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Mamiya, Hiroaki*; Oba, Yojiro; Hiroi, Kosuke; Miyatake, Takayuki*; Gautam, R.*; Sepehri-Amin, H.*; Okubo, Tadakatsu*
IEEE Magnetics Letters, 14, p.7100105_1 - 7100105_5, 2023/02
Times Cited Count：2 Percentile：49.60(Engineering, Electrical & Electronic)Sasaki, Hirokazu*; Akiya, Shunta*; Oba, Yojiro; Onuma, Masato*; Giddings, A. D.*; Okubo, Tadakatsu*
Materials Transactions, 63(10), p.1384 - 1389, 2022/10
Times Cited Count：0 Percentile：0.00(Materials Science, Multidisciplinary)Sasaki, Hirokazu*; Akiya, Shunta*; Oba, Yojiro; Onuma, Masato*; Giddings, A. D.*; Okubo, Tadakatsu*
Do To Dogokin, 60(1), p.309 - 314, 2021/08
no abstracts in English
Nogiwa, Kimihiro; Nishimura, Akihiko; Yokoyama, Atsushi; Otsuka, Satoshi; Kaito, Takeji; Inoue, Masaki; Okubo, Tadakatsu*; Hono, Kazuhiro*
Journal of Nuclear Materials, 417(1-3), p.201 - 204, 2011/10
Times Cited Count：9 Percentile：55.96(Materials Science, Multidisciplinary)Du se 9Cr-ODS (oxide dispersion-strengthened) steel consisting of residual-
ferrite and prime martensite has excellent high-temperature strength. This study describes the microstructure of dual-phase 9Cr-ODS steels characterized by atom-probe tomography in order to compare oxide-particle dispersion states in each phase. This revealed that nano-size oxide particles were of the same chemical composition and that their mean size was about 3 nm in each phase. On the other hand, the number density in the residual- phase was about four times higher than that of the prime phase. These results indicate that the dense distribution of the oxide particles in the residual- phase contribute to the excellent high-temperature strength of 9Cr-ODS steel.
Takeuchi, Tomoaki; Kuramoto, Akira*; Kameda, Jun*; Toyama, Takeshi*; Nagai, Yasuyoshi*; Hasegawa, Masayuki*; Okubo, Tadakatsu*; Yoshiie, Toshimasa*; Nishiyama, Yutaka; Onizawa, Kunio
Journal of Nuclear Materials, 402(2-3), p.93 - 101, 2010/07
Times Cited Count：63 Percentile：96.34(Materials Science, Multidisciplinary)This study reports the effects of the composition and dose on microstructure evolution and hardening in high- and low-impurity A533B-1 steels neutron-irradiated in a wide range from 0.32 to 9.9 
10
n cm
(E 
1 MeV) under a constant high flux at JMTR. The early hardening was found to be caused by mainly matrix defects. The gradual hardening after middle stage of irradiation was found to be caused by the formation of Cu rich clusters (CRCs) and Mn-Ni-Si rich clusters (MNSCs), respectively, in the high- and low-impurity steels. By applying a RB model, it was found that the dislocation-pinning strength of the CRCs and MNSCs is almost the same. Moreover, the high-impurity steel subjected to the highest dose revealed the formation of MNSCs.
Nishimura, Akihiko; Nogiwa, Kimihiro; Otobe, Tomohito; Okubo, Tadakatsu*; Hono, Kazuhiro*; Kondo, Keietsu; Yokoyama, Atsushi
Ultramicroscopy, 109(5), p.467 - 471, 2009/04
Times Cited Count：7 Percentile：28.40(Microscopy)Influence of femtosecond laser pulse condition on the performance of an energy compensated tomographic atom probe (ECOTAP) was investigated. Chirping ratio for laser pulses was controlled by a compressor stage. We have succeeded to get tomographic images of oxide dispersion strengthen steel, which will be used as fast breeder reactors. The ECOTAP successfully observed that the instability of the femtosecond laser pulses make the mass peaks slightly sifted or broadened to higher mass number. To investigate insulator materials, numerical simulation of conductivity increase on diamond has been successfully demonstrated.
