Papers by Author: Akira Yamauchi

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Authors: Kazuya Kurokawa, Akira Yamauchi
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Authors: Hiroyuki Nakata, Akira Yamauchi, Shigeji Taniguchi, Ii Ryoung Sohn, Jin Won Choi, Kazuya Kurokawa
Abstract: Low carbon steels containing Si of 0.1 and 1.0 mass%, and 99.5 mass% pure Fe were oxidized in laboratory air and in a H2O-containing atmosphere at 1173 K. Acoustic emission technique was used to assess the temperature (TF) at which the first major scale failure takes place during cooling. TF of 1.0 %Si steel oxidized in the air was found to increase with an increase in the scale thickness and cooling rate, while TF of 0.1 %Si steel had almost no dependence on these parameters. Moreover, the values of TF of both the steels oxidized in the H2O-containing atmosphere are higher than those in the air. These differences are attributable to the cooling rate, scale structure, and eutectoid reaction. In general, higher cooling rate implies a higher strain rate and there may be a larger temperature gradient across the scale thickness, which additionally enhances the scale failure. The metallographic examinations revealed that eutectoid magnetite particles in the scales formed on 0.1 %Si steel coarsen as the cooling rate decreases and magnetite seam was formed at the bottom of the iron oxide layer. It is clear that the influence of magnetite precipitation increases as the cooling rate decreases and thus the stress in the scale increases.
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Authors: Toto Sudiro, Tomonori Sano, Akira Yamauchi, Shoji Kyo, Osamu Ishibashi, Masaharu Nakamori, Kazuya Kurokawa
Abstract: The objective of this study is to develop an excellent corrosion resistant alloy for high temperature coating applications. The Si-containing alloys consisting of CoNiCrAlY and CrSi2 alloys with varying Si and Ni content respectively were prepared by spark plasma sintering (SPS) technique. The corrosion behavior of these alloys was investigated in the gas phase of air-(Na2SO4+25.7mass%NaCl) at elevated temperatures of 923, 1073 and 1273K. The results showed that CoNiCrAlY alloy with 30mass% Si content and CrSi2 alloy with 10mass% Ni content were the most effective materials for application in the gas phase of air-(Na2SO4+25.7mass%NaCl) due to the formation of protective Al2O3/SiO2 and SiO2 scale, respectively. Therefore, it is realized that CoNiCrAlY-30mass% Si and CrSi2-10mass% Si coating are very effective for improving of high temperature corrosion resistance of STBA21 steel.
272
Authors: Akira Ibano, Kyosuke Yoshimi, Akira Yamauchi, Rong Tu, Kouichi Maruyama, Kazuya Kurokawa, Takashi Goto
Abstract: In this study, the high temperature oxidation behavior of polycrystalline MoSi2 in a low-pressure atmosphere was investigated. Polycrystalline MoSi2 was produced by the spark plasma sintering process. Oxidation tests were carried out at 1500°C at either 10Torr or 760Torr in an Ar-20%O2 atmosphere. For both conditions, the weight change peaked at the initial oxidation stage, and then their weights gradually increased with increasing oxidation time. The sample weight became heavier in the ambient pressure than in the low-pressure, but the evaporation oxidation was not significant in the low-pressure condition. After the low-pressure oxidation tests, the formation of Mo5Si3 in the MoSi2 substrate was identified. The oxidation resistance of MoSi2 at 1500°C is discussed based on the obtained results.
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Authors: Kazuya Kurokawa, Akira Yamauchi, Shinya Matsushita
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Authors: Kazuya Kurokawa, Daichi Goto, Jyunichi Kuchino, Akira Yamauchi, Tamaki Shibayama, Heishichiro Takahashi
Abstract: The microstructures of oxide scales formed on MoSi2 at medium-high temperatures in air were observed by TEM. Based on the observation, relationships between oxidation temperature and formation of MoO3 and crystallization of amorphous SiO2 scales were investigated. At 1273 K and 1373 K, the oxide scales had a structure consisting of amorphous SiO2 with small amounts of fine MoO3 particles. The oxide scales at 1573 K and 1773 K had a structure consisting of amorphous and crystalline SiO2. Growth rate of the oxide scale formed at 1773 K appreciably increased due to crystallization of amorphous SiO2. It was thought that the increase in the oxidation rate at 1773 K was caused by a change in the diffusion mechanism from O2 diffusion to lattice diffusion of O2- through SiO2. In addition, the diffusion coefficient of oxygen was estimated from the growth rate of SiO2 scale.
