발간논문

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An attempt was made to evaluate the change of microstructures and mechanical properties with increasing spheroidizing time in a cold heading steel, containing proeutectoid ferrite and pearlite, by the ultrasonic attenuation and velocity measurements. Ultrasonic velocity was not sensitive to the microstructural changes, resulting from spheroidization heat treatment to produce various mean aspect ratio, mean equivalent size and number of cementite particles per unit area and ferrite grain size. Ultrasonic attenuation was initially decreased and then increased with increasing spheroidizing time. A good correlation between ultrasonic attenuation coefficient and mechanical properties such as hardness and tensile strength was found.

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Continuous low cycle fatigue tests have been conducted on Nb-A286 Fe-base superalloy in an air atmosphere at the temperature range of 25 to 650℃ to investigate the transition of fatigue cracking with test temperature and applied strain range. The intergranular fatigue cracking at high temperatures was due to the precipitation of the η phase at the grain boundary assisted by the applied stress during low cycle fatigue. It is investigated whether the precipitate at the grain boundary provides the site for the grain boundary cavitation, which induces the intergranular cracking in low cycle fatigue. The fatigue cracking mode changes from the transgranular mode to the predominantly intergranular mode with the applied strain range. The change of the fatigue cracking with strain range is induced by the combination of mutually competing factors involving the grain boundary precipitate, the density of the slip band, and the concentration of stress at points where the slip bands impinge upon the grain boundaries.

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High temperature creep behaviour of 20 vol.% SiC particle reinforced 5052 Al, 6061 Al and 7075 Al alloy matrix composites was studied at temperatures of 633, 663, and 693K under the applied stress range of 25∼100 ㎫. The composites were manufactured by the pressureless infiltration technique. The composites showed high apparent stress exponent and activation energy. The 7075 Al matrix composite showed the highest creep strength at 633K, while at 693K the 5052 Al matrix composite had the highest creep resistance. The creep strength of the 7075 Al matrix composite decreased rapidly with increase of the temperature. True stress exponent of the composites was measured as 5 and major creep mechanism of the composites was found to be lattice self-diffusion of the matrix alloy. Different temperature-dependence of the composites` creep strength and the threshold stress (including its origin) were attributed to different amount of precipitates and aluminum nitrides in the matrix alloys.

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In the present study, tensile and corrosion properties of Cu-6Ni-2Mn-2Sn-2Al alloy, with and without Zr addition, were examined. Even though the addition of Zr refined the grain structure of Cu-6Ni-2Mn-2Sn-2Al alloy, the improvement in tensile property was not significant. The addition of Zr, however, greatly improved the corrosion resistance of the alloy in NaCl aqueous solution. The potentiodynamic study clearly demonstrated that the Zr addition substantially reduced the current density of Cu-6Ni-2Mn-2Sn-2Al alloy in 3.5% NaCl solution. The enhanced corrosion resistance with Zr appeared to be due to the reduced Sn segregation during solidification. The stress corrosion cracking study reflected the result of potentiodynamic study in the present study. The Zr-added specimen showed no reduction in tensile ductility after exposure to 3.5% NaCl aqueous solution for 20 days with an applied stress of 80% yield strength, while the tensile elongation of non-Zr added specimen was reduced approximately 10% after the exposure. The reduced Sn segregation during solidification in believed to be the main reason for the improved corrosion property of Zr-containing Cu-6Ni-2Mn-2Sn-2Al alloys.

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The present study is concerned with fabrication and microstructural analysis of TiC/SUS304 surface composites using irradiation of accelerated high-energy electron beam. Three kinds of powder/flux mixtures, i.e., TiC, (Ti+C), and (Ti+SiC) powders with 40 wt.% of CaF₂ flux, were placed on an SUS304 stainless steel substrate, which was then irradiated with electron beam. TiC agglomerates and pores were found in the surface composite layer fabricated by irradiation of TiC powders because of insufficient melting of TiC powders. In the composite layer fabricated by irradiation of Ti and C powders having lower melting points than TiC powders, a number of primary TiC carbides were precipitated without forming TiC agglomerates or pores. This indicated the more effective TiC precipitation obtained from the melting of Ti and C powders, instead of TiC powders. The hardness and wear resistance of the surface composite layer were directly influenced by hard primary TiC carbides, and thus were two to three times greater than those of the stainless steel substrate.

