Chang-Sup Oh

Korea Institute of Science and Technology Information (KISTI), Daiden, Daejeon, South Korea

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Publications (10)5.74 Total impact

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    Chang-Sup Oh, Chang-Suk Han
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    ABSTRACT: The mechanical properties and microstructures of aluminum-matrix composites fabricated by the dispersion of fine alumina particles less than in size into 6061 aluminum alloys are investigated in this study. In the as-quenched state, the yield stress of the composite is 40~85 MPa higher than that of the 6061 alloy. This difference is attributed to the high density of dislocations within the matrix introduced due to the difference in the thermal expansion coefficients between the matrix and the reinforcement. The difference in the yield stress between the composite and the 6061 alloy decreases with the aging time and the age-hardening curves of both materials show a similar trend. At room temperature, the strain-hardening rate of the composite is higher than that of the 6061 alloy, most likely because the distribution of reinforcements enhances the dislocation density during deformation. Both the yield stress and the strain-hardening rate of the T6-treated composite decrease as the testing temperature increases, and the rate of decrease is faster in the composite than in the 6061 alloy. Under creep conditions, the stress exponents of the T6-treated composite vary from 8.3 at 473 K to 4.8 at 623 K. These exponents are larger than those of the 6061 matrix alloy.
    Korean Journal of Materials Research 01/2013; 23(3).
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    Chang-Sup Oh, Chang-Suk Han
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    ABSTRACT: The B2-ordered NiAl has attracted much attention as one of the candidates as a next generation high temperature material, because it has a high melting temperature, a low specific gravity and an excellent high temperature oxidation resistance. However, the application of NiAl to structural materials needs the improvement of its brittleness at room temperature. The study was carried out on the relation between several properties of NiAl and some variation of Ni content within NiAl phase, which means deviations from the stoichiometric composition. The main results were as follows; (i) Good ductility was obtained at the testing temperature more than 1073 K irrespective of Ni content. (ii) Increasing Ni content offered preferable tensile properties. (iii) Every NiAl with varying Ni contents showed the superior oxidation resistance.
    Journal of the Korean Society for Heat Treatment. 01/2013; 26(1).
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    Chang-Sup Oh, Chang-Suk Han
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    ABSTRACT: The purpose of this study is to prepare a high strength fiberglass reinforced metal. Aluminum covering was carried out over carbon materials such as carbon fiber in order to increase their wettability to molten metals such as aluminum. A sputtering apparatus with a cylindrical target was fabricated to carry out the covering. Sputtering was caused by glow discharge between the target and the two anode plates attached to its top and bottom. As the substrate for preliminary test, a thin carbon wire was used instead of carbon fiber, and the wire was placed at the central axis of the target. Aluminium coating was formed on the whole surface of the substrate. The formation rate and structure of coating were varied by controlling the electrical potential of substrate. When the substrate was electrically isolated, coating with columnar structure was formed with a formation rate of . In case of grounded substrate, coating with amorphous structure was formed with a formation rate of .
    Journal of the Korean Society for Heat Treatment. 01/2012; 25(1).
  • Chang-Sup Oh, Chang-Suk Han
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    ABSTRACT: When alloys are vacuum-deposited on cooled substrates, super-rapidly cooled alloy films in the unequilibrium state can be obtained. As an application of this method, Ag-Cu, Ag-Ni and Ag-C alloys were successfully produced, and their mechanical properties with tempering temperature were investigated. The following results were obtained : (1) In case of Ag-Cu alloys, the solid solution was hardened by tempering at . The hardening is considered to occur when the solid solution begins to decompose into and phases. The Knoop hardness number of a 40 at.%Ag-Cu alloy film deposited on a cooled glass substrate was 390 . The as-deposited films were generally very hard but fractured under stresses below their elastic limits. (2) In case of Ag-Ni and Ag-C alloys, after the tempering of 4 at.%Ni-Ag alloy at and of 1 and 2 at.%C-Ag alloys at , they were hardened by the precipitation of fine nickel and carbon particles. The linear relationship between proof stress vs. for bulk silver polycrystals can be applied to vacuum-deposited films up to about 0.1 grain diameter, but the proof stress of ultra-fine grained silver with grain diameters of less than 0.1 was smaller than the value expected from the Petch's relation.
    Journal of the Korean Society for Heat Treatment. 01/2011; 24(5).
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    ABSTRACT: A transmission electron microscopy investigation on the phase decomposition of B2-ordered (Ni,Co)Al supersaturated with Ni and Co has revealed the precipitation of (Ni,Co)2Al which has not been expected from the reported equilibrium phase diagram. The (Ni,Co)2Al phase has a hexagonal structure and takes a rodlike shape with the long axis of the rod parallel to the 〈111〉 directions of the B2 matrix. By aging at temperatures below 873 K, a long period superlattice structure appears in the hexagonal (Ni,Co)2Al phase. The orientation relationship between the (Ni,Co)2Al precipitates and the B2-(Ni,Co)Al matrix is found to be (0001)p//(111)B2 and [$$ \bar 1 $$2$$ \bar 1 $$0]p//[$$ \bar 1 $$10]B2, where the suffix p and B2 denote the (Ni,Co)2Al precipitate and the B2-(Ni,Co)Al matrix, respectively. (Ni,Co)Al hardens appreciably by the fine precipitation of the (Ni,Co)2Al phase.
