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Sato, Yuki*; Abe, Yosuke; Okubo, Kenji*; Tanioka, Takashi*
Philosophical Magazine, 102(12), p.1152 - 1172, 2022/00
Times Cited Count：0 Percentile：0.00(Materials Science, Multidisciplinary)Using in situ observation with high-voltage electron microscope, one-dimensional (1D) migration of self-interstitial atom (SIA) clusters under electron irradiation at 300 K was surveyed for copper materials of five nominal purities and TEM specimens prepared with three methods. In standard (STD) specimens that were prepared through cold rolling and annealing in the vacuum, formation and 1D migration of SIA clusters did not depend much on the nominal purity. In non-annealed (NA) specimens, prepared from high purity materials through mechanical processing and electropolishing, defect structure was coarser than in STD specimens. Results of bulk annealing (BA) specimens showed that annealing of as-received block material had minor effects. Above results were discussed based on the hypothesis that impurity atoms existing in as-received materials and those induced by annealing act as traps for SIA clusters.
-iron investigated using in situ high-voltage electron microscopyAbe, Yosuke; Sato, Yuki*; Hashimoto, Naoyuki*
Philosophical Magazine, 102(12), p.1173 - 1193, 2022/00
Times Cited Count：2 Percentile：30.40(Materials Science, Multidisciplinary)We measured the growth rate of self-interstitial atom (SIA) clusters in electron-irradiated -iron at 275-320 K using in situ high-voltage electron microscopy. To improve the statistical accuracy of the measurement, we used photographic films and video data. This enabled analysis of a considerable amount of data by extracting several SIA clusters and tracking their size growth using image processing techniques. By fitting the temperature-dependent cluster growth rate to the Arrhenius relations derived using rate theory analysis, we obtained vacancy migration energy of 
eV. In addition, the effects of impurities leading to decrease in the cluster growth rate were briefly discussed.
-iron estimated by in situ observation of interstitial clusters at low temperatures using high-voltage electron microscopyAbe, Yosuke; Sato, Yuki*; Hashimoto, Naoyuki*
Philosophical Magazine, 101(14), p.1619 - 1631, 2021/07
Times Cited Count：2 Percentile：16.40(Materials Science, Multidisciplinary)Modeling cluster dynamics or rate theory to describe the microstructural evolution of irradiated materials requires a precise knowledge of the migration energy of a self-interstitial atom (SIA), a product of energetic particle radiation. We measured the evolution of the number density of SIA clusters in electron-irradiated -iron at low temperatures (110-320 K) by in situ observation using high-voltage electron microscopy. We identified temperature-dependent physical quantities, including (1) the peak density of SIA clusters and (2) the critical defect-free zone thickness in a thin foil specimen, associated with interstitial mobility. By fitting these quantities to the Arrhenius relations derived by rate theory analysis, we obtained estimated interstitial migration energy values of 
and 
eV for (1) and (2), respectively.
-ironAbe, Yosuke; Sato, Yuki*; Hashimoto, Naoyuki*; Onuki, Somei*
Philosophical Magazine, 100(1), p.110 - 125, 2020/00
Times Cited Count：7 Percentile：37.52(Materials Science, Multidisciplinary)We derive analytical models associated with the experimentally revealed one-dimensional (1D) migration mechanisms to examine the decreasing behavior of the cluster number density. The model calculation indicates that the detrapping of the stationary SIA clusters causes the surface annihilation of the liberated SIA clusters, leading to the decrease in their number density. The decreasing behavior is in closer accordance with the experimental data when setting the impurity concentration in the same order as the estimation from the previous in situ HVEM experiment. This result suggests that the trapping and detrapping of the SIA clusters are the possible underlying processes for the decreasing behavior.
Ijiri, Yuta*; Ono, Naoko*; Ukai, Shigeharu*; Yu, H.*; Otsuka, Satoshi; Abe, Yosuke; Matsukawa, Yoshitaka*
Philosophical Magazine, 97(13), p.1047 - 1056, 2017/02
Times Cited Count：2 Percentile：11.31(Materials Science, Multidisciplinary)The interaction between oxide particles and dislocations in a 9Cr-ODS ferritic steel is investigated by both static and in-situ TEM observation under dynamic straining conditions and room temperature. The measured obstacle strength () of the oxide particles was no greater than 0.80 and the average was 0.63. The dislocation loops around some coarsened particles were also observed. The calculated obstacle strength by a stress formula of the Orowan interaction is nearly equaled to the average experimental value. Not only cross-slip system but also the Orowan interaction should be considered as the main interaction mechanism between oxide particles and dislocation in 9Cr-ODS ferritic steel.
