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Kobayashi, Keita; Nakamura, Hiroki; Yamaguchi, Akiko; Itakura, Mitsuhiro; Machida, Masahiko; Okumura, Masahiko
Computational Materials Science, 188, p.110173_1 - 110173_14, 2021/02
no abstracts in English
-iron; A Molecular dynamics studySuzudo, Tomoaki; Ebihara, Kenichi; Tsuru, Tomohito
AIP Advances (Internet), 10(11), p.115209_1 - 115209_8, 2020/11
Times Cited Count：0 Percentile：100(Nanoscience & Nanotechnology)The mechanism of their brittle fracture of BCC metals is not fully understood. In this study, we conduct a series of three-dimensional molecular dynamics simulations of cleavage fracture of -iron. In particular, we focus on mode-I loading starting from curved crack fronts. In the simulations, brittle fractures are observed at cleavages on the {100} plane, while the initial cracks become blunted on other planes as a result of dislocation emissions. Our modeling results agreed with a common experimental observation, that is, {100} is the preferential cleavage plane in bcc transition metals.
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：0 Percentile：100(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.
5 grain boundary of -ironEbihara, Kenichi; Suzudo, Tomoaki
Proceedings of Joint International Conference on Supercomputing in Nuclear Applications + Monte Carlo 2020 (SNA + MC 2020), p.65 - 69, 2020/10
Phosphorus (P) is known as one of the elements which cause the grain boundary (GB) embrittlement in steels and its GB segregation is promoted by the increase of vacancies and self-interstitial atoms due to irradiation. Thus we have been developing the rate-theory model for estimating GB P segregation under several temperatures and irradiation conditions. Because the model does not include the trapping and de-trapping processes properly, however, the model cannot calculate GB P coverage which is measured by experiments. As for the de-trapping process, so far, we have considered the migration of a P atom in the GB region of 3 symmetrical tilt GB using molecular dynamics (MD). In the current study, we also simulated the P migration in 5 GB using MD and compared the result with that of 3. As a result, at 800K, it was found that a P atom cannot migrate in 5 without vacancies while a P atom can migrate between iron atoms in 3.
WB-STEM observation of dislocation loop behavior in reactor pressure vessel steel during post-irradiation annealingDu, 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 
loops increased with increasing annealing temperature. We also succeeded in observing the phenomenon that two 
loops collide into a loop. Moreover, a phenomenon in which dislocation loops decorate dislocations was also observed, and the mechanism was successfully explained by molecular dynamics simulation.
-ironEbihara, Kenichi; Suzudo, Tomoaki
TMS 2020; 149th Annual Meeting & Exhibition Supplemental Proceedings, p.995 - 1002, 2020/02
Phosphorus (P) is known as an element which causes grain boundary (GB) embrittlement in steels. In addition, GB P segregation is promoted by the increase of vacancies and self interstitial atoms due to irradiation. Thus, the diffusion rate theory model for estimating irradiation-induced GB P segregation has been developed based on the atomic processes. Since the present model does not include the trapping and de-trapping processes at GBs, however, it cannot calculate the value which is directly compared with experimental results. In this study, we simulated the migration of a P atom in the 3(111) symmetrical tilt GB. In addition, by tracking the migration of the P atom, the diffusion barrier energy was evaluated. As a result, the diffusion barrier energy was almost the same as the P segregation energy of an interstitial site in the GB, and it was found that P atoms migrate via interstitial sites in the GB.
Suzudo, Tomoaki; Onitsuka, Takashi*; Fukumoto, Kenichi*
Modelling and Simulation in Materials Science and Engineering, 27(6), p.064001_1 - 064001_15, 2019/08
Times Cited Count：5 Percentile：33.23(Materials Science, Multidisciplinary)Plasticity of body-centered-cubic (BCC) metals at low temperatures is determined by screw dislocation kinetics. Because the core of screw dislocation in these metals has non-planar structure, its motion is complex and unpredictable. For example, although density functional theory (DFT) predicts slip on a { 110 } plane, the actual slip plane at elevated temperatures departs from the prediction, its mechanism having been a mystery for decades. Here we conduct a series of molecular dynamics simulations to track the screw dislocation motion and successfully reproduced the transition of the slip plane. We then devised an algorithm to scrutinize the activation of dislocation jump over the Peierls barrier and discovered the possible origin of this unexpected phenomenon, i.e., a large fluctuation leads to the kink-pair nucleation for the cross-slip jump without transition of dislocation core structure.
Fedkin, M. V.*; Shin, Y. K.*; Dasgupta, N.*; Yeon, J.*; Zhang, W.*; van Duin, D.*; Van Duin, A. C. T.*; Mori, Kento*; Fujiwara, Atsushi*; Machida, Masahiko; et al.
