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Fe-Al series ferromagnetic damping alloy exhibits good vibration and noise absorbing ability due to the work done in moving and rotating its magnetic domain wall when the alloy is stressed within elastic region. Damping capacity of this alloy was known to be affected by several factors such as the impurities of the specimen, the external magnetic field and applied stress. The applied stress causes the structural inhomogeneity that inhibits the domain wall motion and damping behavior under external stress is neither simple nor straightforward. By designing the special tensile testing device, the effect of pre-strain on SDC(specific damping capacity) within elastic region was studied with aid of modified Bitter magnetic domain wall observation technique. From 0 to 60% level of the yield strain(0.2%), the microstructure did not show any domain wall pinning. The SDC_(max) value of specimen remains as high as the 47% SDC_(max). From 70 to 90% level of the yield strain, the sample showed the magnetic domain wall pinning. The SDC_(max) decreased to 32%. The result advises strongly that the designing safe guard for the Fe-Al damping alloy is around 60% level of the yield strain.

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Tungsten Carbide(WC) coatings were formed on mild steel substrates using the plasma spray process. As a mean to enhance the mechanical properties such as microhardness and wear resistance, as-sprayed coatings were annealed and then subsequently were subjected to the carburization under methane and hydrogen atmospheres. When the heat treatment was carried out under the hydrogen atmosphere, the amounts of WC phase were decreased and the amounts of α-W₂C and W phase were increased with the increase of temperature, which leaded to a slight reduction of the microhardness. In case of the carburization under methane atmosphere, the microhardness of the coatings was increased with the increase of temperature and show the highest value of 1540Hv at 1100℃. The wear resistance also was increased with the increase of the carburization temperature. The wear resistance of the coatings heat treated at 1100℃ were 17 times higher than the as-sprayed coatings. The increase of methane concentration at constant temperature at 900℃ showed the improvement of the microhardness and the wear resistance of coatings. The highest microhardness and wear resistance obtained at the methane concentration of 11%.

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The plasma sprayed NiCoCrAlY coatings on Inconel 713C substrates were boronizeci or boronized and aluminized. The mechanical properties and hot corrosion resistance of the NiCoCrAlY coatings were studied. Mechanical properties were measured by the microhardness and abrasion tests, and hot corrosion resistance was examined by the cyclic hot corrosion test. The microhardness at the 30㎛ from the surface of the boronized samples was about 1200Hv, which is approximately five times, relative to the as-sprayed NiCoCrAlY coatings probably because of the formation of borides such as Ni₂B and Co₂B within the samples. When the samples were aluminized after boronizing, the samples showed the cross sectional hardness of more than 1200Hv because of the homogeneous distribution of the borides and good hot corrosion resistance, relative to the boronized samples. The heat treated samples after the boronizing and aluminizing contained NiAl phase at the surface. The heat treated samples showed increased cyclic hot corrosion resistance, compared with aluminized after boronizing. Also the heat treatment enhanced abrasion resistance significantly, but did not affect the hot corrosion resistance. It can be, therefore, concluded that the heat treatment after boronizing and subsequent aluminizing for the NiCoCrAlY coatings is necessary to improve the mechanical properties and hot corrosion resistance.

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The precipitation behavior in cold-rolled and continuously annealed ultra-low carbon steels containing Ti or Zr has been investigated. The effect of Mn content between 0.2% and 1.0% on the precipitation behavior has been also studied. The annealed sheets of Ti-P steels contained three types of precipitates; TiN, TiS or MnS, TiN+TiS or TiN+MnS. The sulfide type precipitated depended on the Mn and Ti contents. With the increase in the Mn content, the amount of MnS sulfide increased. TiS or MnS in the combined precipitates of TiN+TiS or TiN+MnS, nucleated on a TiN particle and coarsened. The role of Zr in the precipitation behavior of Zr-P steels seemed to be very similar to the role of Ti in the Ti-P steels. The combined precipatates of ZrN and MnS or ZrS were mainly observed in the Zr-P steels. The sulfide type was changed from ZrS to MnS as the Mn content increased.

