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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:0 Percentile:100(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: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 $$alpha$$-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:2 Percentile:39.48(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

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:52.74(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:5 Percentile:36.84(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:92 Percentile:0.96(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

 Times Cited Count:1 Percentile:90.63(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


 Times Cited Count:4 Percentile:36.35(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.

Journal Articles

Improving atomic displacement and replacement calculations with physically realistic damage models

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:93 Percentile:1.39(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 $$sim$$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.

Journal Articles

Recent advances in modeling and simulation of the exposure and response of tungsten to fusion energy conditions

Marian, J.*; Becquart, C. S.*; Domain, C.*; Dudarev, S. L.*; Gilbert, M. R.*; Kurtz, R. J.*; Mason, D. R.*; Nordlund, K.*; Sand, A. E.*; Snead, L. L.*; et al.

Nuclear Fusion, 57(9), p.092008_1 - 092008_26, 2017/06

 Times Cited Count:67 Percentile:0.47(Physics, Fluids & Plasmas)

Under the anticipated operating conditions for demonstration magnetic fusion reactors beyond ITER, structural materials will be exposed to unprecedented conditions of irradiation, heat flux, and temperature. While such extreme environments remain inaccessible experimentally, computational modeling and simulation can provide qualitative and quantitative insights into materials response and complement the available experimental measurements. For plasma facing components such as the first wall and the divertor, tungsten (W) has been selected as the best candidate material due to its superior high-temperature and irradiation properties. In this paper we provide a review of recent efforts in computational modeling of W both as a plasma-facing material as well as a bulk structural material subjected to fast neutron irradiation. We highlight several of the most salient findings obtained via computational modeling and point out a number of remaining future challenges.

Journal Articles

Modeling of Phosphorus Transport by Interstitial Dumbbell in $$alpha$$-Iron Using First-Principles-Based Kinetic Monte Carlo

Ebihara, Kenichi; Suzudo, Tomoaki; Yamaguchi, Masatake

Materials Transactions, 58(1), p.26 - 32, 2017/01

 Times Cited Count:4 Percentile:65.01(Materials Science, Multidisciplinary)

In order to evaluate grain boundary (GB) phosphorous (P) segregation in nuclear reactor pressure vessel steels under irradiation, the rate-theory model based on first-principles calculations is developed. In this study, we evaluated the diffusion coefficient of the mixed interstitial dumbbell of a P atom and an iron(Fe) atom using a kinetic Monte Carlo (kMC) simulation based on first-principles calculations. The evaluated diffusion coefficient was almost the same with the diffusion coefficient of P atoms which migrate via octahedral interstitial sites, and was much faster than that for P transport by vacancies. Furthermore, from the simulation of the irradiation induced GB P segregation using the model which was modified to include P atoms of octahedral interstitial sites, it was found that the boundary condition at GB is not valid for P atoms of octahedral interstitial sites

Journal Articles

Suppression of radiation-induced point defects by rhenium and osmium interstitials in tungsten

Suzudo, Tomoaki; Hasegawa, Akira*

Scientific Reports (Internet), 6, p.36738_1 - 36738_6, 2016/11

 Times Cited Count:15 Percentile:39.32(Multidisciplinary Sciences)

Modeling of the evolution of radiation-induced defects is important for finding radiation-resistant materials, which would be greatly appreciated in nuclear applications. We apply the first principles method combined with kinetic Monte Carlo to indicate a mechanism to mitigate the effect of radiation by adding particular solute elements that change the migration dimension of interstitials in W crystals. The resultant mechanism is applicable to any body-centered-cubic (BCC) metals whose SIAs have one-dimensional (1D) motion and is expected to provide a general guideline for computational design of radiation-resistant alloys in the field of nuclear applications.

Journal Articles

The Two-step nucleation of G-phase in ferrite

Matsukawa, Yoshitaka*; Takeuchi, Tomoaki; Kakubo, Yuta*; Suzudo, Tomoaki; Watanabe, Hideo*; Abe, Hiroaki*; Toyama, Takeshi*; Nagai, Yasuyoshi*

Acta Materialia, 116, p.104 - 113, 2016/09

 Times Cited Count:48 Percentile:3.34(Materials Science, Multidisciplinary)

Atom probe tomography (APT) and TEM were combined for identifying the stage at which solute clusters transform into compounds crystallographically distinct from the matrix, in the precipitation of the G-phase (Ni$$_{16}$$Si$$_{7}$$Mn$$_{6}$$) from ferrite solid solution subjected to isothermal annealing at 673 K. Based on a systematic analysis of solute clusters as a function of annealing time, the nucleation of the G-phase was found to occur via a two-step process. Moreover, the structural change was found to occur via another two-step process. There was a time lag between the end of cluster growth to become a critical size and the start of the structural change. During the incubation period solute enrichment occurred inside the clusters without further size growth, indicating that the nucleation of the G-phase occurs at the critical size with a critical composition. Judging from the results of APT, TEM and the simulation of electron diffraction patterns, the critical composition was estimated to be Ni$$_{16}$$Si$$_{3.5}$$(Fe,Cr)$$_{3.5}$$Mn$$_{6}$$.

Journal Articles

Migration of rhenium and osmium interstitials in tungsten

Suzudo, Tomoaki; Yamaguchi, Masatake; Hasegawa, Akira*

Journal of Nuclear Materials, 467(Part 1), p.418 - 423, 2015/12


 Times Cited Count:29 Percentile:3.12(Materials Science, Multidisciplinary)

Tungsten is expected to be a promising plasma-facing material for future fusion devices, but radiation-induced precipitation (RIP), which leads the material to hardening, is a concern at their practical use. One of the keys to accurate prediction of the emergence of RIP is migration of solute atoms, rhenium and osmium, that are produced by nuclear transmutation through irradiation. We conduct a series of numerical simulations using an atomic kinetic Monte Carlo method and investigate the migration of these solute atoms in the form of tungsten-rhenium and tungsten-osmium mixed dumbbells, considered to be the most efficient "carriers" of the solute atoms. We find that the low rotation energy barrier of these mixed dumbbells leading to three-dimensional migration greatly influences their diffusivities. The result also suggests that, although these dumbbells have three-dimensional motion, one cannot simply reduce their migration behavior to that of vacancy-like spherical objects.

Journal Articles

Simulation of He embrittlement at grain boundaries of bcc transition metals

Suzudo, Tomoaki; Yamaguchi, Masatake

Journal of Nuclear Materials, 465, p.695 - 701, 2015/10

 Times Cited Count:9 Percentile:27.16(Materials Science, Multidisciplinary)

To investigate what atomic properties largely determine vulnerability to He embrittlement at grain boundaries (GB) of bcc transition metals, we introduced a computational model composed of first principles density functional theory and a He segregation rate theory model. Predictive calculations of He embrittlement at the first wall of the future DEMO fusion concept reactor indicated that variation in the He embrittlement is originated not only from He production rate related to neutron irradiation, but also from the He segregation energy at the GB that has a systematic trend in the periodic table.

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