발간논문

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Dry sliding wear tests were carried out on SiC and TiC particle-reinforced 6061 Al alloy matrix composites using different counterpart materials (AISI 52100 bearing steel and alumina) under various loads (1∼12N) and constant sliding velocity (0.2m/sec) conditions. Wear characteristics of the composites against the two counterpart materials were investigated. At low loads (1∼3N), wear rate of composites worn against the steel was lower than that against the alumina. Formation of the MML(Mechanically Mixed Laver) was responsible for the lower wear rate against the steel counterpart. However, at intermediate loads (4∼8N), wear rate of the composites worn against the alumina was lower. Low thermal conductivity of the alumina, which accelerated formation of oxide layers and low adhesive force between the alumina and the matrix alloy were considered to result in the lower rate. At high loads (9∼12N), wear of the composites proceeded by severe deformation of the matrix alloy and subsurface cracking. Wear rates of the composites were similar against the two counterpart materials at the high load regime.

keyword : Sliding wear, Counterpart material, Composite, SiC, TiC, 6061 Al alloy

This study investigated the effect of aging treatment, Co addition and stress on the precipitation of Laves phase in 10Cr-6W ferritic heat-resistant steel for ultra-super critical pressure power generating plants. The hardness change during the aging treatment has shown two peaks. Ductility and toughness rapidly decreased by the very fine disk-like Laves phase precipitates at initial stage of aging. As aging time gets longer, Laves phase has precipitation, which brings out strength`s increase, on the other hand elongation and impact value decrease. Content of Laves phase precipitates increased with addition of Co. The stress during the aging enhanced the Laves phase precipitates, and content of Laves phase precipitates and particle numbers per unite area enlarged by increasing of stress.

keyword : Laves phase, Ferritic heat-resistant steel, High temperature strength, Aging, Strss effect

Thermal stability of Si-doped TiAl alloys with the lamellar microstructure was investigated by partial melting in an optical floating zone(FZ) furnace to ascertain the possibility of usage as a seed material. The lamellar stability of TiAl and TiAl-X(X=Mo, Nb, W) alloys, which proved to be thermally unstable, was improved dramatically by Si addition. In the results of partial melting, it was found that there are two boundaries of Al content(low Al limit and high Al limit) which devide stable and unstable compositions. It is thought that when Al content is lower than low Al limit, recrystallization can be occurred by β-phase formation at high temperature, and when Al content is higher than high Al limit, the lamellar stability will be deteriorated by rapid variation of α/γ phase volume fraction during heating and cooling. The lamellar stability of TiAl-Si alloys can be predicted by drawing above two boundaries at TiAl-Si isothermal section. Moreover, the lamellar stability of TiAl-X-Si alloys can be also predicted by converting the compositions to TiAl-Si compositions. In addition, lamellar orientation control was successfully performed by directional solidification using the Ti-46Al-l.5Mo-1Si alloy as a seed material.

keyword : TiAl, Lamellar microstructure, Thermal stability, Si addition, Stable region

WC powder was synthesized from WO_3 by Self-propagating High-temperature Synthesis process(SHS). The effects of carbon, mole ratio of reducing agent, mole ratio of carbon, and pellet size were investigated in the present study. By optimizing the synthesis conditions, it was possible to fabricate WC powders without secondary phases such as W, W_2C. To remove residual impurities and free carbon, acid leaching and roasting were carried out. Microstructural, compositional and structural characterization of products were done by Scanning Electron Microscope(SEM), ICP and X-Ray Diffractometer, respectively.

keyword : SHS process, WC, Carbon, Acid leaching

The present study is concearned with microstructural analysis of (TiC, SiC)/Ti-6Al-4V surfaces composites fabricated by high-energy electron beam irradiation. The mixtures of TiC, SiC, Ti+SiC, and TiC+SiC powders and CaF_2 flux were evenly 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 flux addition, the surface-alloyed layers of 1.2∼2.1 ㎜ in thickness were homogeneously formed without defects, and contained a large amount (30∼66 vol.%) of precipitates such as TiC and Ti_5Si_3 in the martensitic matrix. Upon electron beam irradiation, (TiC, SiC)/flux powder mixtures and part of the substrate were melted to form a molten metal containing Ti, Si, and C. Here, C and Si atoms interacted with Ti atoms to form TiC and Ti_5Si_3, respectively, and the matrix was composed of martensite because of fast cooling during solidification. Microstructural modifications including melting, solidification, precipitation, and phase transformation processes could be explained well from a Ti-Si-C ternary phase diagram.

keyword : Surface composite, Electron beam irradiation, Ti-6Al-4V, TiC, SiC, Flux, Microstructure

