• Journal of Korea Foundry Society
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• v.36 no.5
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• pp.159-166
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• 2016

The microstructural evolution of a cast Ni base superalloy, IN738LC, has been investigated after long term exposure at several temperatures. Most of the fine secondary ${\gamma}^{\prime}$ particles resolved after 2000 hour exposure at $816^{\circ}C$. At higher temperatures of $871^{\circ}C$ and $927^{\circ}C$, secondary ${\gamma}^{\prime}$ resolved after 1000 hours of exposure, and cuboidal primary ${\gamma}^{\prime}$ grew with exposure time. During the thermal exposure, ${\sigma}$ phase formed at all tested temperatures, and ${\eta}$ phase was observed around interdendritic regions due to carbide degeneration. The influence of microstructural evolution during thermal exposure on the mechanical properties has been analyzed. The effects of ${\gamma}^{\prime}$ particle growth are more pronounced on the high temperature creep properties than on the room temperature tensile properties.

• Korean Journal of Materials Research
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• v.11 no.11
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• pp.915-922
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• 2001

Effects of thermal exposure and rejuvenation treatment on the microstructural evolution and the stress-rupture properties of IN738LC have been investigated. The role of precipitates on the stress-rupture properties has been analyzed through microstructural observations. Thermal exposure at $982^{\circ}C$ for 1000 hours gave rise to precipitation of $\sigma$ phase and coarsening of r'. The microstructural degradation with thermal exposure at $982^{\circ}C$ deteriorated stress rupture properties of the alloy. All the existing phases except MC carbides have completely dissolved into the matrix with homogenization treatment at $1200^{\circ}C$ for 2 hours. Microstructure and stress-rupture properties of the thermal exposed specimens have been successfully rejuvenated by the proposed treatment.

• Korean Journal of Materials Research
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• v.11 no.10
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• pp.833-840
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• 2001

Effects of thermal exposure and rejuvenation treatment on the microstructural evolution and the stress-rupture properties of IN738LC have been investigated. The role of precipitates on the stress- rupture properties has been analyzed through microstructural observations. Thermal exposure at $982^{\circ}C$ for 1000 hours gave rise to precipitation of $\sigma$ phase and coarsening of ${\gamma}$'. The microstructural degradation with thermal exposure at $982^{\circ}C$ deteriorated stress rupture properties of the alloy. All the existing phases except MC carbides have completely dissolved into the matrix with homogenization treatment at $1200^{\circ}C$ for 2 hours. Microstructure and stress-rupture properties of the thermal exposed specimens have been successfully rejuvenated by the proposed treatment.

• Korean Journal of Materials Research
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• v.25 no.10
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• pp.571-576
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• 2015

The effects of coating thickness on the delamination and fracture behavior of thermal barrier coating (TBC) systems were investigated with cyclic flame thermal fatigue (FTF) and thermal shock (TS) tests. The top and bond coats of the TBCs were prepared by electron beam-physical vapor deposition and low pressure plasma spray methods, respectively, with a thickness ratio of 2:1 in the top and bond coats. The thicknesses of the top coat were 200 and $500{\mu}m$, and those of the bond coat were 100 and $250{\mu}m$. FTF tests were performed until 1140 cycles at a surface temperature of $1100^{\circ}C$ for a dwell time of 5 min. TS tests were also done until more than 50 % delamination or 1140 cycles with a dwell time of 60 min. After the FTF for 1140 cycles, the interface microstructures of each TBC exhibited a sound condition without cracking or delamination. In the TS, the TBCs of 200 and $500{\mu}m$ were fully delaminated (> 50 %) within 171 and 440 cycles, respectively. These results enabled us to control the thickness of TBC systems and to propose an efficient coating in protecting the substrate in cyclic thermal exposure environments.

• Journal of Korea Foundry Society
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• v.37 no.5
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• pp.139-147
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• 2017

Microstructural evolution of cast Hastelloy X during thermal exposure has been investigated. OM, SEM, and TEM microscopy were carried out on the as-cast, the standard heat treated, and the thermally exposed conditions. Tensile tests were also conducted to understand the effect of microstructural evolution on the degradation of tensile properties. Coarse $M_6C$ and fine $M_{23}C_6$ carbides were found in as-cast Hastelloy X with fine carbides on sub-boundary. Some of $M_{23}C_6$ carbide dissolved into the matrix during solution heat treatment and dislocation network formed at the interface between the carbide and the matrix due to the misfit strain. There was no significant microstructural difference between the exposed specimens at $400^{\circ}C$ and the solution heat treated specimen. A large amount of $M_{23}C_6$ carbides precipitated along and near grain boundaries and sub-boundaries after exposure at $650^{\circ}C$. Exposure at $870^{\circ}C$ of the alloy caused precipitation of $M_6C$ and ${\mu}$. The strength increased and the elongation decreased by thermal exposure at $650^{\circ}C$ and $870^{\circ}C$ because carbides interfere with the movement of the dislocation. It was found that the precipitation of carbide gave significant effects on the tensile properties of Hastelloy X.