-iron; Molecular dynamics simulationAbe, Yosuke; Tsuru, Tomohito; Shi, S.*; Ono, Naoko*; Ukai, Shigeharu*
Journal of Nuclear Science and Technology, 53(10), p.1528 - 1534, 2016/10
Times Cited Count：11 Percentile：71.04(Nuclear Science & Technology)Various types of nanometric defects such as voids and helium (He) bubbles produced by high energy neutron irradiations are known to degrade the mechanical properties of irradiated materials. In this study, we have evaluated the obstacle strength of He bubbles to the mobility of an edge dislocation in -iron for 2 and 4 nm bubbles with He-to-vacancy (He/V) ratios ranging from 0 to 1 at 300 and 500 K, by molecular dynamics simulation. Results showed that as the He/V ratio increases, the obstacle strength needed for the release of a dislocation from the bubble becomes stronger up to a moderate He/V ratio, and a further increase in the He/V ratio leads to weakening of the obstacle strength. For He/V = 1, the obstacle strengths are 
weaker than those at moderate He/V ratios depending on the bubble size and temperature. The extent of obstacle strength was found to be correlated with the dilation caused by He bubbles depending on the bubble size, He/V ratio, and temperature.
Sato, Yuki*; Abe, Yosuke; Abe, Hiroaki*; Matsukawa, Yoshitaka*; Kano, Sho*; Onuki, Somei*; Hashimoto, Naoyuki*
Philosophical Magazine, 96(21), p.2219 - 2242, 2016/06
Times Cited Count：12 Percentile：50.83(Materials Science, Multidisciplinary)We performed in situ observation of one-dimensional (1D) migration of self-interstitial atom (SIA) clusters in iron under electron irradiation at 110-300 K using high-voltage electron microscopy. Most 1D migration was stepwise positional changes of SIA clusters at irregular time intervals at all temperatures. The frequency of 1D migration did not depend on the irradiation temperature. It was directly proportional to the damage rate, suggesting that 1D migration was induced by electron irradiation. In contrast, the 1D migration distance depended on the temperature: distribution of the distance ranged over 100 nm above 250 K, decreased steeply between 250 and 150 K and was less than 20 nm below 150 K. The distance was independent of the damage rate at all temperatures. Next, we examined fluctuation in the interaction energy between an SIA cluster and vacancies of random distribution at concentrations 
-
, using molecular statics simulations. The fluctuation was found to trap SIA clusters of 4 nm diameter at vacancy concentrations higher than 
. We proposed that 1D migration was interrupted by impurity atoms at temperatures higher than 250 K, and by vacancies accumulated at high concentration under electron irradiation at low temperatures where vacancies are not thermally mobile.
Shi, S.*; Ono, Naoko*; Ukai, Shigeharu*; Abe, Yosuke
Materials Science & Engineering A, 617, p.61 - 65, 2014/11
Times Cited Count：2 Percentile：12.09(Nanoscience & Nanotechnology)Bubble dispersion strengthened copper (BDS-Cu) was synthesized by using the pyrolysis gaseous products of Poly (methyl methacrylate) (PMMA) during spark plasma sintering (SPS) process. Bubbles with an average diameter of 14 nm are densely dispersed in copper matrix with a dispersion distance 119 nm. Vickers' hardness of BDS-Cu is 400 MPa higher than that of pure copper. The strengthening effect is due to the dislocation pinning effect of bubbles.
-ironAbe, Yosuke
Computational Materials Science, 74, p.23 - 26, 2013/06
Times Cited Count：8 Percentile：27.78(Materials Science, Multidisciplinary)The diffusion process of a single self-interstitial atom (SIA) in -iron was studied by a hyper-molecular dynamics (hyper-MD) method that has been demonstrated to efficiently accelerate slow diffusive conformational transitions that occur in various materials. By adding a local bias potential, the acceleration of the dynamics becomes significantly higher at lower temperatures, e.g., on the order of 5 at 300 K, without changing the dynamics of the diffusion process as calculated using a conventional MD simulation. The validity of the results demonstrates that the hyper-MD method with an appropriate local bias potential can be applicable for determination of the diffusion process of SIAs in bulk materials.