Journal of Physical Chemistry A, 123(10), p.2125 - 2141, 2019/03
Times Cited Count：7 Percentile：23.39(Chemistry, Physical)no abstracts in English
Okumura, Masahiko; Kerisit, S.*; Bourg, I. C.*; Lammers, L. N.*; Ikeda, Takashi*; Sassi, M.*; Rosso, K. M.*; Machida, Masahiko
Journal of Environmental Radioactivity, 189, p.135 - 145, 2018/09
Times Cited Count：24 Percentile：9.33(Environmental Sciences)no abstracts in English
3(111) symmetrical tilt grain boundary in -ironEbihara, Kenichi; Suzudo, Tomoaki
Modelling and Simulation in Materials Science and Engineering, 26(6), p.065005_1 - 065005_10, 2018/09
Times Cited Count：1 Percentile：89.52(Materials Science, Multidisciplinary)Irradiation-induced grain boundary phosphorus segregation is an important factor for estimating the embrittlement of nuclear reactor pressure vessel steels, but the physical process of phosphorus migration to grain boundaries is still unclear. We numerically studied phosphorus migration toward 3(111) symmetrical tilt grain boundary in -iron using molecular dynamics. We found that, in the vicinity of the grain boundary within 
1 nm distance, an iron-phosphorus mixed dumbbell and an octahedral interstitial phosphorus atom push a self-interstitial atom into the grain boundary, and the phosphorus atom becomes a substitutional atom. A phosphorus vacancy complex in the region also becomes dissociated, and the vacancy is absorbed in the grain boundary without dragging phosphorus. The results claim that a novel view of the segregation process is required.
Machida, Masahiko; Kato, Koichiro*; Shiga, Motoyuki
Journal of Chemical Physics, 148(10), p.102324_1 - 102324_11, 2018/03
Times Cited Count：9 Percentile：27.94(Chemistry, Physical)The isotopologs of liquid water, H
O, DO, and TO, are studied systematically by first principles PIMD simulations, in which the whole entity of the electrons and nuclei are treated quantum mechanically. The simulation results are in reasonable agreement with available experimental data on isotope effects, in particular, on the peak shift in the radial distributions of HO and DO and the shift in the evaporation energies. It is found that, due to differences in nuclear quantum effects, the H atoms in the OH bonds more easily access the dissociative region up to the hydrogen bond center than the D (T) atoms in the OD (OT) bonds. The accuracy and limitation in the use of the current density-functional-theory-based first principles PIMD simulations are also discussed. It is argued that the inclusion of the dispersion correction or relevant improvements in the density functionals are required for the quantitative estimation of isotope effects.
Nordlund, K.*; Zinkle, S. J.*; Sand, A. E.*; Granberg, F.*; Averback, R. S.*; Stoller, R.*; Suzudo, Tomoaki; Malerba, L.*; Banhart, F.*; Weber, W. J.*; et al.
Nature Communications (Internet), 9, p.1084_1 - 1084_8, 2018/03
Times Cited Count：82 Percentile：1.38(Multidisciplinary Sciences)Atomic collision processes are fundamental to numerous advanced materials technologies such as electron microscopy, semiconductor processing and nuclear power generation. Experimental and computer simulation studies over the past several decades provide the physical basis for understanding the atomic-scale processes occurring during primary displacement events. The current international standard for quantifying this particle damage, the Norgett-Robinson-Torrens displacements per atom (NRT-dpa) model, has nowadays several well-known limitations. In particular, the number of radiation defects produced in energetic cascades in metals is only 1/3 the NRT-dpa prediction, while the number of atoms involved in atomic mixing is about a factor of 30 larger than the dpa value. Here we propose two new complementary displacement production estimators.
Lammers, L.*; Bourg, I. C.*; Okumura, Masahiko; Kolluri, K.*; Sposito, G.*; Machida, Masahiko
Journal of Colloid and Interface Science, 490, p.608 - 620, 2017/03
Times Cited Count：46 Percentile：9.33(Chemistry, Physical)no abstracts in English
Kerisit, S.*; Okumura, Masahiko; Rosso, K. M.*; Machida, Masahiko
Clays and Clay Minerals, 64(4), p.389 - 400, 2016/08
Times Cited Count：16 Percentile：26.28(Chemistry, Physical)no abstracts in English
-butyl phosphate in organic and aqueous environments and its relevance to nuclear extraction processesMu, J.*; Motokawa, Ryuhei; Williams, C. D.*; Akutsu, Kazuhiro*; Nishitsuji, Shotaro*; Masters, A. J.*
Journal of Physical Chemistry B, 120(23), p.5183 - 5193, 2016/06
Times Cited Count：17 Percentile：40.39(Chemistry, Physical)Kaburagi, Masaaki; Yamada, Hironao*; Miyakawa, Takeshi*; Morikawa, Ryota*; Takasu, Masako*; Kato, Takamitsu*; Uesaka, Mitsuru*
Polymer Journal, 48(2), p.189 - 195, 2016/02
Times Cited Count：4 Percentile：76.26(Polymer Science)We performed molecular dynamics (MD) simulations of telomeric single-stranded DNA and POT1 for 100 ns. The distance between 
(POT1) and O5' (telomeric ssDNA) is calculated to verify the binding system for 100 ns MD. We then calculated the distance between the bases of telomeric DNA ends and the root mean square deviation and gyration radius in single and binding states. We compared the root mean square fluctuations between single and binding states and calculated the number of hydrogen bonds between POT1 and telomeric DNA. There are many hydrogen bonds between Gln94 and the first guanine of the closest TTAGGG sequence in telomeric single-stranded DNA. These Gln94 and the guanine have a large difference in root mean square fluctuation between single and binding states. We found that Gln94 and guanine are important components of the binding system, and they are related to its stability.
