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Ijiri, Yuta*; Ono, Naoko*; Ukai, Shigeharu*; Yu, H.*; Otsuka, Satoshi; Abe, Yosuke; Matsukawa, Yoshitaka*
Philosophical Magazine, 97(13), p.1047 - 1056, 2017/02
Times Cited Count：2 Percentile：11.4(Materials Science, Multidisciplinary)The interaction between oxide particles and dislocations in a 9Cr-ODS ferritic steel is investigated by both static and in-situ TEM observation under dynamic straining conditions and room temperature. The measured obstacle strength (
) of the oxide particles was no greater than 0.80 and the average was 0.63. The dislocation loops around some coarsened particles were also observed. The calculated obstacle strength by a stress formula of the Orowan interaction is nearly equaled to the average experimental value. Not only cross-slip system but also the Orowan interaction should be considered as the main interaction mechanism between oxide particles and dislocation in 9Cr-ODS ferritic steel.
Matsukawa, Yoshitaka*; Takeuchi, Tomoaki; Kakubo, Yuta*; Suzudo, Tomoaki; Watanabe, Hideo*; Abe, Hiroaki*; Toyama, Takeshi*; Nagai, Yasuyoshi*
Acta Materialia, 116, p.104 - 113, 2016/09
Times Cited Count：73 Percentile：95.65(Materials Science, Multidisciplinary)Atom probe tomography (APT) and TEM were combined for identifying the stage at which solute clusters transform into compounds crystallographically distinct from the matrix, in the precipitation of the G-phase (Ni
Si
Mn
) from ferrite solid solution subjected to isothermal annealing at 673 K. Based on a systematic analysis of solute clusters as a function of annealing time, the nucleation of the G-phase was found to occur via a two-step process. Moreover, the structural change was found to occur via another two-step process. There was a time lag between the end of cluster growth to become a critical size and the start of the structural change. During the incubation period solute enrichment occurred inside the clusters without further size growth, indicating that the nucleation of the G-phase occurs at the critical size with a critical composition. Judging from the results of APT, TEM and the simulation of electron diffraction patterns, the critical composition was estimated to be NiSi
(Fe,Cr)Mn.
Sato, Yuki*; Abe, Yosuke; Abe, Hiroaki*; Matsukawa, Yoshitaka*; Kano, Sho*; Onuki, Somei*; Hashimoto, Naoyuki*
Philosophical Magazine, 96(21), p.2219 - 2242, 2016/06
Times Cited Count：12 Percentile：51.1(Materials Science, Multidisciplinary)We performed in situ observation of one-dimensional (1D) migration of self-interstitial atom (SIA) clusters in iron under electron irradiation at 110-300 K using high-voltage electron microscopy. Most 1D migration was stepwise positional changes of SIA clusters at irregular time intervals at all temperatures. The frequency of 1D migration did not depend on the irradiation temperature. It was directly proportional to the damage rate, suggesting that 1D migration was induced by electron irradiation. In contrast, the 1D migration distance depended on the temperature: distribution of the distance ranged over 100 nm above 250 K, decreased steeply between 250 and 150 K and was less than 20 nm below 150 K. The distance was independent of the damage rate at all temperatures. Next, we examined fluctuation in the interaction energy between an SIA cluster and vacancies of random distribution at concentrations 
-
, using molecular statics simulations. The fluctuation was found to trap SIA clusters of 4 nm diameter at vacancy concentrations higher than 
. We proposed that 1D migration was interrupted by impurity atoms at temperatures higher than 250 K, and by vacancies accumulated at high concentration under electron irradiation at low temperatures where vacancies are not thermally mobile.
Takeuchi, Tomoaki; Kakubo, Yuta*; Matsukawa, Yoshitaka*; Nozawa, Yasuko*; Toyama, Takeshi*; Nagai, Yasuyoshi*; Nishiyama, Yutaka; Katsuyama, Jinya; Yamaguchi, Yoshihito; Onizawa, Kunio; et al.
Journal of Nuclear Materials, 452(1-3), p.235 - 240, 2014/09
Times Cited Count：40 Percentile：95.02(Materials Science, Multidisciplinary)Microstructures and hardness of stainless steel weld overlay cladding of reactor pressure vessels subjected to the thermal aging at 400 
C for 100-10,000 h were investigated using atom probe tomography and nanoindentation technique. The Cr concentration fluctuation in the 
-ferrite phase caused by spinodal decomposition rapidly progressed by the 100 h aging while NiSiMn clusters increased in number density at 2,000 h and coarsened at 10,000 h. The hardness of the -ferrite phase also rapidly increased at the short aging time. The Cr concentration fluctuation and the hardness were in good correlation with the degree of the Cr concentration fluctuation rather than the formation of the NiSiMn clusters. These results strongly suggested that the dominant factor of the hardening of the -ferrite phase by the thermal aging was Cr spinodal decomposition.
