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Toyama, Takeshi*; Tanno, Takashi; Yano, Yasuhide; Inoue, Koji*; Nagai, Yasuyoshi*; Otsuka, Satoshi; Miyazawa, Takeshi; Mitsuhara, Masatoshi*; Nakashima, Hideharu*; Onuma, Masato*; et al.
Journal of Nuclear Materials, 599, p.155252_1 - 155252_14, 2024/10
Times Cited Count：0 Percentile：0.00(Materials Science, Multidisciplinary)We investigated the stability of oxide nano particles in oxide dispersion-strengthened (ODS) steel, which is a promising candidate material for next-generation reactors, under neutron irradiation at high temperature to high doses. MA957, a 14Cr-ODS steel, was irradiated with Joyo in Japan Atomic Energy Agency under irradiation conditions of 130 dpa at 502
C, 154 dpa at 589C, and 158 dpa at 709C. Three-dimensional atom probe (3D-AP) and transmission electron microscope (TEM) observation were performed to characterize the oxide particles in the ODS steels. A high number density of Y-Ti-O particle was observed in the unirradiated and irradiated samples. Almost no change in the morphology of the oxide particles, i.e. average diameter, number density, and chemical composition, has been observed in the samples irradiated to 130 dpa at 502C and to 154 dpa at 589C. A slight decrease in number density was observed in the sample irradiated to 158 dpa at 709CC. The hardness of any of the irradiated samples was almost unchanged from that of the unirradiated sample. It was revealed that the oxide particles existed stable, and the strength of the material was sufficiently maintained even after being neutron irradiated to high dose of 
160 dpa at high temperature up to 700C. A part of this study includes the results of MEXT Innovative Nuclear Research and Development Program Grant Number JPMXD0219214482.
Miyazawa, Takeshi; Tanno, Takashi; Imagawa, Yuya; Hashidate, Ryuta; Yano, Yasuhide; Kaito, Takeji; Otsuka, Satoshi; Mitsuhara, Masatoshi*; Toyama, Takeshi*; Onuma, Masato*; et al.
Journal of Nuclear Materials, 593, p.155008_1 - 155008_16, 2024/05
Times Cited Count：1 Percentile：57.00(Materials Science, Multidisciplinary)Otsuka, Satoshi; Shizukawa, Yuta; Tanno, Takashi; Imagawa, Yuya; Hashidate, Ryuta; Yano, Yasuhide; Onizawa, Takashi; Kaito, Takeji; Onuma, Masato*; Mitsuhara, Masatoshi*; et al.
Journal of Nuclear Science and Technology, 60(3), p.288 - 298, 2023/03
Times Cited Count：4 Percentile：53.26(Nuclear Science & Technology)JAEA has been developing 9Cr-oxide dispersion strengthened (ODS) tempered martensitic steel(TMS) as a candidate material for the fuel cladding tubes of sodium-cooled fast reactors(SFRs). The reliable prediction of in-reactor creep-rupture strength is critical for implementing the 9Cr-ODS TMS cladding tube in the SFR. This study investigated the quantitative correlation between the creep properties of 9Cr-ODS TMS at 700 C and the dispersions of nanosized oxides by analyzing the creep data and the material's nanostructure. The possibility of deriving a formula for estimating the in-reactor creep properties of 9Cr-ODS TMSs based on an analysis of the nanostructure of neutron-irradiated 9Cr-ODS TMSs was also discussed. The creep properties of 9Cr-ODS TMS at 700 C closely correlated with the dispersion of nanosized oxide particles. The correlation between creep-rupture lives and nanosized oxide particle dispersion was determined using existing creep models. The elucidation of correlation between the stress exponent of secondary creep rate and the nanostructure is essential to enhance future modeling reliability and formulation.
Mitsuhara, Masatoshi*; Kurino, Koichi*; Yano, Yasuhide; Otsuka, Satoshi; Toyama, Takeshi*; Onuma, Masato*; Nakashima, Hideharu*
Tetsu To Hagane, 109(3), p.189 - 200, 2023/03
Times Cited Count：1 Percentile：12.80(Metallurgy & Metallurgical Engineering)Oxide Dispersion Strengthened (ODS) ferritic steel, a candidate material for fast reactor fuel cladding, has low thermal expansion, good thermal conductivity, and excellent resistance to irradiation damage and high temperature strength. The origin of the excellent high-temperature strength lies in the dispersion of fine oxides. In this study, creep tests at 700 or 750C, which are close to the operating temperatures of fast reactors, and high-temperature tensile tests at 900 to 1350 C, which simulate accident conditions, were conducted on 9Cr ODS ferritic steels, M11 and MP23, and 12Cr ODS ferritic steel, F14, to confirm the growth behavior of oxides. In the M11 and F14 creep test samples, there was little oxide growth or decrease in number density from the initial state, indicating that dispersion strengthening by oxides was effective during deformation. After creep deformation of F14, the development of dislocation substructures such as dislocation walls and subgrain boundaries was hardly observed, and mobile dislocations were homogeneously distributed in the grains. The dislocation density increased with increasing stress during the creep test. In the high-temperature ring tensile tests of MP23 and F14, the strength of both steels decreased at higher temperatures. In MP23, elongation decreased with increasing test temperature from 900 to 1100 C, but increased at 1200 C, decreased drastically at 1250 C, and increased again at 1300 C. In F14, elongation decreased with increasing temperature. It was inferred that the formation of the 
-ferrite phase was responsible for this complex change in mechanical properties of MP23 from 1200 to 1300 C.
