• Progress in Superconductivity
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• v.1 no.1
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• pp.14-19
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• 1999

Measurements across single grain boundaries in YBCO thin films and bulk bicrystals have been used to demonstrate the influence of grain boundary structure on the critical current carried across the grain boundary. In particular, we show that one role of grain boundary structure is to change the degree of pinning along the boundary, thereby influencing the critical current. This effect can be used to explain the large difference in critical current density across grain boundaries in thin films compared to that for bulk bicrystal. These differences illustrate the distinction between the intrinsic mechanism of coupling across the grain boundary that determines the maximum possible critical current across a boundary and the measured critical current which is limited by dissipation due to the motion of vortices.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.27 no.1
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• pp.109-118
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• 2003

The recent trend towards miniaturization causes an increased demand for parts with very small dimensions. Milli-structure components are classified as a component group whose size is between macro- and micro-scale. The manufacturing process of these components of thin sheet metal forming has a microscopic properties in addition to a typical phenomenon of bulk deformation because of the forming size. Also, the material properties and the deformation behavior change with miniaturization, which means that, a coarse grained materials show a higher resistance against deformation, when the grain size is in the range of the sheet thickness. In this study, a new numerical approach is proposed to simulate intergranular milli-structure in forming by the finite element method. The grain element and grain boundary element are introduced to simulate the milli-structure in the bending. The grain element is used to analyze the deformation of individual grain while the grain boundary element is for the investigation on the movement of the grain boundary. Also, the result of the finite element analysis is confirmed by a series of milli-sized forming experiments.

• Journal of the Korean Ceramic Society
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• v.32 no.10
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• pp.1117-1122
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• 1995

The effect of grain boundary structure on zigzag migrtion has been studied. Five kinds of a(2110)-m(1010) diffusion couples with different twist angles by 30$^{\circ}$from a [0001] common direction of each plane were prepared. When chromia (Cr2O3) was added to the diffusion couples by a vapor phase, zigzag migration of the grain boundary occurred. The fraction of zigzag migration did not essentially vary with the twist angle, but the magnitude and migration distance of individual migrating segment varied. The variation of CIGM morphology thus appears to result from the change in grain boundary mobility due to microscopic deviation of grain boundary structure out of a macroscopic grain boundary orientation.

• Proceedings of the Korean Society for Technology of Plasticity Conference
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• 2002.02a
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• pp.266-273
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• 2002

Milli-structure components are classified as a component group whose size is between macro and micro scales, that is, about less than 20mm and larger than 1mm. The bending of these components of thin sheets has a typical phenomenon of bulk deformation because of the forming size. The recent trend towards miniaturization causes an increased demand for parts with very small dimensions. The conceptual miniature bending process enables the production of such parts with high productivity and accuracy. The stress values of the flow curve decrease with miniaturization, which means that coarse grained materials show a higher resistance against deformation, when the grain size is in the range of the sheet thickness. In this paper, a new numerical approach is proposed to simulate intergranular milli-structure in forming by the finite element method. The grain element and grain boundary element are introduced to simulate the milli-structure of strip in the bending. The grain element is used to analyze the deformation of individual grain while the grain boundary element is for the investigation on the movement of the grain boundary. Also, the result of the finite element analysis is confirmed by a series of milli-sized forming experiments.

• Transactions of Materials Processing
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• v.11 no.5
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• pp.439-446
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• 2002

This study presents a new computational model to analyze the grain deformation in a polycrystalline aggregate in a discrete manner and based directly in the underlying physical micro-mechanisms. When scaling down a metal forming process, the dimensions of the workpiece decrease but the microstructure of the workpiece remains the similar. Since the dimensions of the workpiece are very small, the microstructure especially the grain size will play an important role in micro forming, which is called size effects. As a result, specific characteristics have to be considered for the numerical analysis. The grains and grain boundary elements are introduced to model individual grains and grain boundary facets, respectively, to consider the size effects in the micro forming. The constitutive description of the grain elements accounts for the rigid-plastic and the grain boundary elements for visco-elastic relationships. The capability of the proposed approach is demonstrated through application of grain element and grain boundary element in the micro forming.