Takeuchi, Tomoaki; Kakubo, Yuta*; Matsukawa, Yoshitaka*; Nozawa, Yasuko*; Toyama, Takeshi*; Nagai, Yasuyoshi*; Nishiyama, Yutaka; Katsuyama, Jinya; Yamaguchi, Yoshihito; Onizawa, Kunio
Journal of Nuclear Materials, 449(1-3), p.273 - 276, 2014/06
Times Cited Count：20 Percentile：81.41(Materials Science, Multidisciplinary)Microstructural changes and hardness of stainless steel weld overlay cladding of reactor pressure vessels subjected to the neutron irradiation with a dose of 7.2 
10
n cm
(E 
1 MeV) and a flux of 1.1 10
n cm s
at 290C were investigated by atom probe tomography and with nanoindentation technique. In order to isolate the effect of the irradiation, we compared the results of the measurements of the irradiated sample with that of the aged one at 300C for the time equivalent to the irradiation. The Cr concentration fluctuation was enhanced in the -ferrite phase of the irradiated sample. In addition, the enhancement of the concentration fluctuation of Si, which was not observed in the aged sample, was observed. The hardening at the -ferrite phase was occurred by both the irradiation and the aging. However, the former was more than that expected from the Cr concentration fluctuation, which suggested that the Si concentration fluctuation and irradiation-induced defects were possible origins of the additional hardening.
Takeuchi, Tomoaki; Kakubo, Yuta*; Matsukawa, Yoshitaka*; Nozawa, Yasuko*; Nagai, Yasuyoshi*; Nishiyama, Yutaka; Katsuyama, Jinya; Onizawa, Kunio; Suzuki, Masahide
Journal of Nuclear Materials, 443(1-3), p.266 - 273, 2013/11
Times Cited Count：16 Percentile：76.47(Materials Science, Multidisciplinary)Investigation on irradiation effects of weld-overlay claddings is necessary for safety assessment of reactor pressure vessels. We investigated microstructural changes in the cladding, which was composed of about 90% austenite and 10% -ferrite phases, subjected to the neutron irradiation to 7.210 n/cm
at 290C, by 3D atom probe tomography technique. In the ferrite phase, the amplitude of the Cr and Si concentration fluctuation was increased by the irradiation and Ni and Mn concentration fluctuations were newly occurred. In the austenite phase, 
'(Ni
Si) -like clusters were formed. In contrast, the results of our previous work on the cladding subjected to thermal aging showed the amplitude of the Cr fluctuation was significantly increased and G (Ni-Si-Mn) phase was formed in the ferrite phase. Moreover, no changes were observed in the austenite by the aging.
Takeuchi, Tomoaki; Nishiyama, Yutaka; Katsuyama, Jinya; Onizawa, Kunio; Matsukawa, Yoshitaka*; Toyama, Takeshi*; Nagai, Yasuyoshi*; Kameda, Jun*
no journal, ,
Microstructure evolution on stainless overlay cladding of reactor pressure vessel was quantitatively investigated by means of three-dimensional local electrode atom probe (3DAP) in nano-scale. The cladding material was neutron-irradiated up to 710n/cm (E 1 MeV)at JMTR. In ferrite phases of cladding material, a 12% of Cr concentration fluctuation, which was arisen through a spinodal decomposition, observed before the irradiation was progressed to 20% after the irradiation while the wave length of the fluctuation was almost not changed about 8-10 nm before and after the irradiation. In addition, Ni, Si and Mn concentration fluctuations were also observed. Correlation between Cr concentration fluctuation and hardness, other elements concentration fluctuation and differences from the thermal aging will be reported in presentation.
Takeuchi, Tomoaki; Nishiyama, Yutaka; Katsuyama, Jinya; Onizawa, Kunio; Nozawa, Yasuko*; Matsukawa, Yoshitaka*; Toyama, Takeshi*; Nagai, Yasuyoshi*; Kameda, Jun*
no journal, ,
Microstructure evolution on stainless overlay cladding of reactor pressure vessel was quantitatively investigated by means of three-dimensional local electrode atom probe (3DAP) in nano-scale. The cladding material was neutron-irradiated up to 710 n/cm (E 1 MeV) at JMTR. In ferrite phases of cladding material, a 12% of Cr concentration fluctuation, which was arisen through a spinodal decomposition, observed before the irradiation was progressed to 20% after the irradiation while the wave length of the fluctuation was almost not changed about 8-10 nm before and after the irradiation. In addition, Ni, Si and Mn concentration fluctuations were also observed. The concentration fluctuations of the elements other than Cr and differences from the thermal aging will be reported in presentation.