Machida, Masahiko; Nakamura, Hiroki; Srinivasan, S. G.*; Van Duin, A. C. T.*
Proceedings of 23rd International Conference on Nuclear Engineering (ICONE-23) (DVD-ROM), 4 Pages, 2015/05
Zircalloy has been widely employed as an excellent material covering the fuel rod. The mechanical and thermal properties have been explored by various experiments. In terms of its use as the fuel cladding, its response to oxidation reactions is an important topic when it is exposed to high temperature and high pressure steam during severe accidents. Especially, the hydrogen production accompanied by the oxidation is critical because it can lead to the crisis of the hydrogen explosion, as observed in the Fukushima Nuclear Power Plant accidents. Silicon carbide (SiC) has been considered as an alternative cladding material owing to an advantage that hydrogen production is much suppressed in the equivalent condition compared to Zircalloy. Therefore, we simulate the oxidation reaction for both materials, i.e. Zirconium metal and SiC in atomistic level by using the ReaxFF reactive force field method to simulate the chemical reaction molecular dynamics. Through such comparative studies between Zirconium and SiC in the same condition, we clarify how the temperature and the steam pressure accelerates the oxidation reaction and the resultant hydrogen production in both materials at typical severe accident conditions. The advantage using ReaxFF is that it allows us to directly trace the oxygen diffusion inside the Zirconium metal and SiC depending on the temperature and vapor pressure together with the oxidation reaction. We can compare the reaction processes in both materials. Especially, we paid attention to the rate of hydrogen production in both materials.
Ebihara, Kenichi; Kaburaki, Hideo; Itakura, Mitsuhiro
"Hagane No Kikaiteki Tokusei Ni Oyobosu Suiso No Koka To Sono Hyoka" Shimpojium Yokoshu (USB Flash Drive), 6 Pages, 2014/09
Since hydrogen(H) embrittlement is one factor causing degradation and/or fracture of steel, understanding its mechanism is required. The grain-boundary(GB) decohesion due to segregation of H is considered to cause the delayed fracture of high strength steels and the cold cracking in welding. In the model based on GB decohesion, information of strength of GBs estimated in the atomic scale is used for the estimation of strength or crack propagation in the macroscopic scale. However the modeling between the atomic and the macroscopic scales is not clear. In particular, the validity of the model using the elastic continuum around nano-cracks for stress concentration at the crack tip is not clear. Thus, we examined the difference of the stress distribution around the nano-crack which was estimated by molecular dynamics and by a continuum calculation. As a result, the discrepancy became remarkable at high strain. The stress concentration was not simulated by the elastic continuum model.
Akimura, Yuka; Maruyama, Toshiki; Yoshinaga, Naotaka*; Chiba, Satoshi
Acta Physica Hungarica A, 27(2-3), p.355 - 358, 2006/10
no abstracts in English
/SiC interfaceMiyashita, Atsumi; Yoshikawa, Masahito; Kano, Takuma; Onuma, Toshiharu*; Sakai, Takayuki*; Iwasawa, Misako*; Soneda, Naoki*
Annual Report of the Earth Simulator Center April 2004 - March 2005, p.287 - 291, 2005/12
Silicon carbide semiconductor device is expected to be used under a severe environment like the nuclear reactor and the space environment. On the semiconductor device interface, the electric charge state of the defect decides an electric characteristic. To emulate interfacial structure the 
interface structure is generated and the electronic geometry is decided by the first-principle molecular dynamics simulation with the earth simulator. The amorphous interface structure is made by medium-scale model of about 400 atoms. The heating temperature is 4000K, the heating time is 3.0ps, the speed of rapid cooling is -1000K/ps, and SiC movable layers in the interface are assumed to be 4 layers. In temperature 2200K the 
terminal was opened to make the layer more amorphous. The model has almost abrupt interface, however, some defects energy levels were still observed in the band gap. The energy levels are originated from interfacial oxygen. The localized electronic distribution of the dangling bond causes defect energy levels.
