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The cold rolled Al-0.2 wt% Sc alloy sheets were pre- heat treated: (1) as solution treated at 630℃ for 1 hour and (2) as artificial aged at 300℃ for 5 hours, and the following accumulative roll bonding was conducted up to the 8 cycles at ambient temperature. The interconnecting boundary spacing of the aged alloys was narrow and maintained constantly at each ARB cycle. While, the spacing of solution treated alloy was relatively broad and decreased rapidly with increasing ARB cycles. The fraction of high angle boundaries in both alloys increased gradually with increasing equivalent strain, however the age-ARB alloys showed relatively higher fraction of HAGBs than the solution treat-ARB alloys at the same strain levels. And the former shows faster development of lamellar structure than the later. These variations are attributed to the fact that the nano-scale, semi-coherent A13Sc hinders the migration of dislocation. However, those precipitates cannot apply pinning effect sufficiently during static recrystallization above 350℃ because the driving pressure for recrystallization overcomes the Zener pinning pressure. (Received September 30, 2004)

keyword : Al3Sc, Accumulative roll bonding, High angle boundary, Lamellar structure, Dynamic recovery, Abnormal grain growth

Continuous indentation test is getting more used to derive indentation flow curve. The overall deriving algorithm is relatively well-established, the details of the algorithm are, however, still argued. One of the arguments is the definition of true strain which constitutes the indentation flow curve. In this study, work-hardening exponent was adopted for determining what definition is appropriate to obtain accurate indentation flow curve since work-hardening exponent is affected by only the definition of true strain not by other constants. The effect of calibrating contact depth was eliminated using finite element analysis. Finally, the definition based on tangent function was determined to be proper for deriving the indentation flow curve. (Received September 8, 2004)

keyword : Continuous indentation test, True strain, Flow curve, Work-hardening exponent, Finite element analysis

Bluntness is inevitable at the tip of a sharp indenter such as the Berkovich indenter used in nanoindentation experiments; this bluntness causes the hardness values observed at low contact depths (<~100 nm) to fall below the Nix and Gao model curve. We present an indentation size effect (ISE) model that extends the available contact depth for ISE application down to several tens of nanometers by considering the tip bluntness effect. The present model reflects the effect of tip bluntness on the distribution of geometrically necessary dislocations needed to accommodate the blunted tip, and the change in the ratio of plastic to total contact depth with contact depth. We applied the present model to nanoindentation results for annealed and strain-hardened Cu, and to previously reported nanoindentation results for surface-nanocrystallized Al-alloy. Good fit was observed between the present model and the hardness results over the full contact depth range. (Received July 22, 2004)

keyword : Geometrically necessary dislocation, Indentation size effect, Nanoindentation, Tip bluntness

Cu-based bulk amorphous alloy billets fabricated by a powder metallurgy route were studied, focusing on the effect of consolidation temperature on the microstructural and compressive properties. Rapidly solidified amorphous powders were produced by commercial atomization method. In order to consolidate amorphous alloy billets, a spark plasma sintering (SPS) equipment was set up, and variables for hot consolidation were investigated. Using the consolidation temperature of 480℃ under the pressure of 80 MPa, a good quality amorphous alloy billet having very few micropores or crystalline phase particles was obtained. Compressive strength of the billet was 1.8 GPa, which was about 6% lower than that of the cast amorphous alloy. This reduction was associated with the existence of few micropores, crystalline phase particles, and oxide layers of prior powder boundaries in the billet. Amorphous matrix composite billets containing Cu particles homogeneously distributed in the amorphous matrix were successfully fabricated by the SPS consolidation. These composite billet showed a considerable amount of plastic strain, although their compressive strength was reduced. (Received June 7, 2004)

keyword : Bulk amorphous alloy, Powder metallurgy, Consolidation, Spark plasma sintering, Composite

The high temperature deformation mechanisms of Ti-6A1-4V with a transformed microstructure were investigated in the context of inelastic-deformation theory. For this purpose, load-relaxation tests were conducted on samples with a lamellar structure containing different alpha-platelet thicknesses at temperatures of 715℃ - 900℃. The flow stress-versus-strain rate curves for all the microstructures were well fit with an inelastic-deformation equation describing grain-matrix deformation (dislocation glide+dislocation climb). However, for heavily pre-deformed specimens, grain-boundary sliding (CBS) as well as grain-matrix deformation was observed to occur. The GBS rate was found to be most rapid for the microstructure with the thinnest alpha laths/platelets. (Received May 31, 2004)

keyword : Ti-6Al-4V, Transformed microstructure, Inelastic deformation, Grain matrix deformation, Grain boundary sliding

In order to improve the surface hardness and fatigue of Ti-6A1-4V alloy, plasma carburization and CrN coating were carried out. After composite surface treatment, the depth of carburized layer was about 150 μm and the thickness of CrN coated layer was about 7.5 pm, respectively. Surface hardness and roughness was significantly improved up to about H_(v)1,960 and 0.045 pm after plasma carburized/CrN coating in comparison with the value hardness and roughness, H_(v), 402 and 0.321 μm of the alloy before surface modification. This result might be associated with the formation of TiC carburized layer and CrN coating on the surface of Ti-6A1-4V alloy. During the scratch test of coating, critical load was as high as 32N as compared with the simply processed CrN coating of a 18N. Such changes of hardness and roughness could be contributed to improving the fatigue life. (Received July 14, 2004)

keyword : Ti-6Al-4V alloy, Plasma-carburizing, CrN thin film, Fatigue, Hardness, Adhesion test