Abe, Yosuke; Tsuru, Tomohito; Jitsukawa, Shiro
Materials Research Society Symposium Proceedings, Vol.1535 (Internet), 7 Pages, 2013/04
In this study, in addition to adequately model the 1D reaction kinetics of SIA loops in the framework of a production bias model, reaction kinetics associated with carbon impurity atoms present in -iron have been formulated to take into account the trapping effect of glissile SIA loops by vacancy-carbon (V-C) complexes that have been shown to have strong bindings with SIA loops by atomistic simulations. Results of calculated defect accumulation behavior of neutron irradiated -iron show that the developed CD model can successfully reproduce the number densities of SIA loops and vacancy clusters when the applied impurity concentration is the same order as experimental one. This indicates that the assumed mechanism for the trapping of glissile SIA loops by V-C complexes is reasonable. The dependences of irradiation dose, dose rate, and temperature are discussed in detail.
-iron under cascade damage conditionAbe, Yosuke; Jitsukawa, Shiro; Okubo, Nariaki; Matsui, Hideki*; Tsukada, Takashi
Effects of Radiation on Nuclear Materials; 25th Volume (ASTM STP 1547), p.313 - 337, 2013/01
Times Cited Count：0 Percentile：0.10(Materials Science, Characterization & Testing)It is known that the mechanical properties degradation of reactor pressure vessel steels caused by neutron irradiation is partly due to the formation of nanometer-size solute and point-defect (PD) clusters. Therefore, the rationalization of radiation induced effects on the microstructure and their consequences on the material properties by developing predictive models is thus of great importance. Cluster dynamics (CD) simulation with rate equations has been used to estimate the long-term evolution of point defect clusters. We have extended the CD simulation code to allow hopping motion for all the self-interstitial atom (SIA) clusters to be mobile. Results of calculation on 316 stainless steel and -iron have been compared. The difference and underlying mechanism of microstructural evolution between austenitic stainless steel and -iron is discussed with regard to the dose dependence.
-ironAbe, Yosuke; Suzudo, Tomoaki; Jitsukawa, Shiro; Tsuru, Tomohito; Tsukada, Takashi
Fusion Science and Technology, 62(1), p.139 - 144, 2012/07
Times Cited Count：10 Percentile：60.07(Nuclear Science & Technology)Static and dynamic interactions between SIA loops and carbon impurities as a major impurity in -iron was studied using atomic-scale computer simulations to clarify possible mechanisms of trapping of SIA loops by carbon impurities. It was found that bindings of carbon-vacancy complexes with SIA loops are so strong when they locate inside the loop habit plane. This situation would be realized below 
450 K at which carbon-vacancy complexes become to dissociate. At higher temperatures, it can be expected that the dissociated interstitial carbon atoms tend to migrate towards the loop periphery due to the internal stress field associated with SIA loops, and this could attribute to the continuous 1D motion of SIA loops with dragging impurities as observed by experiments. A long-term microstructural evolution induced by radiation damage was also simulated by taking into account the trapping and detrapping rates of SIA loops deduced from the knowledge obtained in this study.
Tsuru, Tomohito; Abe, Yosuke; Suzuki, Chikashi; Nakazawa, Tetsuya; Kaji, Yoshiyuki; Tsukada, Takashi
Journal of Nuclear Materials, 417(1-3), p.1054 - 1057, 2011/10
Times Cited Count：1 Percentile：10.66(Materials Science, Multidisciplinary)Reduced activation ferritic steel is one of the leading structural material candidates for a nuclear fusion reactor. Since the solute impurities have an effect on the embrittlement through segregation under irradiation, the stability of impurity elements should be elaborated. In the present study the segregation characteristics of tungsten and some general solute impurities in bcc iron were investigated nonempirically by first principle calculations, where the equilibrium segregation was considered via a regular solution model and the change in enthalpy for segregation were directly evaluated for comparison. Subsequently the energetic stabilities of impurity-impurity and impurity-vacancy pair were evaluated. The segregation enthalpy is influenced by the electronic interaction between the d-electron of Fe and the outer electron of the impurity element, and molybdenum and tungsten tend to prevent from the impurity segregation.
-ironTsuru, Tomohito; Abe, Yosuke; Kaji, Yoshiyuki; Tsukada, Takashi; Jitsukawa, Shiro
Zairyo, 59(8), p.583 - 588, 2010/08
The size- and spacing- dependent obstacle strength due to the Cu precipitation in -Fe is investigated by atomistic simulations, in which the effect on phase transformation of Cu precipitation is considered by a conventional self-guided molecular dynamics (SGMD) method that has an advantage to enhance the conformational sampling efficiency in MD simulations. It was shown that the SGMD method can accelerate calculating the bcc to 9R structure transformation of a small precipitate, enabling the transformation without introducing any excess vacancies. Such metallographic structures increase the obstacle strength through strong pinning effects as a result of the complicated atomic rearrangement within the Cu precipitation.
