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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

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

Molecular dynamics study of phosphorus migration in $$Sigma$$3(111) and $$Sigma$$5(0-13) grain boundaries of $$alpha$$-iron

Ebihara, Kenichi; Suzudo, Tomoaki

Metals, 12(4), p.662_1 - 662_10, 2022/04

Phosphorus atoms in steels accumulate at grain boundaries via thermal and/or irradiation effects and induce grain boundary embrittlement. Quantitative prediction of phosphorus segregation at grain boundaries under various temperature and irradiation conditions is therefore essential for preventing embrittlement. To develop a model of grain boundary phosphorus segregation in $$alpha$$-iron, we studied the migration of a phosphorus atom in two types of symmetrical tilt grain boundaries ($$Sigma$$3[1-10](111) and $$Sigma$$5[100](0-13) grain boundaries) using molecular dynamics simulations with an embedded atom method potential. The results revealed that, in the $$Sigma$$3 grain boundary, phosphorus atoms migrate three-dimensionally mainly in the form of interstitial atoms, whereas in the $$Sigma$$5 grain boundary, these atoms migrate one-dimensionally mainly via vacancy-atom exchanges. Moreover, de-trapping of phosphorus atoms and vacancies was investigated.

Journal Articles

Suppression of vacancy formation and hydrogen isotope retention in irradiated tungsten by addition of chromium

Wang, J.*; Hatano, Yuji*; Toyama, Takeshi*; Suzudo, Tomoaki; Hinoki, Tatsuya*; Alimov, V. Kh.*; Schwarz-Selinger, T.*

Journal of Nuclear Materials, 559, p.153449_1 - 153449_7, 2022/02

To study the effect of the content of chromium (Cr) in the tungsten (W) matrix on the vacancy formation and retention of hydrogen isotopes, the samples of the W-0.3Cr alloy were irradiated with 6.4 MeV Fe ions in the temperature range of 523-1273 K. These displacement-damaged samples were exposed to D$$_{2}$$ gas at a temperature of 673 K. The addition of 0.3% Cr into the W matrix resulted in a significant decrease in the retention of deuterium compared to pure W after irradiation especially at high temperature. Positron lifetime for W-0.3Cr alloy irradiated at 1073 K was almost similar to that for non-irradiated one. These facts indicate the suppression of the formation of vacancy-type defects by 0.3% Cr addition.

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

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

Perspectives on multiscale modelling and experiments to accelerate materials development for fusion

Gilbert, M. R.*; Arakawa, Kazuto*; Suzudo, Tomoaki; Tsuru, Tomohito; 26 of others*

Journal of Nuclear Materials, 554, p.153113_1 - 153113_31, 2021/10

Modelling will continue to do so until the first generation of fusion power plants come on line and allow long-term behaviour to be observed. In the meantime, the modelling is supported by experiments that attempt to replicate some aspects of the eventual operational conditions. In 2019, a group of leading experts met under the umbrella of the IEA to discuss the current position and ongoing challenges in modelling of fusion materials and how advanced experimental characterisation is aiding model improvement. This review draws from the discussions held during that workshop. Topics covering modelling of irradiation-induced defect production and fundamental properties, gas behaviour, clustering and segregation, defect evolution and interactions are discussed, as well as new and novel multiscale simulation approaches, and the latest efforts to link modelling to experiments through advanced observation and characterisation techniques.

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:0 Percentile:0(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

Inclination of self-interstitial dumbbells in molybdenum and tungsten; A First-principles study

Suzudo, Tomoaki; Tsuru, Tomohito

AIP Advances (Internet), 11(6), p.065012_1 - 065012_7, 2021/06

 Times Cited Count:0

In the current study, we analyzed the self-interstitial atoms (SIAs) in BCC molybdenum (Mo) and tungsten (W) in comparison with other BCC transition metals utilizing first-principles method; particularly, we focused on uncommon dumbbells, whose direction are inclined from $$<$$111$$>$$ toward $$<$$110$$>$$ on the {110} plane. Such a direction is not stable neither in the group 5 BCC metals (i.e., vanadium, niobium, and tantalum) nor in $$alpha$$-iron. Our first-principles relaxation simulations indicated that inclined dumbbells were more energetically-favored than common $$<$$111$$>$$ dumbbells in Mo, while this is not necessarily the case for W. However, under a certain degree of lattice strain, such as shear or expansive strain, could make inclined dumbbells more favored also in W, suggesting that the lattice strain can substantially influence the migration barrier of SIAs in these metals because inclined dumbbells generally have a larger migration barrier than $$<$$111$$>$$ dumbbells.