The present study aimed at investigating hardness, wear properties, and fracture toughness of (TiC, SiC)/ Ti-6Al-4V surface composites fabricated by high-energy electron beam irradiation. A large amount (30∼66 vol.%) of precipitates such as TiC and Ti_5Si_3 improved the hardness and wear resistance of the surface composite layer two times and six to twenty five times, respectively, greater than that of the substrate. Particularly in the surface composite fabricated with TiC and SiC powders, the selective wear of the matrix and the cracking or fallen-offs of precipitates were considerably reduced because it had a network-shaped structure of abundant TiC and Ti_5Si_3 precipitates, thereby leading to the best hardness and wear resistance. Observation of the microfracture process of the surface composite layers revealed that microcracks were primarily initiated at precipitates, and that shear bands were formed in the matrix between these microcracks. Thus, fracture toughness was determined mainly by the volume fraction of precipitates working as fracture initiation sites and partly by the matrix property interrupting the crack propagation. These findings suggested that the high-energy electron beam irradiation method was economical and useful for the development of titanium-base surface composites with improved hardness and wear properties.

keyword : Surface composite, Electron beam irradiation, Ti-6Al-4V, TiC, SiC Hardness, Wear resistance, Fracture toughness

The effects of Nb content and final annealing temperature on the corrosion characteristics of the advanced Zr-based alloys for nuclear fuel cladding materials (A : Zr-1.5Nb-0.4Sn-FeCr, B : Zr-1.5Nb-0.4Sn-FeCr, C : Zr-0.4Nb-0.8Sn-FeCrCu) were evaluated in water and LiOH solution at 360℃, and in steam at 400℃, respectively. The corrosion test revealed that the C alloy, which contains Nb content less than its solubility, shows the best corrosion resistance among the which alloys is related to the difference in the size and distribution of precipitates of the alloys and the formation of β-Zr phase after the final annealing at 700℃.

keyword : Zr, Zr alloys, Solubility, Corrosion, Precipitate

Effects of Cr content and the film formation potential an electronic properties of the passive film on Fe-Cr alloys were investigated in pH 9.2 borate buffer solution. The influence of pH on both Fe-Cr and Fe-18%Cr-4%Mo alloys was also examined. According to Mott-Schottky plot results, passive films of Fe-Cr alloys characterized p-type (Cr-oxide) at EE_fb, respectively. The doping density decreased with the increase in Cr content and the film formation potential while the thickness of the space charge layer increased. Cyclic voltammetry results showed that as Cr content increased, the peak current densities of oxidation and reduction of Fe ion species decreased, but the peaks associated with reduction of Cr^6+ to Cr^3+ increased. The reduction peaks were observed only when the passive films were formed above 0.6 V_sce. The donor concentration of Fe-18%Cr-4%Mo alloy tended to be similar to that of Fe-25%Cr alloy as the pH values decreased. AES results showed that as Cr content, the film formation potential and pH values increased, the highest peak of Cr moved from the outside to the inside within the film. AFM images for passive films formed on Fe-18%Cr and Fe-18%-4%Mo alloys showed that Mo affected the smoothness of the passive film.

keyword : Mott-Schottky plot, Cyclic voltammetry, AES, AFM, Passive film, Stainless steel, Flatband potential, Space charge layer

Galvanic corrosion between 6061Al matrix and reinforcements such as graphite, SiC and Al_2O_3 phases has been studied in the deaerated, the open to air and the aerated 0.1N NaCl solutions. Based on the mixed-potential theory, the galvanic currents were measured by using the zero-resistance ammeter and the potentiodynamic polarization curves between 6061Al and reinforcements. Experimental results showed that the galvanic current densities of 6061Al-graphite couple were much higher values than those of 6061Al-SiC and 6061Al-Al_2O_3 couples. As the dissolved oxygen concentration and the surface ratio of reinforcements/6061Al matrix increased, the galvanic current densities were observed to increase.

keyword : Galvanic corrosion, Metal matrix composites, Reinforcement, Mixed-potential theory, Zero-resistance ammeter

The oxidation characteristics with orientation of an unalloyed Zr were investigated. From the Zr crystal bar, the specimen representing a single crystal was prepared to be coarse enough to have its grain be 4×4×1 ㎣ in size. After identifying the crystallographic orientations and planes by XRD, the samples were cut out in such a way as the cutting planes could correspond to the desired crystallographic planes. Oxidation tests were carried out in water at 360℃. The oxidation rate of the (1120) prismatic plane was faster than that of the (0002) basal plane. The analysis of oxide using the synchrotron XRD revealed that the zirconia grown at the basal plane had the preferred (200) plane in mono-ZrO_2, while the zirconia at the prismatic plane grew at both the (200) and (002) planes of mono-ZrO_2. In addition, the oxide layer that had formed at the basal plane having only one preferred growth plane exhibited a high fraction of columnar oxide and a relatively wide range of protective barrier layer. These results mean that the characteristics of the preferred planes of mono-ZrO_2 play an important role on the oxidation rate of Zr single crystal.

keyword : Zr, Oxidation, Oxide, Basal plane, Prism plane, Synchrotron X-ray, mono-ZrO2