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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

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.

Oral presentation

Simulation for temperature dependence of irradiation-induced grain-boundary phosphorus segregation using first-principles-based rate theory model

Ebihara, Kenichi; Suzudo, Tomoaki; Yamaguchi, Masatake; Nishiyama, Yutaka

no journal, , 

Since grain-boundary (GB) embrittlement by phosphorus (P) under irradiation is a crucial problem in steels, it is necessary to evaluate irradiation-induced P GB segregation in order to secure the integrity of nuclear structural materials. Recently, partial diffusion coefficients including the vacancy (V) drag effect, which are obtained by the kinetic Monte Carlo method incorporating the barrier energy estimated by the first-principles calculation, are built into the diffusion rate model, and the model is applied to the evaluation of the P GB segregation. Since the amount of V is affected by temperature, we evaluated the P GB segregation by the model. As a result, although the model could not simulate the increase of P GB segregation in the high temperature region which is shown by the McLean's diffusion model, the model incorporating the process of capturing and releasing P at GB simulated the increase. Hence, such a process is significant for simulating the temperature dependence.

Oral presentation

Numerical estimation of phosphorus transport for different migration modes in $$alpha$$-iron

Ebihara, Kenichi; Suzudo, Tomoaki; Yamaguchi, Masatake

no journal, , 

It is desired that segregation of phosphorus(P), which is known to to cause GB embrittlement in neutron-irradiated reactor pressure vessel steels, to grain-boundary is estimated for various irradiation conditions. In order to evaluate GB P segregation using the first-principles-based rate theory model, we evaluated the diffusion coefficient(DC) for mixed interstitial dumbbell(MID) in $$alpha$$-iron using the first-principles-based kinetic Monte Carlo(kMC) simulation, and compared it with those both for octahedral interstitial P(octP) atoms and for the vacancy(V) migration mode. As results, it was confirmed that DCs of MIDs and octP atoms are much larger than that of V migration mode. In addition, by applying the model incorporating the evaluated DCs to simulation of GB P segregation, it was found that the model needs to be modified. Furthermore, we will describe the investigation of the process of GB P segregation using molecular dynamics simulation.

Oral presentation

Simulation of irradiation-induced grain boundary phosphorus segregation by first-principles-based rate theory model including trapping and detrapping processes

Ebihara, Kenichi; Suzudo, Tomoaki; Yamaguchi, Masatake

no journal, , 

The quantitative estimation of phosphorus (P) segregation at grain boundaries (GBs) under irradiation is required to secure the integrity of nuclear structural materials. The model for estimating irradiation-induced GB P segregation is developed by considering P atoms migrating via octahedral interstitial sites and using the diffusion coefficients evaluated by kMC simulations based on the first-principles calculation. Because the trapping and the detrapping processes of P atoms at GBs are excluded in the model, however, the model cannot estimate the GB P coverage. In this study, we considered incorporating these processes into the model on the basis of the result of the MD simulations. We also examined the temperature dependence of GB P coverage. As results, the GB P segregation was successfully simulated by the model. We also obtained the result showing thermal segregation in the high-temperature region which cannot be simulated by the previous model.

Oral presentation

Computational study of phosphorus migration to grain boundary in $$alpha$$-iron

Ebihara, Kenichi; Suzudo, Tomoaki

no journal, , 

Grain boundary (GB) phosphorus (P) segregation is one factor for understanding embrittlement of reactor pressure vessel steels due to aging. Thus, the rate theory model for estimating GB P segregation by irradiation has been developed. In this study, for elucidating the segregation mechanism of P atoms to GBs, the P migration to a GB was simulated by molecular dynamics (MD), and the result of MD simulations was verified by the first-principles calculation. As a result, in the region near GB, a vacancy or a self-interstitial atom which migrates along with a P atom was absorbed by the GB and the P atom became substitutional. This phenomenon was confirmed by the first-principles calculation in the case of the mixed-interstitial dumbbell. This presentation is based on the paper [Modelling and Simulation in Materials Science and Engineering 26(2018) 065005].

Oral presentation

Rate theory model of phosphorus grain boundary segregation considering atomistic processes

Ebihara, Kenichi; Suzudo, Tomoaki; Yamaguchi, Masatake

no journal, , 

Phosphorus (P) atoms bring about grain boundary (GB) embrittlement in steel materials and can influence the rise of ductile-brittle transition temperature in reactor pressure vessel steels. Thus, a rate theory model for analyzing irradiation-induced GB P segregation is developed based on the atomistic processes. So far, we have incorporated the trapping process to the model based on the result of molecular dynamics (MD) simulations. However, the conventional model is used for the trapping process. In this study, we simulated the migration of a P atom in a GB. In addition, based on the consideration of the MD results, we modified the de-trapping model and applied the rate theory model to the temperature dependence of irradiation-induced GB P segregation. It was found that P atoms migrate through a gap in the GB region. In the calculated GB P segregation, the GB P coverage increased at T$$>$$600$$^{o}$$C and that the increase depended on the GB P segregation energy.

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