Journal Articles

Computational study of solute effects in tungsten under irradiation

Suzudo, Tomoaki

Materials Science Forum, 1024, p.87 - 94, 2021/03

Tungsten (W) is suitable for solid targets of spallation neutron source due to its high neutron yield. The prediction of radiation effects of W is, therefore, of importance; especially, the influence of solute elements are complex and are not clearly known to date. We discuss here the solute effects using the first principles and kinetic Monte Carlo calculations and show that rhenium (Re) and osmium (Os), which are nuclear transmutation products of W, can largely change the stability and mobility of radiation defects. Such influences of the solute elements seem to explain the unsolved mechanism of the microstructural evolution of W-based materials under irradiation.

Journal Articles

Numerical interpretation of hydrogen thermal desorption spectra for iron with hydrogen-enhanced strain-induced vacancies

Ebihara, Kenichi; Sugiyama, Yuri*; Matsumoto, Ryosuke*; Takai, Kenichi*; Suzudo, Tomoaki

Metallurgical and Materials Transactions A, 52(1), p.257 - 269, 2021/01

 Times Cited Count:2 Percentile:45.37(Materials Science, Multidisciplinary)

We simulated the thermal desorption spectra of a small-size iron specimen to which was applied during charging with hydrogen atoms using a model incorporating the behavior of vacancies and vacancy clusters. The model considered up to vacancy clusters $$V_9$$, which is composed of nine vacancies and employed the parameters based on atomistic calculations, including the H trapping energy of vacancies and vacancy clusters that we estimated using the molecular static calculation. As a result, we revealed that the model could, on the whole, reproduced the experimental spectra except two characteristic differences, and also the dependence of the spectra on the aging temperature. By examining the cause of the differences, the possibilities that the diffusion of clusters of $$V_2$$ and $$V_3$$ is slower than the model and that vacancy clusters are generated by applying strain and H charging concurrently were indicated.

Journal Articles

Brittle-fracture simulations of curved cleavage cracks in $$alpha$$-iron; A Molecular dynamics study

Suzudo, Tomoaki; Ebihara, Kenichi; Tsuru, Tomohito

AIP Advances (Internet), 10(11), p.115209_1 - 115209_8, 2020/11

 Times Cited Count:2 Percentile:24.67(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 $$alpha$$-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.

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:1 Percentile:39.17(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

Molecular dynamics study of phosphorus migration in $$Sigma$$5 grain boundary of $$alpha$$-iron

Ebihara, 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 $$Sigma$$3 symmetrical tilt GB using molecular dynamics (MD). In the current study, we also simulated the P migration in $$Sigma$$5 GB using MD and compared the result with that of $$Sigma$$3. As a result, at 800K, it was found that a P atom cannot migrate in $$Sigma$$5 without vacancies while a P atom can migrate between iron atoms in $$Sigma$$3.

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

Molecular dynamics simulations of phosphorus migration in a grain boundary of $$alpha$$-iron

Ebihara, Kenichi; Suzudo, Tomoaki

TMS 2020; 149th Annual Meeting & Exhibition Supplemental Proceedings, p.995 - 1002, 2020/02

 Times Cited Count:0 Percentile:0.11

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 $$Sigma$$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.

Journal Articles

Anomalous solution softening by unique energy balance mediated by kink mechanism in tungsten-rhenium alloys

Tsuru, Tomohito; Wakeda, Masato*; Suzudo, Tomoaki; Itakura, Mitsuhiro; Ogata, Shigenobu*

Journal of Applied Physics, 127(2), p.025101_1 - 025101_9, 2020/01

 Times Cited Count:1 Percentile:24.67(Physics, Applied)

We explored softening/strengthening behavior in various solute in W matrix by density functional theory (DFT) calculations combined with solid solution model. As a result of DFT calculations for various solute, a clear trend was observed in the interaction energy between a solute and a screw dislocation, which has predominant influence on solid solution behavior. The predictions based on the solid solution model with DFT can reasonably reproduce the complicated softening/strengthening as a function of temperature and solute concentration. We conclude that this specific balance is the origin of macroscopic solid solution softening.