Nogiwa, Kimihiro; Nishimura, Akihiko; Oba, Hironori; Yokoyama, Atsushi; Okubo, Tadakatsu*; Hono, Kazuhiro*
Materia, 47(12), P. 626, 2008/12
no abstracts in English
Kondo, Keietsu; Nemoto, Yoshiyuki; Miwa, Yukio; Kaji, Yoshiyuki; Tsukada, Takashi; Nagai, Yasuyoshi*; Hasegawa, Masayuki*; Okubo, Tadakatsu*; Hono, Kazuhiro*
JAEA-Research 2006-013, 39 Pages, 2006/12
JAEA-Research-2006-013.pdf:4.57MB
There has been an increasing number of stress corrosion cracking (SCC) incidents on low carbon austenitic stainless steels used in boiling water reactor (BWR) environments. To reveal the acceleration factor of intergranular crack propagation from the viewpoint of solute distribution in stainless steels, the material extracted from a core shroud of Japanese BWR was analyzed by the three dimensional atom probe (3DAP), which has the highest spatial resolution among the various microanalytical techniques. It was revealed by statistical analysis on 3DAP data that solute elements, such as Fe, Cr, Ni, Mo, Mn, Si, are randomly distributed in matrix of the shroud material. This result means that solute was not segregated or precipitated and was not form spinodal decomposition during the service. The concentration profile in the vicinity of grain boundary obtained from 3DAP dataset showed the random distribution of Cr. This result shows that degradation of the corrosion resistance induced by depletion of Cr was not responsible for the crack propagation along grain boundaries in low carbon stainless steel. On the other hand, enrichment of Mo and Si was observed at grain boundary. The width of the enriched zone was about 2 nm across the grain boundary, and the concentration of those elements could be much higher than the concentration obtained by field emission transmission electron microscopy/energy dispersive X-ray spectroscopy (FE-TEM/EDS). Therefore, it is necessary to study about the effects of enrichment of Mo and Si as a potential contributor to SCC.
Nogiwa, Kimihiro; Nishimura, Akihiko; Inoue, Masaki; Otsuka, Satoshi; Yokoyama, Atsushi; Okubo, Tadakatsu*; Hono, Kazuhiro*
no journal, ,
Japan Atomic Energy Agency (JAEA) has developed 9Cr-ODS martensitic steel and has succeeded in manufacturing thin-walled tubes that overcome highly anisotropic properties. Previous studies revealed that the 9Cr-ODS steel has a dual phase structure consisting of residual-ferrite and martensite. Improvement in creep strength is apparently caused by the retention of the phase, in which ultrafine oxide particles are dispersed. In this study, we have investigated the nanometer-scale microstructure of a dual phase 9Cr-ODS steel by the 3D atom probe operated with femtosecond laser pulses and compared the dispersion state of oxide particles in residual-ferrite and martensite phases.
Nogiwa, Kimihiro; Nishimura, Akihiko; Oba, Hironori; Yokoyama, Atsushi; Okubo, Tadakatsu*; Hono, Kazuhiro*
no journal, ,
no abstracts in English
Nishimura, Akihiko; Nogiwa, Kimihiro; Kondo, Keietsu; Otobe, Tomohito; Yokoyama, Atsushi; Okubo, Tadakatsu*; Hono, Kazuhiro*
no journal, ,
We successfully improved the performance of 3- Dimensional atom probe by using femtosecond laser ionization. A reflectoron type atom probe was applied due to its high mass spectroscopy. Laser system has a specially designed pulse compressor to change the laser pulse duration. An OPA wave length converter was installed to the laser system. We selected ODS steel and Zr metal for 3D atom probe analysis with numerical simulation. Femtosecond laser assisted 3D atom probes can overcome the performance of voltage pulsing 3D atom probes completely.
Sasaki, Hirokazu*; Akiya, Shunta*; Oba, Yojiro; Onuma, Masato*; Okubo, Tadakatsu*; Uzuhashi, Jun*
no journal, ,
no abstracts in English
