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Yamamoto, Yojiro*; Hayakawa, Sho*; Okita, Taira*; Itakura, Mitsuhiro
Computational Materials Science, 229, p.112389_1 - 112389_9, 2023/10
Times Cited Count:2 Percentile:29.99(Materials Science, Multidisciplinary)He bubbles are characteristic microstructures under fusion reactor conditions. They approach and coalesce through their own migration, which significantly impacts the microstructure and material properties. However, these processes, which involve multiple migrations of metal atoms, cannot be treated by molecular dynamics (MD) due to its timescale limitation. In this study, self-evolving atomistic kinetic Monte Carlo (SEAKMC) was used to expand the timescale and reproduce bubble coalescences in Fe. To enhance selections of events that led to the process by avoiding trivial events with an extremely low activation energy such as tiny vibrations of a He atom or short-range displacements of the Fe atom, we introduced two algorithms into SEAKMC, a two-step saddle point search for the former measure and setting a threshold for a displacement distance of the Fe atom for the latter. Furthermore, by adding another algorithm to set an upper bound for the activation energy to prevent selections of events with an impractically high activation energy, we succeeded to reproduce the change in the configuration from dumbbell to elliptical up to a simulated time of s, 8 orders longer than MD timescales. The developed method is effective for analyzing microstructures of metallic materials containing light elements and is the only method that can reach timescales comparable to those of experiments.
Hayakawa, Sho*; Yamamoto, Yojiro*; Okita, Taira*; Itakura, Mitsuhiro; Suzuki, Katsuyuki*
Computational Materials Science, 218, p.111987_1 - 111987_10, 2023/02
Times Cited Count:1 Percentile:7.18(Materials Science, Multidisciplinary)Tsugawa, Kiyoto*; Hayakawa, Sho*; Okita, Taira*; Aichi, Masaatsu*; Itakura, Mitsuhiro; Suzuki, Katsuyuki*
Computational Materials Science, 215, p.111806_1 - 111806_8, 2022/12
Times Cited Count:5 Percentile:32.41(Materials Science, Multidisciplinary)Tsugawa, Kiyoto*; Hayakawa, Sho*; Iwase, Yuki*; Okita, Taira*; Suzuki, Katsuyuki*; Itakura, Mitsuhiro; Aichi, Masaatsu*
Computational Materials Science, 210, p.111450_1 - 111450_9, 2022/07
Times Cited Count:15 Percentile:75.60(Materials Science, Multidisciplinary)Mori, Sho*; Matsuda, Nayuta*; Okita, Taira*; Aichi, Masaatsu*; Itakura, Mitsuhiro; Suzuki, Katsuyuki*
Materialia, 21, p.101371_1 - 101371_6, 2022/03
Okita, Taira*; Terayama, Satoshi*; Tsugawa, Kiyoto*; Kobayashi, Keita; Okumura, Masahiko; Itakura, Mitsuhiro; Suzuki, Katsuyuki*
Computational Materials Science, 202, p.110865_1 - 110865_9, 2022/02
Times Cited Count:7 Percentile:37.34(Materials Science, Multidisciplinary)Terayama, Satoshi*; Iwase, Yuki*; Hayakawa, Sho*; Okita, Taira*; Itakura, Mitsuhiro; Suzuki, Katsuyuki*
Computational Materials Science, 195, p.110479_1 - 110479_12, 2021/07
Times Cited Count:9 Percentile:46.15(Materials Science, Multidisciplinary)Hayakawa, Sho*; Doihara, Kohei*; Okita, Taira*; Itakura, Mitsuhiro; Aichi, Masaatsu*; Suzuki, Katsuyuki*
Journal of Materials Science, 54(17), p.11509 - 11525, 2019/09
Times Cited Count:17 Percentile:55.11(Materials Science, Multidisciplinary)Hayakawa, Sho*; Okita, Taira*; Itakura, Mitsuhiro; Kawabata, Tomoya*; Suzuki, Katsuyuki*
Journal of Materials Science, 54(16), p.11096 - 11110, 2019/08
Times Cited Count:13 Percentile:45.79(Materials Science, Multidisciplinary)Nakanishi, Daiki*; Kawabata, Tomoya*; Doihara, Kohei*; Okita, Taira*; Itakura, Mitsuhiro; Suzuki, Katsuyuki*
Philosophical Magazine, 98(33), p.3034 - 3047, 2018/09
Times Cited Count:10 Percentile:43.41(Materials Science, Multidisciplinary)By using the six sets of interatomic potentials for face-centredcubic metals that differ in the stacking fault energy (SFE) while most of the other material parameters are kept almost identical, we conducted molecular dynamics simulations to evaluate the effects of SFE on the defect formation process through collision cascades. The ratio of glissile SIA clusters tends to decrease with increasing SFE. This is because perfect loops, the edges of which split into two partial dislocations with stacking fault structures between them in most cases, prefer to form at lower SFEs. The enhanced formation of glissile SIA clusters at lower SFEs can also be observed even at increased temperature.
Hayakawa, Sho*; Okita, Taira*; Itakura, Mitsuhiro; Aichi, Masaatsu*; Suzuki, Katsuyuki*
Philosophical Magazine, 98(25), p.2311 - 2325, 2018/06
Times Cited Count:8 Percentile:36.78(Materials Science, Multidisciplinary)We conduct kinetic Monte Carlo simulations for the conservative climb motion of a cluster of self-interstitial atoms towards another SIA cluster in BCC Fe; the conservative climb velocity is inversely proportional to the fourth power of the distance between them, as per the prediction based on Einstein's equation. The size of the climbing cluster significantly affects its conservative climb velocity, while the size of the cluster that originates the stress field does not. The activation energy for the conservative climb is considerably greater than that derived in previous studies and strongly dependent on the climbing cluster size.