Journal Articles

Analyzing the cross slip motion of screw dislocations at finite temperatures in body-centered-cubic metals; Molecular statics and dynamics studies

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:7 Percentile:63.45(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.

Journal Articles

Primary radiation damage; A Review of current understanding and models

Nordlund, K.*; Zinkle, S. J.*; Sand, A. E.*; Granberg, F.*; Averback, R. S.*; Stoller, R. E.*; Suzudo, Tomoaki; Malerba, L.*; Banhart, F.*; Weber, W. J.*; et al.

Journal of Nuclear Materials, 512, p.450 - 479, 2018/12

 Times Cited Count:141 Percentile:99.04(Materials Science, Multidisciplinary)

Scientific understanding of any kind of radiation effects starts from the primary damage. We consider the extensive experimental and computer simulation studies that have been performed over the past several decades on what the nature of the primary damage is. We review both the production of crystallographic or topological defects in materials as well as radiation mixing, i.e. the process where atoms in perfect crystallographic positions exchange positions with other ones in non-defective positions. We also consider the recent effort to provide alternatives to the current international standard for quantifying this energetic particle damage, the Norgett-Robinson-Torrens displacements per atom (NRT-dpa) model for metals. We present in detail new complementary displacement production estimators ("athermal recombination corrected dpa": arc-dpa) and atomic mixing ("replacements per atom": rpa) functions that extend the NRT-dpa, and discuss their advantages and limitations.

Journal Articles

Atomistic simulation of phosphorus segregation to $$Sigma$$3(111) symmetrical tilt grain boundary in $$alpha$$-iron

Ebihara, Kenichi; Suzudo, Tomoaki

Modelling and Simulation in Materials Science and Engineering, 26(6), p.065005_1 - 065005_10, 2018/09

AA2018-0168.pdf:2.74MB

 Times Cited Count:2 Percentile:16.24(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 $$Sigma$$3(111) symmetrical tilt grain boundary in $$alpha$$-iron using molecular dynamics. We found that, in the vicinity of the grain boundary within $$sim$$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.

Journal Articles

First-principles calculations of interaction between solutes and dislocations in tungsten

Tsuru, Tomohito; Suzudo, Tomoaki

Nuclear Materials and Energy (Internet), 16, p.221 - 225, 2018/08

Changes in mechanical properties due to transmutation products (Re and Os) in W alloys is a central issue for plasma-facing materials in fusion reactors. We implemented density function theory calculations to investigate the effect of Re, Os, and other 5d solutes on dislocation core structure and motion associated with plastic deformation. Ir, Pt, Au, and Hg solutes show strong attractive interactions with screw dislocations, causing solution strengthening by the pinning mechanism. On the other hand, Hf, Ta, and Re cause softening by facilitating dislocation motion around solutes. This prediction corresponds well with the experimental observation of softening behavior in W-Re alloys.

Journal Articles

First-principles study of solvent-solute mixed dumbbells in body-centered-cubic tungsten crystals

Suzudo, Tomoaki; Tsuru, Tomohito; Hasegawa, Akira*

Journal of Nuclear Materials, 505, p.15 - 21, 2018/07

AA2017-0485.pdf:0.51MB

 Times Cited Count:6 Percentile:56.63(Materials Science, Multidisciplinary)

Tungsten (W) is considered as a promising candidate for plasma-facing materials for future nuclear fusion devices, and selecting optimal alloying constituents is a critical issue to improve radiation resistance of the W alloys as well as to improve their mechanical properties. We conducted in the current study a series of first-principles calculations for investigating solvent-solute mixed dumbbells in W crystals. The results suggested that titanium (Ti), vanadium (V), and chromium (Cr) are favorable as solutes for W alloys from irradiation-effect perspectives because these elements are expected to promote vacancy-interstitial recombination without causing radiation-induced precipitation that reduces ductility of irradiated materials.

193 (Records 1-20 displayed on this page)