    Metals and Materials International 01/2010; 16(2):175-183. · 1.43 Impact Factor
  • Journal of Chemical Engineering of Japan - J CHEM ENG JPN. 01/2008; 41(7):627-630.
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    ABSTRACT: We have developed a novel route to monolithic silica aerogels via ambient pressure drying by the acid–base sol–gel polymerization of methyltrimethoxysilane (MTMS) precursor. An extent of silica polymerization in the alcogels plays a crucial role in obtaining the monolithic aerogels which could be optimized by a proper control over the MeOH/MTMS molar ratio (S) during the sol–gel synthesis. The alcogel undergoes the distinct “spring-back effect” at the critical stage of the drying and thereby preserving the highly porous silica network without collapse. The process yields silica aerogels exhibiting very low bulk density and high specific surface area of 0.062 g/cm3 and 520 m2/g, respectively. The average pore diameter and the cumulative pore volume varied from 4.5 to 12.1 nm and 0.58 to 1.58 cc/g, respectively. In addition, the aerogels are superhydrophobic with contact angle as high as 152°. We anticipate that the new route of the monolithic silica aerogel production will greatly expand the commercial exploitation of these materials.
    Microporous and Mesoporous Materials. 03/2007;
  • Chang-Sup Oh, Chang-Suk Han
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    ABSTRACT: Transmission electron microscopy (TEM) examinations of Al3Ti with an L12-ordered structure have revealed the precipitation of D023-Al11Ti5 and L10-TiAl upon aging after quenching from higher temperatures. TEM observations revealed that fine uniform precipitation of Al2Ti occurs when the supersaturation is sufficiently high, and, a preferential precipitation at the antiphase boundaries can be observed in alloy with a low supersaturation. When L12-Al3Ti is supersaturated with DO22-Al3Ti, DO23-Al11Ti5 with a multidomain structure is formed during aging. On the other hand, plate-like L10-TiAl precipitates lie on the {001} planes of (Al,Ag)3Ti matrix in the short aging period and the habit plane changed from {001} to {hhl} after a long period aging or higher temperature aging and finally to {225} of the matrix lattice. The Ll2 phase field in the Al-Ti-Ag system is severely skewed with respect to the temperature axis and is restricted into a much smaller field at lower temperatures. Appreciable hardening and overage softening during aging can be explained in terms of microstructural variations.
    Metals and Materials International 18(3). · 1.43 Impact Factor
  • Chang-Sup Oh, Chang-Suk Han
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    ABSTRACT: Microstructural control to produce a multiphase structure and there by improve the high temperature strength as well as low temperature ductility of intermetallics has received much attention. A transmission electron microscopy investigation has been performed in the present work on the precipitation of supersaturated B2-ordered (Ni,Co)Al and α-Cr in B2-ordered β-NiAl with different stoichiometry. Precipitation behavior and hardening were investigated by measuring the hardness variation. The hardness of (Ni,Co)Al and β-NiAl increases appreciably by the fine precipitation of (Ni,Co)2Al and α-Cr, and overage softening occurs after prolonged aging. In the case of B2-ordered (Ni,Co)Al, the (Ni,Co)2Al phase has a hexagonal structure and takes a rod-like shape with the long axis of the rod parallel to the 〈111〉 directions of the B2 matrix. By aging at temperatures below 873 K, a long period superlattice structure appears in the hexagonal (Ni,Co)2Al phase. The orientation relationship between the (Ni,Co)2Al precipitates and the B2-(Ni,Co)Al matrix is found to be (0001)p//(111)B2 and [[`1]\bar 12[`1]\bar 10]p//[[`1]\bar 110]B2, where the suffixes p and B2 denote the (Ni,Co)2Al precipitate and the B2-(Ni,Co)Al matrix, respectively. (Ni,Co)Al hardens appreciably by fine precipitation of the (Ni,Co)2Al phase. On the other hand, in the case of B2-NiAl, perfect lattice coherency is retained at the interfaces between the α-Cr particles and the matrix during the initial stage of aging. After prolonged aging, a loss of coherency occurs by the attraction of matrix dislocations to the particle/matrix interface followed by climbing around the particles. Keywordsintermetallics–aging–mechanical properties–transmission electron microscopy (TEM)–fine precipitation
    Metals and Materials International 17(2):215-222. · 1.43 Impact Factor
  • Chang-Sup Oh, Chang-Suk Han
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    ABSTRACT: Fine dispersion of disordered phases is obtained in a Ni-Al-Co and Fe-Al-Co ternary system. A transmission electron microscopy investigation has been performed in the present work on the precipitation of supersaturated B2-ordered (Ni,Co)Al and α-Fe in B2-ordered FeAl(Co) with different stoichiometries. Precipitation behavior and hardening were investigated by measuring the hardness variation. The hardness of (Ni,Co)Al and B2-FeAl(Co) increased appreciably by the fine precipitation of (Ni,Co)2Al, α-Fe, and overage softening occurred after prolonged aging. In case of B2-ordered (Ni,Co)Al, the (Ni,Co)2Al phase had a hexagonal structure and took a rod-like shape with the long axis of the rod parallel to the 〈111〉 directions of the B2 matrix. By aging at temperatures below 873 K, a long period superlattice structure appeared in the hexagonal (Ni,Co)2Al phase. The orientation relationship between the (Ni,Co)2Al precipitates and the B2-(Ni,Co)Al matrix was (0001)p//(111)B2 and $[\bar 12\bar 10]_p //[\bar 110]_{B2}$ , where the suffix p and B2 denote the (Ni,Co)2Al precipitate and the B2-(Ni,Co)Al matrix, respectively. (Ni,Co)Al hardened appreciably by the fine precipitation of the (Ni,Co)2Al phase. On the other hand, in case of B2-FeAl(Co), the disordered α-Fe phase was present as a precipitate in a B2-FeAl(Co) matrix and had a cubic-cubic orientation with the matrix. At the early aging periods, prismatic dislocation loops formed in the B2-FeAl(Co) matrix. B2-FeAl(Co) matrix was typically hardened by the precipitation of α-Fe.
    Metals and Materials International 19(2). · 1.43 Impact Factor