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Journal Articles

Reaction of Np, Am, and Cm ions with CO$$_{2}$$ and O$$_{2}$$ in a reaction cell in triple quadrupole inductively coupled plasma mass spectrometry

Kazama, Hiroyuki; Konashi, Kenji*; Suzuki, Tatsuya*; Koyama, Shinichi; Maeda, Koji; Sekio, Yoshihiro; Onishi, Takashi; Abe, Chikage*; Shikamori, Yasuyuki*; Nagai, Yasuyoshi*

Journal of Analytical Atomic Spectrometry, 38(8), p.1676 - 1681, 2023/07

 Times Cited Count:0 Percentile:0.02

Journal Articles

Defect analysis of matrix damage in reactor pressure vessel steel using WB-STEM

Yoshida, Kenta*; Toyama, Takeshi*; Inoue, Koji*; Nagai, Yasuyoshi*; Shimodaira, Masaki

Materia, 62(3), p.154 - 158, 2023/03

no abstracts in English

Journal Articles

${it In situ}$ TEM observation and MD simulation of frank partial dislocation climbing in Al-Cu alloy

Chen, J.*; Yoshida, Kenta*; Suzudo, Tomoaki; Shimada, Yusuke*; Inoue, Koji*; Konno, Toyohiko*; Nagai, Yasuyoshi*

Materials Transactions, 63(4), p.468 - 474, 2022/04

 Times Cited Count:1 Percentile:24.45(Materials Science, Multidisciplinary)

In situ electron irradiation using high-resolution transmission electron microscopy (HRTEM) was performed to visualize the Frank loop evolution in aluminium-copper (Al-Cu) alloy with an atomic-scale spatial resolution of 0.12 nm. The ${it in situ}$ HRTEM observation along the [110] direction of the FCC-Al lattice, Frank partial dislocation bounding an intrinsic stacking fault exhibited an asymmetrical climb along the $$<$$112$$>$$ direction opposed to those in the reference pure Al under an electron irradiation, with a corresponding displacement-per-atom rate of 0.055-0.120 dpa/s. The asymmetrical climb of the partial dislocation was described as pinning effects due to Cu-Cu bonding in Guinier-Preston zones by a molecular dynamics simulation.

Journal Articles

The Role of silicon on solute clustering and embrittlement in highly neutron-irradiated pressurized water reactor surveillance test specimens

Takamizawa, Hisashi; Hata, Kuniki; Nishiyama, Yutaka; Toyama, Takeshi*; Nagai, Yasuyoshi*

Journal of Nuclear Materials, 556, p.153203_1 - 153203_10, 2021/12

 Times Cited Count:0 Percentile:0.01(Materials Science, Multidisciplinary)

Solute clusters (SCs) formed in pressurized water reactor surveillance test specimens neutron-irradiated to a fluence of 1 $$times$$ 10$$^{20}$$ n/cm$$^{2}$$ were analyzed via atom probe tomography to understand the effect of silicon on solute clustering and irradiation embrittlement of reactor pressure vessel steels. In high-Cu bearing materials, Cu atoms were aggregated at the center of cluster surrounded by the Ni, Mn, and Si atoms like a core-shell structure. In low-Cu bearing materials, Ni, Mn, and Si atoms formed cluster and these solutes were not comprised core-shell structure in SCs. While the number of Cu atoms in clusters was decreased with decreasing nominal Cu content, the number of Si atoms had clearly increased. The cluster radius ($$r$$) and number density ($$N_{d}$$) decreased and increased, respectively, with increasing nominal Si content. The shift in the reference temperature for nil-ductility transition ($$Delta$$RT$$_{NDT}$$) showed a good correlation with the square root of volume fraction ($$V_{f}$$) multiplied by r ($$sqrt{V_{f}times {r}}$$). This suggested that the dislocation cutting through the particles mechanism dominates the precipitation hardening responsible for irradiation embrittlement. The negative relation between the nominal Si content and $$Delta$$RT$$_{NDT}$$ indicated that increasing of nominal Si content reduces the degree of embrittlement.

Journal Articles

Radiation-enhanced diffusion of copper in iron studied by three-dimensional atom probe

Toyama, Takeshi*; Suzudo, Tomoaki; Nagai, Yasuyoshi*; 9 of others*

Journal of Nuclear Materials, 556, p.153176_1 - 153176_7, 2021/12

 Times Cited Count:2 Percentile:36.99(Materials Science, Multidisciplinary)

We performed a high-precision investigation of radiation-enhanced diffusion (RED) using electron irradiation and three-dimensional atom probe (3D-AP). Cu-Fe diffusion pairs were created using high-purity Fe and Cu as base materials, and irradiated by 2 MeV electron. Cu diffusion into the Fe matrix was observed at the atomic level using 3D-AP, and the diffusion coefficient was obtained directly using Fick's law. RED was clearly observed, and the ratio of diffusion under irradiation to thermal diffusion was enhanced at low temperature. RED was quantitatively evaluated using the reaction kinetics model, and the model which consider only vacancies gave a good agreement. This gave experimental clarification that RED was dominated by irradiation-induced vacancies. In addition, the direct experimental results on the effect of irradiation on the solubility limits of Cu in Fe was obtained; solubility limits under irradiation were found to be lower than those under thermal aging.