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The microstructure and coarsening kinetics were studied in modified MAR-M247 type nickel-base superalloy as a function of alloy composition and aging time at 982℃, In this study, Ta concentrations were changed to characterize the effects of the element on the microstructure and coarsening kinetics during the aging. The morphology of gamma prime particles was observed to be sensitive to alloy composition and gamma prime particle size. Gamma prime particles changed from spherical to cuboidal, or cuboidal to a plate-like morphology during the aging at 982℃. The coarsening kinetics of the alloys followed the t^⅓ dependence predicted by LSW theory. The addition of Ta decreased the coarsening rate of gamma prime.

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Isothermal and thermal fatigue tests were carried out for plasma sprayed and laser treated MCrAly/ZrO₂ 8wt%Y₂O₃ thermal barrier coatings(TBCs) to discuss the failure mechanism and the effects of laser-treatment. Phase transformation of the ceramic coating was not occurred during testing. In thermal fatigue test, at the temperature range of 1100℃∼RT, specimens were investigated prior to and after failure. Cracks initiated, grew and combined by stresses due to thermal expansion mismatch and oxidation of the bond coating near the bond/ceramic coating interface. So, failure of the TBC without laser-treatment occurred suddenly. In case of laser-treated TBCs, vertical cracks arisen from laser-treatment and cracks which initiated and grew near the interface-combined. But failure of the laser-treated TBC occurred gradually. Thermal fatigue Iife of the laser-treated TBC was increased 1.7 times than the TBC without laser-treatment.

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An in -situ observation of fracture behavior, together with direct measurement of fracture toughness, was adopted to verify microfracture mechanism of a high speed steel (HSS) roll material using a compact tension (CT) type loading stage inside a scanning electron microscope (SEM). The speciment used in the present study was a small and thin CT speciment which had a sharply machined notch and was grooved along its central region. The observation of fracture process indicated that the interfaces between hard primary carbide and matrix were separated first to form microcracks at low stress intensity factor levels. Then, the microoracks were easily propagated along the intercellular carbide networks, leading to relatively low fracture toughness. This in-situ SEM fracture technique was found useful for studying the microstructural aspects of fracture initiation and for correlating the microscopic fracture behavior with macroscopic fracture toughness of brittle materials.

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Corrosion tests of various Zr-based alloys were performed at 350℃ in pure water and aqueous solution containing 220 ppm Li. The alloys were standard Zircaloy-4, Zirealoy type alloys(ZrSnFeCr), ZrNbFeCr, ZrSnNbFeCr and ZrFeCr alloys. For some of them Cr was replaced by V. In pure water Zircaloy-type alloys showed higher corrosion resistance than standard Zicaloy-4, while most Nb-containing alloys showed lower corrosion resistance. All of the alloys tested in this study exhibited the Li-accelerated corrosion in 220 ppm Li solution. The influence of Li addition on corrosion was small for the Zircaloy-type alloys, but significant for the Nb-containing alloys. Especially, C-2(Zr0.1Nb0.25Sn0.4Fe0.2Cr) which had superior corrosion resistance in water showed much higher corrosion rate in 220 ppm Li solution than 1.4Sn Zircaloy-type alloy(A-1). Addition of tin in the range of 0.8∼1.1wt% imparted good corrosion resistance to Zircaloy-type alloys in pure water and 220 ppm Li solution. The addition of 0.1∼0.4wC%Nb which is below the solubility limit in Zr matrix improved the corrosion resistance in low tin alloys.

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Microstructures, thermal stability and deformation characteristics of amorphous Al_(88)Ni_(10-x)Fe_xNd₂(x=0,3,5 at %) alloys were examined during tensile testing in the temperature range from room temperature to 680K. Three alloys experienced the two-stage crystallization process of Am→Am+Al→Al+compound at the temperature range. The particle sizes were about 10nm and 30-50nm for Al phase and the compound, respectively. The onset temperature of the precipitation for Al and intermetallic compound phases increased with the replacement of Ni by Fe, indicating an enhancement of thermal stability of the amorphous phase. All the amorphous alloys subjected to tensile testing at elevated temperature exhibited large elongations of 22-42% and 40-54% near precipitation peak temperature of Al and intermetallic compound phases, respectively. The large elongation is interpreted to be induced by crystallization of these phases. Tensile fracture occurred through shear slip of amorphous phase below T_(x2), respectively.

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