Abe, Yosuke; Jitsukawa, Shiro
Philosophical Magazine Letters, 89(9), p.535 - 543, 2009/09
Times Cited Count：5 Percentile：36.20(Materials Science, Multidisciplinary)The self-guided molecular dynamics (SGMD) method which can enhance the conformational sampling efficiency in MD simulations, was applied in investigating the phase transformation of Cu precipitate in -iron that took place during thermal ageing. It was shown that the SGMD method can accelerate calculating the bcc to 9R structure transformation of a small precipitate (even 4.0 nm in size), enabling the transformation without introducing any excess vacancies. The size dependence of the transformation also agreed with that seen in previous experimental studies.
-iron from a combination of Langevin molecular dynamics and the basin-hopping techniqueAbe, Yosuke; Jitsukawa, Shiro
Philosophical Magazine, 89(4), p.375 - 388, 2009/02
Times Cited Count：9 Percentile：48.27(Materials Science, Multidisciplinary)A combination of simulated annealing with Langevin molecular dynamics and basin-hopping with occasional jumping (BHOJ) technique was used to systematically determine the most stable configurations of self-interstitial atom (SIA) clusters 
(
1-38) in -iron. In addition to the original BHOJ technique, we introduced an additional long jumping process in which a randomly selected less-bounded atom is moved to a neighboring site of another SIA in the cluster to enhance the probability of locating the global minimum structure. With the obtained putative lowest energy structures, the binding energies as a function of cluster size were estimated. We also determined the sizes of particular stable clusters based on their geometrical symmetry. Furthermore, the values were extrapolated based on accurately determined formation energies, and are available for immediate use in kinetic Monte Carlo or rate theory models.
Abe, Yosuke; Jitsukawa, Shiro; Tsukada, Takashi
no journal, ,
In the present study, one-dimensional (1D) motion of SIA-clusters was modeled in the framework of production bias model because the 1D motion of SIA clusters is known to greatly affect the microstructural evolution in irradiated materials. The trapping effect of 1D migrating SIA clusters by impurity-vacancy clusters was also taken into account to model the detailed underlying physical process which can take place at relatively low temperature in real materials. This model has no fitting procedures that are often applied in previous studies to reproduce experimental data. It was shown that the developed model with the impurity concentration of 10-100 appm, that is the same order as experimental one, can accurately reproduce experimental data, suggesting that the agglomeration and growth processes of point-defect clusters and the underlying mechanism of microstructural evolution are well described in this model.
-ironAbe, Yosuke; Tsuru, Tomohito; Jitsukawa, Shiro
no journal, ,
A combination of simulated annealing with Langevin molecular dynamics and basin-hopping with occasional jumping (BHOJ) technique has been applied to global optimization and used to determine systematically the most stable configurations of self-interstitial atom (SIA) clusters with and without carbon (C) impurity in -iron. With the obtained putative lowest energy structures, the binding energies with and without C atoms as a function of cluster size were estimated. As a consequence, we have derived the extrapolated values based on the accurately determined binding energies which are more relevant conditions to real materials.
Abe, Yosuke; Jitsukawa, Shiro
no journal, ,
We investigated effects of the mobility of SIA clusters on the microstructural evolution under cascade irradiation conditions in austenitic stainless steel using the cluster dynamics simulation code extended in this study. It was shown that taking into account the mobility of relatively small SIA clusters can significantly improve the accuracy of predicting the irradiation dose and temperature dependences of the number density and size of SIA clusters.
Abe, Yosuke; Jitsukawa, Shiro
no journal, ,
During ageing, Cu precipitates were observed initially coherent and on further growth they begin to transform by the sequence of bcc-9R-3R-fcc. Since structures of Cu precipitates are considered to be relevant to embrittlement, it is important to understand the mechanisms and conditions of precipitate transformation. In this work we investigate the dependency of the size of Cu precipitate and vacancy concentration on the structures of the precipitate by using the combination of the simulated annealing and molecular dynamics method. We firstly demonstrated that the phase transformation to 9R takes place in diameter of 5-6 nm for pure-Cu precipitate. Moreover, for the precipitate with a vacancy concentration 5at.%, the phase transformation can occur even in 4-5 nm assisted by the diffusional contribution of vacancies and nucleation of fcc embryos by them. This precipitate size that the phase transformation starts to occur is in good agreement with the experimentally observed one.
