Initialising ...

Initialising ...

Initialising ...

Initialising ...

Initialising ...

Initialising ...

Initialising ...

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

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：0 Percentile：0(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：6 Percentile：50.98(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：3 Percentile：28.74(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：4 Percentile：36.14(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：3 Percentile：28.45(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：9 Percentile：63.65(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：12 Percentile：73.42(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：69 Percentile：96.99(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.

Okita, Taira*; Koshiishi, Masato*; Hashimoto, Tsuneyuki*; Tanaka, Shigeaki*; Kodama, Mitsuhiro*; Kondo, Keietsu; Tsukada, Takashi

no journal, ,

no abstracts in English

Itakura, Mitsuhiro; Miyashiro, Satoshi*; Okita, Taira*

no journal, ,

There are very few studies for the development of interatomic potential for Austenitic steel, owing to the complexity of ternary system of Iron, Chromium and Nickel. In the present study we develop a preliminary version of an embedded atom method type potential for this ternary system. We have carried out first-principles calculations of 50 cases of randomly generated Fe23Cr5Ni4 configurations and observed relaxed crystal structure and total energy. Both the electron density and the crystal structure is found to be very close to that of a perfect FCC lattice system. By a least square fitting, we found that the energy difference between the different configurations is mainly explained by the number of nearest neighbor Cr-Cr and Ni-Ni pairs. The interaction energy between these atoms is on the order of 0.08 eV, and this result endorses the use of randomly generated configurations.

Itakura, Mitsuhiro; Miyashiro, Satoshi*; Okita, Taira*; Yamaguchi, Masatake; Okuda, Hiroshi*

no journal, ,

An embedded atom method (EAM) type interatomic potential for Fe-Cr-Ni alloys is presented. The potential is fitted to reproduce the density functional theory (DFT) calculation results of bulk properties in the FCC structure, such as lattice constant and elastic constants, based on the DFT results of randomly generated 60 atomic configurations. We show that the magnetic ordering of the periodicity (2a, 0, 0) in stabilizes the FCC lattice structure at zero temperature. To obtain the bulk properties of the stable FCC structure, all the elements are treated as non-magnetic in the DFT calculations. The validity of this non-magnetic treatment will also be discussed.

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 Zircaloy 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 interaction between solute iron atoms and self-interstitial Zr atoms to deduce the fundamental parameters for the modeling of irradiation effect. 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 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, ,

In this research we have constructed an atomistic model of three-dimensional motion of an interstitial cluster via a conservative climb toward a dislocation, which is a key process for void swelling, one of the serious material degradation for nuclear power plant. We also conducted a comparison of results obtained by the model with that by the molecular dynamics. Details of the prediction model and the comparison results will be given in the presentation. This study leads to provide new atomistic insight about the three-dimensional motion of an interstitial cluster, which had been unclear for a long time, and would certainly contribute to the prediction model of mechanical property degradation of nuclear materials.

Hayakawa, Sho*; Okita, Taira*; Itakura, Mitsuhiro; Haixuan, X.*; Osetsky, Y.*

no journal, ,

no abstracts in English

Okita, Taira*; Itakura, Mitsuhiro; Nakanishi, Daiki*; Kawabata, Tomoya*

no journal, ,