• Proceedings of the Korean Society for Technology of Plasticity Conference
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• 2004.05a
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• pp.186-189
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• 2004

Although magnesium has high potential for structural material due to the lightweight and high specific strength, the structural application has been limited by the low ductility at room temperature. The reason of the poor ductility is few activated slip systems of magnesium (HCP structure) during deformation. As temperature increases, however, additional non-basal slip systems are incorporated to exhibit higher ductility comparable to aluminum. In the present study, a series of tensile tests of Mg-Al alloy has been carried out to study deformation behavior with temperature variation. Analysis of load relaxation test results based on internal variable approach gave information about relationship between the micromechanical character and corresponding deformation behavior of magnesium. Especially, the material parameter, p representing dislocation permeability through barriers was altered from 0.1 to 0.15 as the non-basal slip systems were activated at high temperature.

• Proceedings of the Korean Society For Composite Materials Conference
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• 2004.10a
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• pp.23-26
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• 2004

Ag/Ni multilayers with different bilayer thickness between 3 and 100 nm produced by DC magnetron puttering have been studied by cross-sectional TEM and nanoindentation. The micrograph shows perfect layered structure with sharp interfaces between Ag and Ni layers. Absolute hardness is calculated as a reference value to compare hardness of specimens regardless of indent depth. A hardness enhancement of nearly $100\%$ over the rule-of-mixtures values, calculated from the measured hardness of single Ag and Ni thin films, is observed. The hardness increases with decreasing bilayer thickness until 8nm. This enhancement shows a good agreement with Hall-Petch relation using grain size (one half of the bilayer thickness) confined within a layer. The deformation behavior can be explained by dislocation pile-up in smaller grains.

• Proceedings of the KSME Conference
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• 2003.04a
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• pp.637-642
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• 2003

The Modified 9Cr-1Mo steel identified as T91, P91 and F91 in the ASME specification has been widely used for the construction of modern power plants. The available data on the influence of process parameters during manufacturing and fabrication on its properties are not sufficient. In this study, the influence of various thermal cycles on the hardness and the creep rupture strength was analyzed in the base metal and the weldments made in tube and pipe of a Mod.9Cr-1Mo steel. The low hardness, 155Hv, showed low creep rupture strength below the allowable stresses of T91 base metal in the ASME specification. This low value was attributed to the fully recovered dislocation structure and the weakening of precipitation hardening associated with the abnormal thermal cycles.

• Proceedings of the Optical Society of Korea Conference
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• 2007.07a
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• pp.237-238
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• 2007

A new strain analysis model, so called the stress matched model, in an epitaxial multilayer system is proposed. The model makes it possible to know the strain, the stress, the elastic strain energy in each epitaxial layer. Analytical formulas of strain parameters in each epitaxial layer are derived under assumptions that the substrate thickness is finite and the in-plane lattice constant is the same for all epitaxial layers for dislocation free growth. As an example, the model is applied to a 405nm InGaN/InGaN multiple quantum well laser diode. Analysis result shows that AlxGa1-xN layer with Al mole fraction of 0.06 and the thickness of 6${\mu}m$ is one of good templates for a laser. In fact, this layer structure coincides with experimentally optimized one.

• Proceedings of the Korean Society of Marine Engineers Conference
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• 2005.06a
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• pp.331-338
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• 2005

The grain structure, dislocation density and second phase particles in various regions including the stir zone(SZ), thermo-mechanically affected zone(TMAZ), and heat affected zone(HAZ) of a friction stir weld 6.35mm thick aluminum 7075-T651 alloy were investigated and compared with the base metal. The microstruectures of nugget zone were compared according to tool rotation speeds and tool transition speeds. The hardness profiles of nugget zone were increased, while decreasing rotation speed and increasing welding speed. The optimal microstructure was gained at the low rotation speed 800rpm and th high welding speed 124mm/min. The nugget microstructures of fracture surface, transgranular dimple and quasicleavage type were showed different fracture type with the HAZ, shear fracture type.