Journal Articles

Investigation of Cu diffusivity in Fe by a combination of atom probe experiments and kinetic Monte Carlo simulation

Zhao, C.*; Suzudo, Tomoaki; Toyama, Takeshi*; Nishitani, Shigeto*; Inoue, Koji*; Nagai, Yasuyoshi*

Materials Transactions, 62(7), p.929 - 934, 2021/07

 Times Cited Count:2 Percentile:20.18(Materials Science, Multidisciplinary)

We succeeded in measuring the diffusion coefficient of Cu in Fe in a low temperature range that had not been measured so far. Since the diffusion couple, which is a general method for measuring the diffusion coefficient, can be applied only at high temperature, atom probe tomography and Cu precipitation rate theory were used in this study. The estimated diffusion coefficient was found to be more reliable than that obtained in previous studies. Therefore, it is considered that the estimation by the atom probe provided higher accuracy. Furthermore, the kinetic Monte Carlo simulation revealed that the diffusion coefficient estimated by this method tends to be slightly overestimated as the temperature decreases.

Journal Articles

Grain-boundary phosphorus segregation in highly neutron-irradiated reactor pressure vessel steels and its effect on irradiation embrittlement

Hata, Kuniki; Takamizawa, Hisashi; Hojo, Tomohiro*; Ebihara, Kenichi; Nishiyama, Yutaka; Nagai, Yasuyoshi*

Journal of Nuclear Materials, 543, p.152564_1 - 152564_10, 2021/01

 Times Cited Count:9 Percentile:92.2(Materials Science, Multidisciplinary)

Reactor pressure vessel (RPV) steels for pressurized water reactors (PWRs) with bulk P contents ranging from 0.007 to 0.012wt.% were subjected to neutron irradiation at fluences ranging from 0.3 to 1.2$$times$$10$$^{20}$$ n/cm$$^{2}$$ (E $$>$$ 1 MeV) in PWRs or a materials testing reactor (MTR). Grain-boundary P segregation was analyzed using Auger electron spectroscopy (AES) on intergranular facets and found to increase with increasing neutron fluence. A rate theory model was also used to simulate the increase in grain-boundary P segregation for RPV steels with a bulk P content up to 0.020wt.%. The increase in grain-boundary P segregation in RPV steel with a bulk P content of 0.015wt.% (the maximum P concentration found in RPV steels used in Japanese nuclear power plants intended for restart) was estimated to be less than 0.1 in monolayer coverage at 1.0$$times$$10$$^{20}$$ n/cm$$^{2}$$ (E $$>$$ 1 MeV). A comparison of the PWR data with the MTR data showed that neutron flux had no effect upon grain-boundary P segregation. The effects of grain-boundary P segregation upon changes in irradiation hardening and ductile-brittle transition temperature (DBTT) shifts were also discussed. A linear relationship between irradiation hardening and the DBTT shift with a slope of 0.63 obtained for RPV steels with a bulk P content up to 0.026wt.%, which is higher than that of most U.S. A533B steels. It is concluded that the intergranular embrittlement is unlikely to occur for RPV steels irradiated in PWRs.

Journal Articles

Atomistic modeling of hardening in spinodally-decomposed Fe-Cr binary alloys

Suzudo, Tomoaki; Takamizawa, Hisashi; Nishiyama, Yutaka; Caro, A.*; Toyama, Takeshi*; Nagai, Yasuyoshi*

Journal of Nuclear Materials, 540, p.152306_1 - 152306_10, 2020/11

 Times Cited Count:8 Percentile:74.98(Materials Science, Multidisciplinary)

Spinodal decomposition in thermally aged Fe-Cr alloys leads to significant hardening, which is the direct cause of the so-called 475C-embrittlement. To illustrate how spinodal decomposition induces hardening by atomistic interactions, we conducted a series of numerical simulations as well as reference experiments. The numerical results indicated that the hardness scales linearly with the short-range order (SRO) parameter, while the experimental result reproduced this relationship within statistical error. Both seemingly suggest that neighboring Cr-Cr atomic pairs essentially cause hardening, because SRO is by definition uniquely dependent on the appearance probability of such pairs. A further numerical investigation supported this notion, as it suggests that the dominant cause of hardening is the pinning effect of dislocations passing over such Cr-Cr pairs.