The objective of this study is to understand the effect of W and Mo on the corrosion property of the Ca-modified weathering steel and to determine the optimum chemical composition to maximize the corrosion resistance at seaside corrosion environment. The effects of various alloying elements on the weathering steel were evaluated through a series of electrochemical test and analysis of the corrosion product formed on the steel surface. The polarization resistance of Ca-modified weathering steel can be increased by the addition of Ni, W and Mo, thus the corrosion resistance of Ca^(-)modified weathering steel containing Ni, Mo and W is increased at the initial stage of corrosion in a solution with high NaCl concentration. From the measurement on the permeability of Cl^(-) ions, the rust membranes formed from weathering steel of thin plate shape showed the anion-selectivity and the addition of various alloying elements (Ni, W and Mo) contributed to lower the permeability of Cl^(-) ions toward rust membranes. Particularly, Mo and W are more effective than Ni to suppress the permeation of Cl^(-) ions. It has an industrial significance for alloy design of the weathering steel since design of a lower cost weathering steel with higher corrosion resistance can be possible with addition of Mo or W instead of increasing the high cost Ni content to 2% or even higher. (Received May 19, 2004)

keyword : Ca-modified weathering steel, Corrosion resistance, α-FeOOH, Permeability of Cl(-) ion

A study was made to investigate the corrosion and corrosion-wear performances in strong acid (5 wt% H₂SO_(4)) solution for the high-velocity oxygen-fuel (HVOF) sprayed coatings of tungsten carbide (WC) cermets. Four different alloy compositions were used as WC cermet coating materials in this study, and they were WC-Co, WCNi, WC-Co-Cr and WC-CrC-Ni. The immersion test and electrochemical test showed that galvanic corrosion occurred between WC particle and binder metal in the coating layer and general corrosion occurred on metallic binder for non-Cr added WC-Co and WC-Ni coatings. The best anti-corrosion properties were observed in the metallic Cr containing WC-Co-Cr layer, which was presumably due to the formation of beneficial chromium oxide on the coating surface. It was found that the corrosion resistance of WC cermet coatings was dependent on the compositions of binder metals as well as the micro-crack ratio of coated layer. (Received June 12, 2004)

keyword : WC cermet coating, HVOF, Corrosion, Wear

The oxidation characteristics of Zr-xSn (x=0, 0.4, 1.5, 3.0 wt.%) alloys were investigated in the LOCA temperature ranges of 800 to 1200℃ under the steam condition, using a modified thereto-gravimetric analyzer. At the pre-transition regime, the parabolic rate constants could be applied to the Zr-xSn alloys although the discontiuety of rate constants was revealed at 950?1050℃ in the alloys more than 1.5 wt.% of Sn contents. Below 1000℃, the rate constants increased as the Sn contents in Zr-xSn system increased. Above 1100℃, however, the rate constants did not depend upon the Sn contents. In the case of the Zr-Sn alloys more than 1.5 wt.% Sn, the oxidation rate would be ehanced by the formation of laterial cracks in oxide layer. The Sn-enriched line, which was observed within the oxide layer at the temperatures above 1100℃, was moved toward the metal/oxide interface with increasing the Sn content and would play a role in the reradation of oxidation rate due to the inhibition of oxygen diffusion and/or migration at the particle line. (Received April 9, 2004)

keyword : Zirconium alloy, Zr-Sn system, High-temperature oxidation, Steam oxidation, Oxidation kinetics, Oxidation rate constant, Loss-of-coolant accident, LOCA

In order to apply a sacrificial anode cathodic protection system for automotive muffler materials, determination of reliable sacrificial anode and its protection ability were evaluated by laboratory tests in synthetic condensed water. Three kinds of sacrificial anodes were selected: aluminum (AI), zinc (Zn) and magnesium (Mg), and the muffler materials were Al-coated steel and uncoated steel. The corrosion behavior in synthetic condensed water for the muffler materials and sacrificial anodes was investigated by potentiodynamic tests. The cathodic protection ability was evaluated by galvanic corrosion test and weight-loss test. The results of the potentiodynamic test indicated that Al was less suitable for sacrificial anode because it became passive. Mg was also unqualified due to the high dissolution rate. However, Zn was adequate for sacrificial anode because of its low dissolution rate, high stability and suitable potential difference. Results of weight-loss test demonstrated that the corrosion rate of Al-coated steel was reduced from 0.028 to 0.018 mm/year, and that of uncoated steel was 0.08 to 0.002 mm/year by coupling with Zn anode. In addition, the proper area ratio of anode to cathode was 1 : 10. From these results, it can be concluded that Zn anode can protect both the Al-coating with passivation, and steel substrate with low dissolution rate. (Received January 8, 2004)

keyword : Muffler, Condensed water, Sacrificial anode, Cathodic protection system, Galvanic effect prediction, Computer simulation