• Journal of the Korean Vacuum Society
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• v.3 no.3
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• pp.337-340
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• 1994

기판의 크기에 따른 Strained Layer에서의 응력 풀림 현상을 연구하기 위하여 마이크론 이하에 서 50 마이크론의 크기를 가지는 메사 구조를 제작하였다. 제작된 메사 구조위에 성장시킨 GeSi층은 마 이크로 Raman, XPS, I-V와 C-V측정에 의하여 응력의 풀림현상을 관찰하였다. 이 실험을 통하여 메사 구조 위에 성장시킨 GexSi1-x 층에서의 응력풀림이 편평한 기판위에 성장시킨 경우에 비하여 원활하였 다. 이것은 메사 구조의 가장자리에 Strain Dislocation 이 편중되는 것으로 이해될수 있다.

• Proceedings of the Korean Vacuum Society Conference
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• 2011.08a
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• pp.132-132
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• 2011

The sheet resistance (Rs) of undoped GaN films on AlN/c-plane sapphire substrate was investigated in which the AlN films were grown by R. F. magetron sputtering method. The Rs was strongly dependent on the AlN layer thickness and semi-insulating behavior was observed. To clarify the effect of crystalline property on Rs, the crystal structure of the GaN films has been studied using x-ray scattering and transmission electron microscopy. A compressive strain was introduced by the presence of AlN nucleation layer (NL) and was gradually relaxed as increasing AlN NL thickness. This relaxation produced more threading dislocations (TD) of edge-type. Moreover, the surface morphology of the GaN film was changed at thicker AlN layer condition, which was originated by the crossover from planar to island grains of AlN. Thus, rough surface might produce more dislocations. The edge and mixed dislocations propagating from the interface between the GaN film and the AlN buffer layer affected the electric resistance of GaN film.

• Corrosion Science and Technology
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• v.15 no.5
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• pp.237-244
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• 2016

2205 duplex stainless steels have been used for the construction of the marine environment, because of their excellent corrosion resistance and high strength. However, the resistance to hydrogen embrittlement (HE) may be less than that of 316L austenitic stainless steel. The reason why 316L stainless steels have better resistance to HE is associated with crystal structure (FCC, face centered cubic) and the higher stacking faults energy than 2205 duplex stainless steels. Furthermore 2205 stainless steels with or without tungsten were also examined in terms of HE. 2205 stainless steels containing tungsten is less resistible to HE. It is because dislocation tangle was formed in 2205 duplex stainless steels. Slow strain-rate tensile test (SSRT) was conducted to measure the resistance to HE under the cathodic applied potential. Hydrogen embrittlement index (HEI) was used to evaluate HE resistance through the quantitative calculation.

• Transactions of Materials Processing
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• v.12 no.4
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• pp.284-289
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• 2003

The effects of nitrogen on the deformation behavior of duplex stainless steel have been studied. The variation of strength was correlated with the characteristic microstructures pertaining to nitrogen. Analysis based on Hall-fetch relation confirmed that nitrogen enhances phase-boundary strengthening effect. The evolution of dislocation structure, slip traces and misorientation distribution during deformation were also characterized to elucidate the effect of nitrogen on inelastic deformation mechanism. It has been verified in this study that the higher nitrogen content provides a dual-phase microstructure with smaller strength difference between austenite and ferrite resulting into the earlier transfer of inelastic deformation from austenite to ferrite.

• Proceedings of the Korean Powder Metallurgy Institute Conference
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• 2002.04b
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• pp.11-11
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• 2002

Plastic deformation was observed by TEM around the intragranular SiC particles in the $Al_2O_3$ matrix for $Al_2O_3/SiC$ nanocomposite system. The dislocations are generated at selected planes and there is a tendency for the dislocations to form a subgrain boundary structure with low-angel grain boundaries and networks. In this study, dislocation generated in the $Al_2O_3$ matrix during cooling down from sintering temperatures by the highly localized thermal stresses within and/or around SiC particles caused from the thermal expansion mismatch between $Al_2O_3$ matrix and SiC particle was observed. In monolithic $Al_2O_3$ and $Al_2O_3/SiC$ microcomposite system. These phenomena is closely related to the plastic relaxation of the elastic stress and strain energy associated with both thermal misfitting inclusions and creep behaviors. The plastic relaxation behavior was explained by combination of yield stress and internal stress.