Tokunaga, Tomonori*; Watanabe, Hideo*; Yoshida, Naoaki*; Nagasaka, Takuya*; Kasada, Ryuta*; Lee, Y.-J.*; Kimura, Akihiko*; Tokitani, Masayuki*; Mitsuhara, Masatoshi*; Hinoki, Tatsuya*; et al.
Journal of Nuclear Materials, 442(1-3), p.S287 - S291, 2013/11
Times Cited Count：16 Percentile：71.45(Materials Science, Multidisciplinary)Mitsuhara, Masatoshi*; Harada, Erika*; Yamasaki, Shigeto*; Ikeda, Kenichi*; Hata, Satoshi*; Nakashima, Hideharu*; Otsuka, Satoshi; Kaito, Takeji
Kashika Joho Gakkai-Shi, 31(122), p.98 - 103, 2011/07
no abstracts in English
Mitsuhara, Masatoshi*; Ikeda, Kenichi*; Hata, Satoshi*; Nakashima, Hideharu*; Wakai, Takashi
Kyushu Daigaku Cho Koatsu Denkenshitsu Kenkyu Hokoku, 32, 2 Pages, 2008/08
no abstracts in English
Mitsuhara, Masatoshi*; Ikeda, Kenichi*; Hata, Satoshi*; Nakashima, Hideharu*; Wakai, Takashi
Materia, 47(6), P. 301, 2008/06
no abstracts in English
Mitsuhara, Masatoshi*; Ikeda, Kenichi*; Hata, Satoshi*; Nakashima, Hideharu*; Wakai, Takashi
Materia, 46(12), P. 800, 2007/12
High Cr ferritic steels bearing V have high creep strength due to the precipitation hardening. In order to enhance the efficiency of the precipitation hardening, the elemental components, shape and distribution of the precipitates should be comprehended. In this study, we investigated that of the precipitates in the 10Cr ferritic steel bearing V using the STEM-EDS analysis and the three-dimensional electron tomography (3D-ET) observation. The two types of precipitates on the lath boundaries could be observed, which were lump-shaped precipitates and film-shaped precipitates. They had the different elemental component. From the 3D-ET observation, it revealed that the film-shaped precipitates covered the lath boundary widely. Therefore, the film-shaped precipitates are expected to be effective for the stabilization of martensitic microstructure during the creep deformation.
Mitsuhara, Masatoshi*; Yoshida, Yu*; Ikeda, Kenichi*; Nakashima, Hideharu*; Wakai, Takashi
Kyushu Daigaku Cho Koatsu Denkenshitsu Kenkyu Hokoku, 31, p.78 - 79, 2007/00
The effects of V and Nb on creep strength of high Cr ferritic steel were investigated based on the observation results using STEM. As a result, it is clarified that Nb contributes to the improvement of creep strength by encumber the movement of the lath boundary by fine dispersed precipitation as MX particles and V contributes as film precipitation on the lath boundary.
Yoshida, Yu*; Mitsuhara, Masatoshi*; Ikeda, Kenichi*; Nakashima, Hideharu*; Wakai, Takashi
Tainetsu Kinzoku Zairyo Dai-123-Iinkai Kenkyu Hokoku, 47(10), p.109 - 113, 2006/07
This paper studies the influence of V and Nb on the creep strength and behavior of high Cr ferritic steels. A series of creep tests of 11 steels having various V and Nb contents is conducted. The test results are analyzed by omega-method and the microstructures of the damaged specimens are examined by EBSP. As a result, it is shown that omega-method gives good prediction of creep lives of the steels and that the influence of V and Nb on the omega-parameters is significant. In addition, it is clarified that the martensitic microstructure of the steels depends upon Nb content.
Nakashima, Hideharu*; Ikeda, Kenichi*; Yoshida, Fuyuki*; Otsuka, Satoshi; Ukai, Shigeharu
JNC TY9400 2004-019, 56 Pages, 2004/08
JNC-TY9400-2004-019.pdf:5.22MB
The grian boundary structure of Oxide Dispersion Strengthened (ODS) steel was analysed. Based on the analysis results, the grain boundary control procedure for creep strength improvement was discussed.