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The present study is concerned with the microstructural analysis and the evaluation of hardness and wear properties of Ti-6Al-4V surface-alloyed materials fabricated by high-energy electron beam. The mixtures of TiC, TiN, or TiC+TiN powders and CaF₂ flux were deposited on a Ti-6Al-4V substrate, and then electron beam was irradiated on these mixtures using an electron beam accelerator. In the specimens processed with a flux mixing ratio of 50 wt.%, the surface-alloyed layers of 1 ㎜ in thickness were homogeneously formed without defects, and contained a large amount (over 30 vol.%) of precipitates such as TiC, TiN, (Ti_xAl(1-x).)N, and TiC_xN(1-x) in the martensitic or N-rich acicular α-Ti matrix. This microstructural modification including the formation of hard precipitates and hardened matrices in the surface-alloyed layers improved hardness and wear resistance. Particularly in the TiN/Ti surface-alloyed material, the wear resistance was greatly enhanced by ten times higher than that of the Ti alloy substrate. These findings suggested that the surface-alloying using high-energy electron beam irradiation was economical and useful for the development of titanium-base surface-alloyed materials with improved hardness and wear properties.

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In this study, the microstructure and mechanical properties of A1100 aluminum alloy sheets manufactured by the Accumulative Roll Bonding(ARB) process were investigated. Hot rolled A1100 sheets were highly strained up to a true strain of 6.32 at room temperature by the ARB process. These sheets were recrystallized at the temperature range of 150℃ to 400℃ for 1hr. The ARB process was repeated up to 6 cycles and well bonded sheets were obtained successfully. The hardness and tensile strength of 6th ARBed specimen(true strain : 6.32) are three times higher than those of a hot rolled sheet. After recrystallization treatment at 200℃, the specimen has ultra-fine grain structures with grain size of less than 1 ㎛. In case of annealing at 300℃ for 1hr., the microstructures of the specimens consist of fully recrystallized grain structure with the grain size less than 7 ㎛.

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The microstructure and mechanical properties have been investigated on a commercial 5083 aluminum alloy processed by the equal-channel angular pressing (ECAP) at 473K. The microstructure of a single pass sample consisted of elongated subgrains. Increasing the number of pass led to very fine equilibrium grains having high angle grain boundaries. The 0.2% yield stress and hardness after a single pass increased greatly, and then kept constant with additional straining. On the other hand, the elongation to failure of a single pass sample decreased a little to compare with sample before ECAP, but there was no further reduction with additional straining. Recrystallization partially occurred in ECAPed 5083 aluminum alloy when annealed at 523K for 1 hr, and completely finished when annealed only at 573K for 1 hr. With increasing the number of pass, the onset temperature of recrystallization decreased and the rate of softening increased.

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Recently Zn-Co alloy coating has been studied because minute additions of cobalt to zinc decrease its rate of corrosion. The corrosion behavior of electrodeposited Zn-Co alloy coatings with various contents of Co was compared with pure Zn coating using electrochemical test and electrochemical impedance spectroscopy (EIS) measurement in 3.5 wt% NaCl solution and surface analysis. In immersion test, the corrosion potential of all Zn-Co coatings was higher than that of pure Zn coating. Corrosion current density for the 0.9 wt%Co and 5.0 wt%Co was lower than that for pure Zn coating. Dezincification which occurred in the corrosion process made a Co-rich barrier in the coating layer and only 0.9 wt%Co and 5.0 wt%Co had an effective Co-rich barrier. The 58.0 wt%Co exhibited the highest current density owing to the localized corrosion at large cracks. The 0.9 wt%Co and 5.0 wt%Co, with a more active corrosion potential, were galvanically corroded at cracks in the coating to protect the exposed substrate cathodically. Whereas on the 58.0 wt%Co, anodic dissolution and galvanic corrosion of the substrate were concentrated at coating defects by a large cathode(58.0 wt%Co coating) and anode(substrate) surface area ratio. EIS and EPMA results confirmed that cobalt enrichment, together with the zinc corrosion products, acts as a barrier layer reducing the total corrosion rate.

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Indium and tin layers were used as the diffusion barriers between the indium-tin oxide (ITO) and poly-Si in order to reduce the contact resistance. ITO/Si contacts should be adopted in thin-film transistor liquid crystal display (TFT LCD) for simplifying the fabrication process. For 5 ㎚ thick In and Sn layers, 10-minute annealing at 250℃ was performed to minimize the loss in the optical transmittance. With In and Sn layers, contact resistance values of 5×10^(-3) ∼ 4×10^(-3)Ω㎠ were obtained. These values were higher than those measured from the conventional ITO/Mo/Al/Si contacts (5×10^(-5) Ω㎠∼4 ×10^(-4)Ω㎠) but. lower than those obtained from ITO/Si contacts (about 1×10^(-1)Ω㎠) Sn was found to be stable after the annealing but In lost its function as a diffusion barrier by diffusion into Si.

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