case of lab data application to actual continuous casting process. (Received November 22, 2004)

keyword : Hot ductility, Continuous casting, Strain rate, Remelting, Cast structure, R/A

The reorientation of hydrides and its effect on the fracture of Zircaloy-4 cladding tubes with the hydrogen content of 220 wt. ppm were investigated. The reorientation of hydrides along the radial directions was most pronounced if the tube was cooled from 300℃ to 200℃ under circumferential loading. Reorientation occurred much less frequently in either higher (cooled from 400℃ to 300℃) or lower (cooled from 200℃ to 100℃) temperature range. The reorientation of hydrides was found to increase with the increase of loading time, suggesting time dependent stress-aided dissolution of circumferential hydrides (hydride platelets in the circumferential-axial plane) and reprecipitation of radial hydrides (hydride platelets lying in to radial-axial plane). The strength and ductility decreased with the reorientation of hydrides. The strength decreased to ~380 MPa and the ductility decreased to zero in the tube which was reorientation-treated from 300℃ to 200℃ (R43AC). Fracture surface of R32AC sample exhibited flat cleavage fracture along the radial hydride platelet. (Received October 29, 2004)

keyword : Hydrides, Zircaloy-4, Reorientation, Ductility, Strength

In the present study, the effect of Mo contents on the microstructure, tensile and corrosion behaviors of as-solutionized CD4MCU cast duplex stainless steel was examined. The polarization test was conducted in 3.5% NaCl +5% H₂SO_(4) aqueous solution for general corrosion resistance and the slow strain rate tests were also conducted in air and 3.5% NaC1+5% H₂SO_(4) aqueous solution to study the stress corrosion cracking (SCC) of the present alloy. A substantial microstructural evolution of CD4MCU cast duplex stainless steel was observed with different Mo contents, which in turn affected the tensile and corrosion behaviors significantly. The beneficial effect of Mo on improving the corrosion and the SCC resistances was largely overwhelmed by this variation of microstructural characteristics. The relationship between the microstructural evolution and the tensile and corrosion behavior of CD4MCU cast duplex stainless steels with different Mo contents was discussed based on the optical and SEM micrographic and fractographic observations. (Received September 2, 2004)

keyword : CD4MCU cast duplex stainless steel, Mo contents, Tensile behavior, Corrosion behavior, Slow strain rate test

In a method to improve resistance to wear, corrosion and mechanical properties of protective coating, DLC coatings have been extensively studied over the past several years due to their unique combination of properties. Especially, Si-DLC coatings have been attracting increasing interest of researchers since they have a great potential for solving some of the major disadvantages of pure DLC coatings. Si-incorporated diamond-like carbon (Si-DLC) coatings ranging from 0 to 2 at.% were deposited on Ti alloy substrate by means of r.f. plasma-assisted chemical vapor deposition (r.f. PACVD) technique, using mixtures of benzene (C_(6)H_(6)) and silane (SiH_(4)) as the reaction gases. Corrosion and wear behavior of DLC coatings was investigated by electrochemical techniques (potentiodynamic polarization test and electrochemical impedance spectroscopy), tribological techchnique(wear test) and surface analyses (atomic force microscopy, scanning electron microscopy and Raman spectroscopy). It was found that DLC coatings with increasing Si content could acquire improved corrosion and wear resistance in the simulated body fluid environment (0.89 wt.% NaCl of pH 7.4 at 37℃). (Received December 6, 2004)

keyword : DLC, Corrosion, Wear, Si effect

In the present study, corrosion behavior of friction stir welded (FSWed) Al 6061-T651 with varying welding parameters, including rotating and welding speeds, were examined. The 4 nun thick Al 6061-T651 alloy plates were friction stir welded. The resistance to corrosion was slightly improved, or at least did not decrease, for FSWed AI 6061-T651 alloy in 3.5% NaCl solution compared to the parent material. The effect of different welding conditions on the corrosion resistance was minimal. The micrographic and fractographic observations strongly suggested that the change in corrosion behavior of FSWed Al 6061-T651 was largely related to the clustering of coarse Mg₂Si precipitates, due to the whirling and casting action by severe plastic flow in the weld zone. Low welding speed and/or high rotating speed tended to encourage the plastic flow per unit time and consequently the clustering of coarse precipitates. The resistance to SCC in 3.5% NaCl solution was considerably higher for the FSWed AI 6061-T651 compared to that of the PM counterpart. The improved SCC resistance for the FSWed specimen was due to the fact that the stress corrosion cracks were enhanced at the boundary between the DXZ and the HAZ due to the greatest galvanic corrosion potential difference between the two phases, while the tensile fracture occurred at the other area where the strength level was the lowest. (Received October 29, 2004)

keyword : Friction stir welding, Al 6061-T651, Corrosion behavior, Stress corrosion cracking

Corrosion characteristics of Cu-6%Ni-4%Sn-x%Cr alloys in a acid-chloride solution (0.1M H₂SO₄+ 0.0056M NaCl) at pH = 6, were studied as a function of chromium addition. Three kinds of Cu-6%Ni-4%Sn-x%Cr alloys were prepared by melting, and casting, followed by thermomechanical treatments. The corrosion behavior of Cu-6%Ni-4%Sn-x%Cr alloys was investigated by electrochemical corrosion tests (potentiodynamic polarization test, electrochemical impedance spectroscopy) and surface analyses (optical microscopy, electron probe micro analysis, scanning electron microscopy). Cr additions were found to prevent the localized corrosion behavior. This is the reason that the small addition of Cr promoted the decomposition behaviors of the supersaturated solid solution a in Cu-6%Ni-4%Sn alloy, i.e., the number of like-Sn-rich carbide impurities (LCI), which increased the galvanic effect between the matrix and the LCI, decreased with increasing the Cr addition. (Received September 8, 2004)

keyword : Cu-Ni-Sn alloy, Aacid-chloride solution, Localized corrosion, Sn-rich, Galvanic effect

Hydrogen embrittlement (HE) mitigation on the low alloy steel was studied using electrochemical technique. This method can be applied to reduce HE susceptibility. Hydrogen induced cracking (HIC) characteristics of the low alloy steel was investigated in aerated and deaerated borate buffer solution (0.3M H₃BO₃ + 0.074M N₂B_(4)O_(7), pH 8.4). The electrochemical properties of the low alloy steel showed passive behavior in this environment. By performing galvanostatic and potentiostatic test, hydrogen was charged and discharged to specimens respectively. By applying the potential of +630 mV_(SCE), the accumulated hydrogen was eliminated electrochemically. A specimen discharged for 24h indicated the efficiency of the hydrogen discharge up to 99.9%. Slow strain rate tests (SSRT) results showed that a specimen with higher hydrogen concentration revealed inferior mechanical properties such as shorter time to fracture, lower elongation, ultimate tensile strength and yield strength values. Furthermore, SEM fractographs revealed that the higher the hydrogen concentration is, the more probable the features of brittle fracture are. (Received July 26, 2004)

keyword : Hydrogen charge/discharge, SSRT, HIC, Potentiostatic/Galvanostatic test