• Proceedings of the KWS Conference
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• 2002.10a
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• pp.250-254
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• 2002

Intergranular corrosion of austenitic stainless steels is a conventional and momentous problem during welding and high temperature use. One of the major reasons for such intergranular corrosion is so-called sensitization, i.e., chromium depletion due to chromium carbide precipitation at grain boundaries. Conventional methods for preventing sensitization of austenitic stainless steels include reduction of carbon content in the material, stabilization of carbon atoms as non-chromium carbides by the addition of titanium, niobium or zirconium, local solution-heat-treatment by laser beam, etc. These methods, however, are not without drawbacks. Recent grain boundary structure studies have demonstrated that grain boundary phenomena strongly depend on the crystallographic nature and atomic structure of the grain boundary, and that grain boundaries with coincidence site lattices are immune to intergranular corrosion. The concept of "grain boundary design and control", which involves a desirable grain boundary character distribution, has been developed as grain boundary engineering. The feasibility of grain boundary engineering has been demonstrated mainly by thermomechanical treatments. In the present study, a thermomechanical treatment was tried to improve the resistance to the sensitization by grain boundary engineering. A type 304 austenitic stainless steel was pre-strained and heat-treated, and then sensitized, varying the parameters (pre-strain, temperature, time, etc.) during the thermomechanical treatment. The grain boundary character distribution was examined by orientation imaging microscopy. The intergranular corrosion resistance was evaluated by electrochemical potentiokinetic reactivation and ferric sulfate-sulfuric acid tests. The sensitivity to intergranular corrosion was reduced by the thermomechanical treatment and indicated a minimum at a small roll-reduction. The frequency of coincidence-site-lattice boundaries indicated a maximum at a small strain. The ferric sulfate-sulfuric acid test showed much smaller corrosion rate in the thermomechanically-treated specimen than in the base material. An excellent intergranular corrosion resistance was obtained by a small strain annealing at a relatively low temperature for long time. The optimum parameters created a uniform distribution of a high frequency of coincidence site lattice boundaries in the specimen where corrosive random boundaries were isolated. The results suggest that the thermomechanical treatment can introduce low energy segments in the grain boundary network by annealing twins and can arrest the percolation of intergranular corrosion from the surface.

• Proceedings of the Korean Society for Technology of Plasticity Conference
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• 1999.03b
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• pp.173-178
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• 1999

In this study MD simulations have been performed to observe the behavior of a grain boundary in an a-Fe plate under 2-dimensional loading. In MD simulation the acceleration of every molecule can be achieved from the potential energy and the force interacting between each molecule and the integration of the motion equation by using Verlet method gives the displacement of each molecule. Initially four a-Fe rectangular plates having different misorientation angles of grain boundary were modeled by using the Johnson potential and Morse potential We compared the potential energy of the grain boundary system with that of the perfect structure model. Also we could obtain the width of the grain boundary by investigating the local potential energy distribution. The tensile loading for each grain boundary models was applied and the behavior of grin boundary was studied. From this study it was clarified that in the case using Johnson potential the obvious fracture mechanism occurs along the grain boundary in the case of Morse potential the diffusion of the grain boundary appears instead of the grain boundary fracture.

• Journal of the Korean Ceramic Society
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• v.44 no.12
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• pp.683-689
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• 2007

Grain-boundary conduction in the fluorite-structure solid oxide electrolytes such as acceptor-doped zirconia and ceria were reviewed. The siliceous impurity, even several hundreds ppm, affects the ionic conduction across grain boundary to a great extent. Various approaches to improve grain-boundary conduction in fluorite-structure oxide electrolytes have been investigated, which include (1) the scavenging of siliceous phase by the reaction with second phase, (2) the gathering of intergranular siliceous phase into a discrete configuration and (3) the dewetting of intergranular liquid phase by post-sintering heat treatment.

• Proceedings of the Korean Society for Technology of Plasticity Conference
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• 2003.05a
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• pp.439-442
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• 2003

This study presents a new computational model to analyze the grain deformation in a polycrystalline aggregate in a discrete manner and based directly in the underlying physical micro-mechanisms. As a result, specific characteristics have to be considered for the numerical analysis. The grains and grain boundary elements are introduced to model individual grains and grain boundary facets, respectively, to consider the size effects in the micro forming. The constitutive description of the grain elements accounts for the rigid-plastic and the grain boundary elements for elastic relationships. The capability of the proposed approach is demonstrated through application of grain element and grain boundary element in the micro forming.

• Journal of the Korean Vacuum Society
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• v.7 no.2
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• pp.112-117
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• 1998

TiN has been investigated as a good candidate for a diffusion barrier of Cu. Therefore, in this study, the grain boundary diffusion of Cu in TiN film was investigated by X-ray photoelectron spectroscopy(XPS). In general, TiN has a columnar grain structure. In the relatively lower temperature, less than 1/3 of the melting point, it was observed that Cu diffused into TiN mainly along the grain boundaries of TiN. The grain size of TiN was measured by atomic force microscope (AFM). In order to estimate the grain boundary diffusion constants, we used the modified surface accumulation method. The activation energy, $Q_b$ was 0.23 eV, and the diffusivity, $D_{bo}$ was $5.5\times10^{-12{\textrm{cm}^2$/sec.