• Transactions of Materials Processing
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• v.33 no.3
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• pp.193-199
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• 2024

The friction welding characteristics of stainless steels, mainly used in energy and chemical plant industries due to its excellent corrosion resistance and high strength, was evaluated in this study. Friction welding was introduced and conducted at a rotation speed of 2,000 RPM, friction pressure of 30 MPa, burn-off length of 5 mm and upset pressure of 110 ~ 200 MPa on rod typed specimens. The grain boundary characteristics distributions such a grain size, shape, misorientation angle and kernel average misorientation of the welds were clarified by electron backscattering diffraction method. The application of friction welding on SUS304 alloy resulted in a significant refinement of the grain size in the weld zone (5.11 mm) compared to that of the base material (48.09 mm). The mechanical properties of the welds, on the other hand, appeared to be relatively low or similar to those of the base material, which were mainly caused by dislocation density in the initial material and grain refinement in the welds.

• 한국초전도학회:학술대회논문집
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• v.9
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• pp.358-362
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• 1999

Magnetic levitation forces and trapped magnetic fields of top-seeded melt growth-processed YBCO superconductors with multiseeding were studied. The number of seeds was varied from one to six. The surface magnetic field of the prepared YBCO samples was dependent on the number of the seeds. The trapped magnetic field of the top surface decreased with increasing the number of the seed. Particularly, it drastically decreased at the YBCO grain boundary, probably due to the weak rink nature of the boundary. The magnetic levitation force also decreased with increasing the number of the seeds, similar to the variation of the surface magnetic field.

• International Journal of Safety
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• v.8 no.1
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• pp.6-9
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• 2009

The crack in a J85 engine V.G. actuator arm shaft for a bell crank on the engine compressor was investigated. The crack was observed in twenty two shafts during the inspection of 238 shafts. The failure analysis of shaft cracks was performed by chemical composition analysis using ICP(Inductively Coupled Plasma) and by fracture surface and microstructure analysis using FE-SEM and optical microscope. The crack initiated from the top and bottom and propagated to the center along the grain boundaries. From the chemical composition analysis, the fractography of the fracture surface and the microstructure, it was found that the failure mechanism of the shafts is the inclusion-related intergranular decohesion crack. The inclusion was found out from MnS particle by EDS(Energy Dispersive Spectroscopy). The crack initiated MnS inclusion in the grain boundary and propagated with the increase of applied shear stress during long operation. In order to prevent the fracture, NDI(Nondestructive inspection) is needed periodically as recommended.

• The Korean Journal of Ceramics
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• v.7 no.1
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• pp.6-10
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• 2001

Ultrafine $(CeO_2)_{0.9}(Gd_2O_3)_{0.1}$ solid solution powders synthesized by the glycine-nitrate process, with specific surface areas of $19-23\;\textrm{m}^2$/g were sintered at $1500^{\circ}C$ for various sintering times and then their electrical characteristics were investigated using AC impedance and four-point probe measurements. The electrical resistivity of the sintered $(CeO_2)_{0.9}(Gd_2O_3)_{0.1}$ bodies showed the minimum value at the sintering time of 10 hrs. The minimum total resistivity of the $(CeO_2)_{0.9}(Gd_2O_3)_{0.1}$ bodies sintered at $1500^{\circ}C$ for 10 hrs seems to result from the lowest activation energy for the electrical resistivity by the combination between the activation energies for the resistivities at the grain interior and grain boundary.

• Solar Energy
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• v.17 no.4
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• pp.3-11
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• 1997

Because grain boundaries in polycrystalline silicon act as potential barriers and recombination centers for the photo-generated charge carriers, these defects degrade conversion effiency of solar cell. To reduce these effects of grain boundaries, we investigated various influencing factors such as thermal treatment, various grid pattern, selective wet etching for grain boundaries, buried contact metallization along grain boundaries, grid on metallic thin film. Pretreatment above $900^{\circ}C$ in $N_2$ atmosphere, gettering by $POCl_3$ and Al treatment for back surface field contributed to obtain a high quality poly-Si. To prevent carrier losses at the grain boundaries, we carried out surface treatment using Schimmel etchant. This etchant delineated grain boundaries of $10{\mu}m$ depth as well as surface texturing effect. A metal AI diffusion into grain boundaries on rear side reduced back surface recombination effects at grain boundaries. A combination of fine grid with finger spacing of 0.4mm and buried electrode along grain boundaries improved short circuit current density of solar cell. A ultra-thin Chromium layer of 20nm with transmittance of 80% reduced series resistance. This paper focused on the grain boundary effect for terrestrial applications of solar cells with low cost, large area, and high efficiency.

• Korean Journal of Materials Research
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• v.29 no.4
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• pp.233-240
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• 2019

This study analyzes the mechanical properties, including the attrition rate, of $50{\mu}m$ size yttria-stabilized zirconia (YSZ) beads with different microstructures and high-energy milling conditions. The yttria distribution in the grain and grain-boundary of the fully sintered beads relates closely to Vickers hardness and the attrition rate of the YSZ beads. Grain size, fractured surfaces, and yttrium distribution are analyzed by electronic microscopes. For standardization and a reliable comparison of the attrition rate of zirconia beads with different conditions, Zr content in milled ceramic powder is analyzed and calculated by X-ray Fluorescence Spectrometer(XRF) instead of directly measuring the weight change of milled YSZ beads. The beads with small grain sizes sintered at lower temperature exhibit a higher Vickers hardness and lower attrition rate. The attrition rate of $50{\mu}m$ YSZ beads is measured and compared with the various materials properties of ceramic powders used for high-energy milling. The attrition rate of beads appears to be closely related to the Vickers hardness of ceramic materials used for milling, and demonstrates more than a 10 times higher attrition rate with Alumina(Hv ~1650) powder than $BaTiO_3$ powder (Hv ~315).