595
Authors: Akira Yamauchi, Kyosuke Yoshimi, Yoshihiro Murakami, Kazuya Kurokawa, Shuji Hanada
Abstract: Isothermal oxidation behavior of Al added Mo-Si-B in-situ composites was investigated under Ar-20%O2 and air atmosphere over the temperature range of 1073–1673 K. The Al added Mo-Si-B composites ((Mo-8.7mol%Si-17.4mol%B)-1mol%Al) were prepared by arc-melting, and homogenized at 2073 K for 24 h in an Ar-flow atmosphere. The ternary Mo-Si-B in-situ composite exhibited a rapid mass loss at the initial oxidation stage and then the passive oxidation after the substrates were sealed with borosilicate glass in the temperature range of 1173–1473 K, whereas it exhibited a rapid mass gain around 1073 K. On the other hand, the Al addition significantly improved the oxidation resistance of Mo-Si-B in-situ composites at temperatures from 1073–1573 K. These excellent oxidation resistances are considered to be due to the rapid formation of a continuous, dense scale of Al-Si-O complex oxides.
215
Authors: Akira Yamauchi, Xue Mei Yi, Tomohiro Akiyama, Kazuya Kurokawa
Abstract: The oxidation behavior of β-SiAlONs (Si6-zAlzOzN8-z, z = 1, 3, and 4) was investigated at temperature ranging from 1473 to 1673 K in a (N2-3%O2)-20%H2O atmosphere. Oxidation kinetics was followed on the basis of the mass gains, and the oxidized specimens were characterized by FE-SEM, XRD, and EPMA. The mass gain was found to clearly increase with temperature and the z value. For oxidation at 1473 and 1573 K, the gain in mass was observed to be small. During oxidation at 1573 K, mass loss occurred. This loss may be because of the formation of volatile SiO(OH)2. The changes in mass depended on the z value. On the other hand, the oxidation rates at 1673 K were found to be higher than those at and below 1573 K by more than one order of magnitude. The oxidation kinetics at 1673 K followed an almost linear rate law. The XRD and EPMA results showed that the oxide formed on β-SiAlON (z = 1) was composed of a mixture of amorphous aluminosilicate and mullite, whereas that on β-SiAlONs (z = 3 and 4) was composed only of mullite.
395
Authors: Kyosuke Yoshimi, Minseok Sung, Sadahiro Tsurekawa, Akira Yamauchi, Ryusuke Nakamura, Shuji Hanada, Koichi Kawahara, Tadao Watanabe
Abstract: Substructure development through aging and annealing treatments was studied for rapidly solidified TiCo ribbons using TEM. In as-spun ribbons, equiaxed grain structure was developed and its crystal structure was B2-ordered immediately after melt-spinning, while a small amount of fine precipitates existed as second phase. Some grains were dislocation-free but others contained a certain amount of curved or helical dislocations and loops. The dislocation density in the ribbons annealed at 700 °C for 24 h was obviously higher than those in the as-spun ribbons and the ribbons aged at 200 °C for 100 h. The increase of the dislocation density in the annealed ribbons would result from the absorption of excess vacancies. Therefore, the obtained results indicated that a large amount of supersaturated thermal vacancies were retained in TiCo as-spun ribbons by the rapid solidification.
849
Authors: Akira Yamauchi, Yuji Yamauchi, Yuko Hirohata, Tomoaki Hino, Kazuya Kurokawa
Abstract: Hydrogen dissolved in the Cr2O3 scale formed on the stainless steel in the H2O-containing atmospheres is observed by TDS (thermal desorption spectroscopy) measurements. The amount of dissolved hydrogen in the Cr2O3 scale reaches a maximum about 0.32 mol% when the H2O concentration in the gas reaches 20%. It was found from GDS (glow discharge spectroscopy) measurements that hydrogen may exist at the oxide scale / substrate interface or in Cr2O3 scale bounded that interface. However, results from the Vickers hardness and the observation of scale morphology by SEM (scanning electron microscopy), hydrogen dissolved in the Cr2O3 scale would have little effect on a decrease in the mechanical property of the Cr2O3 scale. Therefore, hydrogen dissolved in the Cr2O3 scale may not be main factor of the deterioration of the Cr2O3 scale.
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