Yoshida, Fuyuki*; Nakashima, Hideharu*
PNC TJ9606 97-001, 43 Pages, 1997/03
None
Yoshida, Fuyuki*; ; ; Nakashima, Hideharu*;
PNC TJ9606 95-002, 62 Pages, 1995/04
In order to clarify the origin of strength anisotropy in ODS ferritic steel with Y
O
particles at high temperatures, a threshold stress measured by stress abruptly loading test (SAL test) were compared with the Orowan and the void-hardening stresses calculated from dispersion parameters and subjected to the high-temperature compression test at temperature from 600 to 700 C and at strain rates from 1
10
to 110
s
, and the effect of grain-boundary sliding on the high-temperature strength was clarified. The results are summarized as follows. (1) The Orowan stress and the void-hardening stress calculated from dispersion parameters approximately agreed with the threshold stress obtained by SAL test. In addition, as the time under load elapsed, the transition of the threshold stress obtained by SAL test was observed, it is concluded that the originating mechanism of the threshold stress in ODS ferritic steel is the Srolovitz's one (2) The grain-boundary sliding was clearly observed for grain boundaries inclined by 45 to the stress axis. The flow stress was changed significantly by aspect ratio of grain size. Therefore, it is concluded that the strength anisotropy in ODS ferritic steel induces the grain-boundary sliding behaviour.
Nakashima, Hideharu*; ; ; Yoshida, Fuyuki*;
PNC TJ9606 94-001, 59 Pages, 1994/03
In order to clalify the origin of strength anisotropy in ODS ferritic steel with YO particles at high temperatures, the threshold stress was measured by the stress abruptly loading test at 650 C and the effect of grain-boundary sliding on the high-temperature strength was clarified. The results are summalized as follows. (1)The threshold stress obtained by the stress abruptly loading test decreased from the Orowan stress to the void-hardening stress as the time under load elapsed. This behavior is well explained by the attractive interaction between dislocations and particles. (2)The grain-boundary sliding was clearly observed for grain boundaries in-clined by 45 to the stress axis. The threshold stress for the high-temperature deformation accompanying grain-boundary sliding agreed with the threshold stress obtained by the stress abruptly loading test. Therefore, it is considered that grain interior deformation induces the grain-boudary sliding of the ODS ferritic steel at high temperatures.
Nakashima, Hideharu*;
PNC TJ9606 93-001, 36 Pages, 1993/03
It has been examined about mechanisms of strength anisotropy and ductility dip of ODS ferritic steel induded with YO particles at high temperatures. The results are summarized as follows. (1)Above 650C, 0.2% proof stress measured by tensile test and axial cleep rupture strength agree with the threshold stress for dislocation motion evaluated by the theory of the dispersion strengthening. But the bi-axial creep rupture stress is smaller than the threshold stress. It is presumed that grain boundaries in transverse section of cladding tube slide by the hoop stress below the threshold stress. It is considered that the strength anisotropy comes from an elongated grains formed by fabrication. (2)The ductility decreases in two different temperature ranges which are around 600C and 1100C respectively. The ductility-minimum temperature in the lower temperature range is independent of the strain rate in a high strain rate range, whereas that temperature is dependent on the strain rate in a low strain rate range. The reason why the strain rate dependence of the ductility-minimum temperature is different between high and low strin rate ranges has not been clarified. But it is suggested that the dip in the lower temperature range should be related to the grain boundary sliding and the grain-boundary precipitation. In the higher ductility-dip temperature range, this alloy has an 
-
duplex structure. Therefore, it is considered that the ductility dip arises from the concentration of the deformation in phase.
Mitsuhara, Masatoshi*; Ikeda, Kenichi*; Hata, Satoshi*; Nakashima, Hideharu*; Wakai, Takashi
no journal, ,
no abstracts in English
Mitsuhara, Masatoshi*; Ikeda, Kenichi*; Hata, Satoshi*; Nakashima, Hideharu*; Wakai, Takashi
no journal, ,
no abstracts in English
Mitsuhara, Masatoshi*; Yoshida, Yu*; Ikeda, Kenichi*; Nakashima, Hideharu*; Wakai, Takashi
no journal, ,
The effect of V and Nb on creep strengthening was studied in 10% Cr steels. In a steel added 0.06% V where no particles were observed in the lath, creep strength was slightly increased, indicating that it was not strengthened by particle hardening effect in the lath. Lump-shaped precipitates and film-shaped precipitates including Cr and V, which were observed on lath boundaries in the steel added V, are expected to inhibit the lath boundaries from migrating. This inhibition is the mechanism of the improvement in the steel added V. In the case of steels added Nb individually, creep strength was found to be increased by dispersion hardening due to fine precipitates in the lath. These results revealed that V and Nb have each role to improve creep strength. In steels added both V and Nb, the effect on creep strength was expressed by the sum of the effects due to each element, which were the retardation of the lath boundary migrations and the pinning of the dislocations in the lath.
Yoshida, Yu*; Mitsuhara, Masatoshi*; Ikeda, Kenichi*; Nakashima, Hideharu*; Wakai, Takashi
no journal, ,
no abstracts in English