• Journal of the Korean Institute of Electrical and Electronic Material Engineers
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• v.17 no.1
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• pp.65-69
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• 2004

We synthesized photoalignment material of high thermal resistance with hydroxyl aromatic polyimide, and studied the liquid crystal (LC) aligning capabilities on the photopolymer layers. Also, electro-optical (EO) performances for the twisted-nematic (TN)-liquid crystal display (LCD) photoaligned with linearly polarized UV exposure were investigated. A good LC alignment with UV exposure on the photopolymer surface can be obtained. However, the low pretilt angles were obtained below 1$^{\circ}$. The Voltage-transmittance (V-T) curve without backflow bounce in the photoaligned TN cell with UV exposure was observed. The response time of photoaligned TN cell was measured about 24 ms. Finally, The photoaligned TN cell has few hysteresis, and shows the residual DC voltage that is less.

• Proceedings of the Korean Institute of Electrical and Electronic Material Engineers Conference
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• 2004.05a
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• pp.210-213
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• 2004

We synthesized photoalignment material of high thermal resistance with hydroxyl aromatic polyimide, and studied the liquid crystal (LC) aligning capabilities on the photopolymer layers. Also, electro-optical (EO) performances for the twisted-nematic (TN)-liquid crystal display (LCD) photoaligned with linearly polarized UV exposure were investigated. A good LC alignment with UV exposure on the photopolymer surface can be obtained. However, the low pretilt angles were obtained below $1^{\circ}$. The Voltage-transmittance (V-T) curve without backflow bounce in the photoaligned TN cell with UV exposure was observed. The response time of photoaligned TN cell was measured about 24 ms. Finally, The photoaligned TN cell has few hysteresis, and shows the residual DC voltage that is less.

• Proceedings of the Korean Society For Composite Materials Conference
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• 2004.10a
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• pp.39-42
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• 2004

Carbon nanofiber/Cu composite powder has been fabricated by electroless plating process. Microstructural evolution of the composite powder after heat treatment under vacuum, hydrogen and air environment was investigated. A dispersed carbon nanofiber coated by copper was produced at the as-plated condition. Carbon nanofiber is coated uniformly and densely with the plate shaped copper particles. The copper plates on the carbon nanofiber aggregate during the thermal exposure at elevated temperature in vacuum and hydrogen in order to reduce surface energy. The thermal exposure of the composite powder in air at $400^{\circ}C$ for 3 hours leads to the spherodization of the composite powder owing to oxidation of copper.

• Journal of Welding and Joining
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• v.17 no.4
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• pp.46-52
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• 1999

Some of the pressurized reactor pressure vessels used in many chemical plants are made of low alloy carbon steel plates internally clad with an austenitic stainless steel for improved anti-corrosion properties. In this study, metallurgic structure of the weld interface of A 387 Grade12Class1 low alloy carbon steel claded with A182-F321 austenitic stainless steel after thermal exposure simulation heat treatment was investigated to display a characteristic behavior of dissimilar metal weld interface with thermal exposure during service at high temperature and pressure. EPMA, STEM, vickers-hardness test were performed and the results were correlated with the microstructure. To estimate the depth of the carburized/decarburized bands quantitatively, a model for carbon diffusion was proposed. The validity of the proposed theoretical relationships was confirmed by the directly measured data from the welded parts failed during service.

• Proceedings of the Korean Institute of Electrical and Electronic Material Engineers Conference
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• 2003.11a
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• pp.502-505
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• 2003

We synthesized high thermal photoalignment material with hydroxyl aromatic polyimide, and studied the liquid crystal (LC) aligning capabilities on the photopolymer layers. Also, electro-optical (EO) performances for the twisted-nematic (TN)-liquid crystal display (LCD) photoaligned with linearly polarized UV exposure were investigated. A good LC alignment with UV exposure on the photopolymer surface can be obtained. However, the low pretilt angles were obtained below $1^{\circ}$. The Voltage-transmittance (V-T) curve without backflow bounce in the photoaligned TN cell with UV exposure was observed. The response time of photoaligned TN cell was measured about 24 ms.