Journal Articles

${it In situ}$ WB-STEM observation of dislocation loop behavior in reactor pressure vessel steel during post-irradiation annealing

Du, Y.*; Yoshida, Kenta*; Shimada, Yusuke*; Toyama, Takeshi*; Inoue, Koji*; Arakawa, Kazuto*; Suzudo, Tomoaki; Milan, K. J.*; Gerard, R.*; Onuki, Somei*; et al.

Materialia, 12, p.100778_1 - 100778_10, 2020/08

In order to ensure the integrity of the reactor pressure vessel in the long term, it is necessary to understand the effects of irradiation on the materials. In this study, irradiation-induced dislocation loops were observed in neutron-irradiated reactor pressure vessel specimens during annealing using our newly developed WB-STEM. It was confirmed that the proportion of $$<100>$$ loops increased with increasing annealing temperature. We also succeeded in observing the phenomenon that two $$frac{1}{2}$$$$<111>$$ loops collide into a $$<100>$$ loop. Moreover, a phenomenon in which dislocation loops decorate dislocations was also observed, and the mechanism was successfully explained by molecular dynamics simulation.

Journal Articles

Predoping effects of boron and phosphorous on arsenic diffusion along grain boundaries in polycrystalline silicon investigated by atom probe tomography

Takamizawa, Hisashi; Shimizu, Yasuo*; Inoue, Koji*; Nozawa, Yasuko*; Toyama, Takeshi*; Yano, Fumiko*; Inoue, Masao*; Nishida, Akio*; Nagai, Yasuyoshi*

Applied Physics Express, 9(10), p.106601_1 - 106601_4, 2016/10

 Times Cited Count:0 Percentile:0(Physics, Applied)

Journal Articles

The Two-step nucleation of G-phase in ferrite

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:67 Percentile:95.48(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$$_{16}$$Si$$_{7}$$Mn$$_{6}$$) 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 Ni$$_{16}$$Si$$_{3.5}$$(Fe,Cr)$$_{3.5}$$Mn$$_{6}$$.

Journal Articles

Hardening in thermally-aged Fe-Cr binary alloys; Statistical parameters of atomistic configuration

Suzudo, Tomoaki; Nagai, Yasuyoshi*; Schwen, D.*; Caro, A.*

Acta Materialia, 89, p.116 - 122, 2015/05

 Times Cited Count:12 Percentile:52.44(Materials Science, Multidisciplinary)

By exploiting Monte Carlo methodology and molecular dynamics, we computationally simulate the spinodal decomposition of iron-chromium binary alloys and analyze the relationship between the increase of yield stress induced by the phase separation phenomenon, and statistical parameters of the atomistic configuration. We successfully model the experimentally-discovered proportional relationship between the hardness and the variation parameter (or V), and also found that the adequacy of the parameter V as an empirical indicator of hardening is limited, because it does not properly capture short-range atomistic configurations that influence the hardening. We suggest that the short-range-order parameter has more potential to become universal descriptor of the phenomenon.

Journal Articles

Effects of thermal aging on microstructure and hardness of stainless steel weld-overlay claddings of nuclear reactor pressure vessels

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:37 Percentile:94.6(Materials Science, Multidisciplinary)

Microstructures and hardness of stainless steel weld overlay cladding of reactor pressure vessels subjected to the thermal aging at 400 $$^{circ}$$C for 100-10,000 h were investigated using atom probe tomography and nanoindentation technique. The Cr concentration fluctuation in the $$delta$$-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 $$delta$$-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 $$delta$$-ferrite phase by the thermal aging was Cr spinodal decomposition.

Journal Articles

Effects of neutron irradiation on microstructures and hardness of stainless steel weld-overlay cladding of nuclear reactor pressure vessels

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:18 Percentile:82.3(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 $$times$$ 10$$^{19}$$n cm$$^{-2}$$ (E $$>$$ 1 MeV) and a flux of 1.1 $$times$$ 10$$^{13}$$n cm$$^{-2}$$ s$$^{-1}$$ at 290$$^{circ}$$C 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 300$$^{circ}$$C for the time equivalent to the irradiation. The Cr concentration fluctuation was enhanced in the $$delta$$-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 $$delta$$-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.

Journal Articles

Effect of neutron irradiation on the microstructure of the stainless steel electroslag weld overlay cladding of nuclear reactor pressure vessels

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:77.2(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% $$delta$$-ferrite phases, subjected to the neutron irradiation to 7.2$$times$$10$$^{19}$$ n/cm$$^{2}$$ at 290$$^{circ}$$C, 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, $$gamma$$'(Ni$$_{3}$$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.