• Korean Journal of Metals and Materials
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• v.50 no.9
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• pp.659-667
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• 2012

Alloy 617 is a Ni-base superalloy and a candidate material for the intermediate heat exchanger (IHX) of a very high temperature gas reactor (VHTR) which is one of the next generation nuclear reactors under development. The high operating temperature of VHTR enables various applications such as mass production of hydrogen with high energy efficiency. Alloy 617 has good creep resistance and phase stability at high temperatures in an air environment. However, it was reported that the mechanical properties decreased at a high temperature in an impure helium environment. In this study, high-temperature corrosion tests were carried out at $850^{\circ}C-950^{\circ}C$ in a helium environment containing the impurity gases $H_2$, CO, and $CH_4$, in order to examine the corrosion behavior of Alloy 617. Until 250 h, Alloy 617 specimens showed a parabolic oxidation behavior at all temperatures. The activation energy for oxidation in helium environment was 154 kJ/mol. The SEM and EDS results elucidated a Cr-rich surface oxide layer, Al-rich internal oxides and depletion of grain boundary carbides. The thickness and depths of degraded layers also showed a parabolic relationship with time. A normal grain growth was observed in the Cr-rich surface oxide layer. When corrosion tests were conducted in a pure helium environment, the oxidation was suppressed drastically. It was elucidated that minor impurity gases in the helium would have detrimental effects on the high-temperature corrosion behavior of Alloy 617 for the VHTR application.

• Korean Journal of Materials Research
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• v.26 no.12
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• pp.688-695
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• 2016

Recrystallization behavior has been investigated for commercial purity AA1050 (99.5wt%Al) and high purity 3N Al (99.9wt% Al). Samples were cold rolled with 90% of thickness reduction and were annealed isothermally at 290, 315, and 350o C for various times until complete recrystallization was achieved. Hardness measurement and Electron Backscatter Diffraction(EBSD) analyses, combined with Grain Orientation Spread(GOS), were employed to investigate the recrystallization behavior. EBSD analysis combined with GOS were distinctly revealed to be a more useful method to determine the recrystallization fraction and to characterize the recrystallization kinetics. As the annealing temperature increased, recrystallization in AA1050 accelerated more than that process did in Al 3N. Both AA1050 and Al 3N showed the same temperature dependence of the n value of the Johnson-Mehl-Avrami-Kolmogorov equation(JMAK equation), i.e., n values increased as annealing temperature increased. Activation energy of recrystallization in AA1050 is about 176 kJ/mol, which is comparable with the activation energy of grain boundary migration in cold-rolled AA1050. This value is somewhat higher than the activation energy of recrystallization in Al 3N.

• Korean Journal of Materials Research
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• v.4 no.7
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• pp.828-836
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• 1994

In order to investigate the high temperature deformation characteristics in YBaiCu307-, oxide superconductor, the compression test was performed at temperatures from $890^{\circ}C$ to $930^{\circ}C$ at initial strain rate between $1.0 x 10^{-5}s^{-1}\; and \; 1.0^{-4}s^{-1}$. As the temperature increased and the initial strain rate decreased, the flow stress decreased. The strain rate sensitivity exponent measured as 0.41-0.46, supporting occurence of a superplastic deformation. The activation energy for superplastic deformation was calculated as 500-580KJ/mol, which decreased with increasing Ag content. Microstructure of the superplastically-deformed specimens showed that a grain growth occurred during deformation, and it appeared to be considerable when Ag content increased, but most grains still remained equiaxed after deformation. In this study, the deformation mechanism of YBCO superconductor was the grain boundary sliding with the diffusional accommodation and the contribution of the gram boundary sliding to the total strain was estimated to be 65%.

• Journal of Powder Materials
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• v.23 no.5
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• pp.379-383
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• 2016

A thick film of $Li_7La_3Zr_2O_{12}$ (LLZO) solid-state electrolyte is fabricated using the tape casting process and is compared to a bulk specimen in terms of the density, microstructure, and ion conductivity. The final thickness of LLZO film after sintering is $240{\mu}m$ which is stacked up with four sheets of LLZO green films including polymeric binders. The relative density of the LLZO film is 83%, which is almost the same as that of the bulk specimen. The ion conductivity of a LLZO thick film is $2.81{\times}10^{-4}S/cm$, which is also similar to that of the bulk specimen, $2.54{\times}10^{-4}S/cm$. However, the microstructure shows a large difference in the grain size between the thick film and the bulk specimen. Although the grain boundary area is different between the thick film and the bulk specimen, the fact that both the ion conductivities are very similar means that no secondary phase exists at the grain boundary, which is thought to originate from nonstoichiometry or contamination.