Takeuchi, Tomoaki; Nishiyama, Yutaka; Katsuyama, Jinya; Onizawa, Kunio; Nozawa, Yasuko*; Matsukawa, Yoshitaka*; Toyama, Takeshi*; Nagai, Yasuyoshi*; Kameda, Jun*
no journal, ,
Microstructure evolutions on two types, namely electroslag (ESW) and submerged-arc (SAW) welding, of stainless overlay claddings of reactor pressure vessel were quantitatively investigated by means of three-dimensional local electrode atom probe (3DAP) in nano-scale. The cladding materials were neutron-irradiated up to about 710n/cm (E 1 MeV) at JMTR. In ferrite phases of cladding materials, a 12% of Cr concentration fluctuation, which was arisen through a spinodal decomposition, observed before the irradiation was progressed to 20% after the irradiation while the wave length of the fluctuation was almost not changed about 8-10 nm before and after the irradiation. In addition, Ni, Si and Mn concentration fluctuations were also observed. Correlation between Cr concentration fluctuation and hardness, other elements concentration fluctuation and differences between the neutron irradiation and the thermal aging, the ESW and SAW will be reported in presentation.
Matsukawa, Yoshitaka*; Kakubo, Yuta*; Nozawa, Yasuko*; Toyama, Takeshi*; Nagai, Yasuyoshi*; Takeuchi, Tomoaki; Yamaguchi, Yoshihito; Katsuyama, Jinya; Nishiyama, Yutaka
no journal, ,
The / duplex stainless steel overlay-welded on the inner wall of light-water reactor pressure vessels for protection from corrosion is known to exhibit a complex phase transformation under thermal aging. Our atom-probe tomography analysis revealed that in the -ferrite /' spinodal decomposition and precipitation of nanometer-sized Ni-Mn-Si clusters occur together at 673 K after 2000 hrs. The Ni-Mn-Si clusters visualized in the 3-D atom map are most likely G-phase, which is an intermetallic compound commonly observed in this type of duplex stainless steels; however, since crystallographic information is largely lost in atom-probe data, it is unknown if they are certainly G-phase accompanied with a specific supper-lattice structure or simple agglomerates of solute atoms without any crystal structure change. In the present study crystal structure of the Ni-Mn-Si nano-clusters has been analyzed by transmission electron microscopy to determine the fraction of G-phase over all clusters.
Kakubo, Yuta*; Nozawa, Yasuko*; Matsukawa, Yoshitaka*; Toyama, Takeshi*; Nagai, Yasuyoshi*; Kameda, Jun; Takeuchi, Tomoaki; Yamaguchi, Yoshihito; Katsuyama, Jinya; Nishiyama, Yutaka; et al.
no journal, ,
no abstracts in English
/ duplex stainless steel weld overlay cladding of light-water reactor pressure vessel steelsMatsukawa, Yoshitaka*; Kakubo, Yuta*; Nozawa, Yasuko*; Toyama, Takeshi*; Nagai, Yasuyoshi*; Takeuchi, Tomoaki; Yamaguchi, Yoshihito; Katsuyama, Jinya; Nishiyama, Yutaka
no journal, ,
Crystal structure change and composition change of G-phase (NiSiMn) precipitates in a duplex stainless steel welded on an RPV steel were analyzed by transmission electron microscopy combined with atom-probe tomography. Based on TEM crystallographic analysis data and atom-probe composition analysis data, together with simulation results of electron diffraction patterns, it was found that those precipitates turned G-phase in structure-wise before they turned stoichiometric composition.
Kishita, Shiori*; Abe, Hiroaki*; Abe Tomonori*; Kunii, Daichi*; Udagawa, Yutaka; Matsukawa, Yoshitaka*; Sato, Yuki*
no journal, ,
no abstracts in English
Matsukawa, Yoshitaka*; Takeuchi, Tomoaki; Nagai, Yasuyoshi*
no journal, ,
no abstracts in English
Hamasato, Tsunehiko*; Matsukawa, Yoshitaka*; Tsujimura, Soyo*; Sakaguchi, Chisato*; Tsurekawa, Sadahiro*; Homma, Yoshiya*; Yabuuchi, Kiyohiro*; Yamaguchi, Masatake
no journal, ,
no abstracts in English
SiMnMatsukawa, Yoshitaka*; Watanabe, Daiki*; Tsurekawa, Sadahiro*; Abad, N. M.*; Muta, Hiroaki*; Yoshida, Kenta*; Kasada, Ryuta*; Yamaguchi, Masatake; Kumano, Hideki*; Endo, Minako*
no journal, ,
It is now generally believed that when a light water reactor is operated for nearly 80 years, Ni-Si-Mn ternary intermetallic compounds such as the G phase and the 
phase precipitate in the pressure vessel steel, which contribute to irradiation embrittlement. We have started a basic study focusing on the relationship between these compounds and diffusion coefficients. In order to investigate the characteristics of interdiffusion at the precipitate/matrix interface of pressure vessel steels, bulk materials of the compounds were prepared by arc melting and bonded to bulk materials of pure iron to produce so-called diffusion pairs. In this report, we describe our findings for the G phase.