Journal Articles

Study on microstructural changes in thermally-aged stainless steel weld-overlay cladding of nuclear reactor pressure vessels by atom probe tomography

Takeuchi, Tomoaki; Kameda, Jun*; Nagai, Yasuyoshi*; Toyama, Takeshi*; Nishiyama, Yutaka; Onizawa, Kunio

Journal of Nuclear Materials, 415(2), p.198 - 204, 2011/08

 Times Cited Count:34 Percentile:92.22(Materials Science, Multidisciplinary)

Microstructural changes by thermal aging in stainless steel weld overlay cladding of nuclear reactor pressure vessels were investigated using atom probe tomography. The cladding material was composed of about 90% austenite phase and 10% $$delta$$-ferrite phase and thermally aged at 400$$^{circ}$$C for 10,000 h. In the ferrite phase, the thermal aging increased a fluctuation of Cr concentration due to spinodal decomposition and caused the precipitation of G phase with chemical composition of Ni:Si:Mn = 16:7:6. Moreover, significant hardening of the ferrite phase was induced by the thermal aging. On the other hand, the thermal aging did not affect on the microstructures and the hardness in the austenite phase, which indicates the microstructural changes were responsible for the hardening in the ferrite phase. The analyses of the magnitude of the spinodal decomposition and the hardness implied that the spinodal decomposition was the main cause of the hardening.

Journal Articles

Effects of chemical composition and dose on microstructure evolution and hardening of neutron-irradiated reactor pressure vessel steels

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:58 Percentile:96.38(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 $$times$$ 10$$^{19}$$ n cm$$^{-2}$$ (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.

Journal Articles

Interlaboratory comparison of positron annihilation lifetime measurements

Ito, Kenji*; Oka, Toshitaka*; Kobayashi, Yoshinori*; Shirai, Yasuharu*; Wada, Kenichiro*; Matsumoto, Masataka*; Fujinami, Masanori*; Hirade, Tetsuya; Honda, Yoshihide*; Hosomi, Hiroyuki*; et al.

Materials Science Forum, 607, p.248 - 250, 2009/00

So far no standard procedure for the positron annihilation lifetime (PAL) technique has been established. A lack of the standards has led to difficulty in ensuring the equivalency and reliability of data from different laboratories. As a first, we conducted an interlaboratory comparison of PAL measurements for metal, polymer and silica glass with agreed procedures for data recording and analysis. The PAL data recorded at different laboratories were analyzed with a single lifetime component for the metal sample and with three components for the others, respectively. Based on the results of the reported positron and ortho-positronium lifetimes, the possible sources of the uncertainties in the PAL measurements are discussed. To reduce the effect of scattered $$gamma$$ rays, a lead shield was placed between the detectors. The uncertainty was significantly decreased, signifying that placing lead shields between the detectors effectively reduced the false signals due to the scattered $$gamma$$ rays.

Journal Articles

Impact of advanced experimental method on nuclear materials R&D; The Progress of technology has brought about a deep understanding !

Kimura, Akihiko*; Nagai, Yasuyoshi*; Fujii, Katsuhiko*; Nishiyama, Yutaka; Soneda, Naoki*

Nihon Genshiryoku Gakkai-Shi ATOMO$$Sigma$$, 50(10), p.630 - 633, 2008/10

no abstracts in English

Journal Articles

Effects of neutron-irradiation-induced intergranular phosphorus segregation and hardening on embrittlement in reactor pressure vessel steels

Nishiyama, Yutaka; Onizawa, Kunio; Suzuki, Masahide; Anderegg, J. W.*; Nagai, Yasuyoshi*; Toyama, Takeshi*; Hasegawa, Masayuki*; Kameda, Jun*

Acta Materialia, 56(16), p.4510 - 4521, 2008/09

 Times Cited Count:64 Percentile:91.99(Materials Science, Multidisciplinary)

The effects of intergranular P segregation and hardening on the ductile-to-brittle transition temperature (DBTT) in several neutron-irradiated reactor pressure vessel steels with different bulk contents of P and Cu have been investigated using a scanning Auger microbe, a local electrode atom probe and positron annihilation spectroscopy. Increasing the neutron fluence at 563 K promotes intergranular P segregation. The content of P more significantly affects irradiation hardening than that of Cu due to distinct formation of P-rich precipitates arising from the stabilization of vacancies. Analyzing the correlations between the P segregation, hardening, fraction of intergranular fracture and DBTT, it is found neutron irradiation mitigates an embrittling effect of segregated P, and therefore the hardening more strongly affects the DBTT shift than the P segregation.

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