Doihara, Kohei*; Okita, Taira*; Itakura, Mitsuhiro; Aichi, Masaatsu*; Suzuki, Katsuyuki*
Philosophical Magazine, 98(22), p.2061 - 2076, 2018/05
Times Cited Count:22 Percentile:70.02(Materials Science, Multidisciplinary)In this study, molecular dynamics simulations were performed to elucidate the effects of stacking fault energy (SFE) on the physical interactions between an edge dislocation and a spherical void in the crystal structure of face-centred cubic metals at various temperatures and for different void sizes. Four different types of interaction morphologies were observed, in which (1) two partial dislocations detached from the void separately, and the maximum stress corresponded to the detachment of the trailing partial; (2) two partial dislocations detached from the void separately, and the maximum stress corresponded to the detachment of the leading partial; (3) the partial dislocations detached from the void almost simultaneously without jog formation; and (4) the partial dislocations detached from the void almost simultaneously with jog formation. With an increase in void size or SFE, the interaction morphology changed in the above-mentioned order. It was observed that the magnitude of the critical resolved shear stress (CRSS) and its dependence on the SFE were determined by these interaction morphologies. The value of the CRSS in the case of interaction morphology (1) is almost equal to an analytical one based on the linear elasticity by employing the Burgers vector of a single partial dislocation. The maximum value of the CRSS is also obtained by the analytical model with the Burgers vector of the two partial dislocations.
Okita, Taira*; Itakura, Mitsuhiro
Nihon Genshiryoku Gakkai-Shi ATOMO, 59(12), p.712 - 716, 2017/12
Molecular simulations for nuclear materials aim to reproduce atomistic-scale phenomena induced by irradiation and infer the change in material properties. In the present work, recent progress in this field is presented. In particular, the following three topics are explained: (1) Quantification of lattice defects formation process induced by fast neutron collision. (2) Identification of dislocation-channeling mechanism induced by interactions between defect clusters and dislocations. (3) Modeling of the three dimensional movement of defect clusters using molecular dynamics and kinetic Monte Carlo simulations.
Asari, Keisuke*; Hetland, O. S.*; Fujita, Satoshi*; Itakura, Mitsuhiro; Okita, Taira*
Journal of Nuclear Materials, 442(1), p.360 - 364, 2013/11
Times Cited Count:24 Percentile:85.58(Materials Science, Multidisciplinary)Molecular dynamics simulations were conducted using a set of six interatomic potentials for FCC metals that differed only in stacking fault energy (SFE), to clarify the effect of SFE on interactions between a dissociated edge dislocation and a void. There are two different types of interaction mechanism: separate depinning of the individual partial dislocations and almost simultaneous depinning of the combined partial dislocations. The interaction mechanism depends on both the SFE and void size, and changes the absolute value of the critical resolved shear stress (CRSS) and its dependence on the SFE. In the separate depinning case, the CRSS is relatively low and is almost independent of the SFE, while in the simultaneous case, the CRSS is increases with SFE. The void size for which the change in interaction mechanism occurs increases with decreasing SFE.
Itakura, Mitsuhiro; Kaburaki, Hideo; Yamaguchi, Masatake; Okita, Taira*
Acta Materialia, 61(18), p.6857 - 6867, 2013/10
Times Cited Count:143 Percentile:98.62(Materials Science, Multidisciplinary)The interaction between dislocations and impurity atoms in metals determines various properties of plastic deformation, such as the dependence of the yield stress on the impurity contents. Since the direct observation of atomistic structure of screw dislocation is almost impossible, several hypothetical assumptions have been employed to explain conveniently experimental observations. Recent advancement of computational hardware, as well as the development of elaborated techniques to reduce the size-effect in the first-principles calculation, enabled direct calculations of dislocation-impurity interaction. We have succeeded to evaluate the effect of hydrogen atoms on the dislocation mobility in iron.
Mori, Sho*; Matsuda, Nayuta*; Okita, Taira*; Itakura, Mitsuhiro
no journal, ,
no abstracts in English
Itakura, Mitsuhiro; Okita, Taira*
no journal, ,
Time to hydride precipitation in Zircalloy fuel cladding determines the lifetime of the fuel cladding of nuclear reactors. For the design of a new reactor with increased irradiation to transmute TRU elements and reduce waste, the effect of irradiation on the hydride formation must be evaluated. We carried out ab-initio calculations of vacancy cluster and the segregation of hydrogen to the cluster to deduce the fundamental parameters for the modeling of hydride formation. The results are indispensable for the design of the new reactor.
Itakura, Mitsuhiro; Okita, Taira*
no journal, ,
Time to hydride precipitation in Zircalloy fuel cladding determines the lifetime of the fuel cladding of nuclear reactors. For the design of a new reactor with increased irradiation to transmute TRU elements and reduce waste, the effect of irradiation on the hydride formation must be evaluated. We carried out ab-initio calculations of diffusion and interaction properties of lattice defects and solute atoms to deduce the fundamental parameters for the modeling of hydride formation. The results are indispensable for the design of the new reactor.
Hayakawa, Sho*; Okita, Taira*; Itakura, Mitsuhiro; Haixuan, X.*; Osetsky, Y.*
no journal, ,
no abstracts in English
Itakura, Mitsuhiro; Okita, Taira*; Nakamura, Hiroki
no journal, ,
no abstracts in English