Kinetic Monte Carlo Simulations of Helium Cluster Nucleation in Tungsten with Preexisting Vacancies

Yang, Zhangcan; Blondel, Sophie; Hammond, Karl D.; Wirth, Brian D.

doi:10.13182/fst16-111

Cited by 22 publications

(17 citation statements)

References 44 publications

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“…For efficient computation of material modification on time and length scales relevant for ITER, these nano-scale processes must be parameterised in such a way that they can be incorporated into higher scale models without the need to model them explicitly. This requires detailed atomistic modelling, typically with Monte-Carlo simulations performed on systems of hundreds or thousands of atoms [1][2][3], combined with empirical verification of those models to ensure that the conclusions are relevant in real materials that are invariably much larger and more complex.…”

Section: Introductionmentioning

confidence: 99%

Effect of W self-implantation and He plasma exposure on early-stage defect and bubble formation in tungsten

et al. 2018

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To determine the effect of pre-existing defects on helium-vacancy cluster nucleation and growth, tungsten samples were self-implanted with 1 MeV tungsten ions at varying fluences to induce radiation damage, then subsequently exposed to helium plasma in the MAGPIE linear plasma device. Positron annihilation lifetime spectroscopy was performed both immediately after self-implantation, and again after plasma exposure. After self-implantation vacancies clusters were not observed near the sample surface (<30 nm). At greater depths (30-150 nm) vacancy clusters formed, and were found to increase in size with increasing W-ion fluence. After helium plasma exposure in the MAGPIE linear plasma device at ~300 K with a fluence of 10 23 Hem -2 , deep (30-150 nm) vacancy clusters showed similar positron lifetimes, while shallow (<30 nm) clusters were not observed. The intensity of positron lifetime signals fell for most samples after plasma exposure, indicating that defects were filling with helium. The absence of shallow clusters indicates that helium requires pre-existing defects in order to drive vacancy cluster growth at 300 K. Further samples that had not been pre-damaged with W-ions were also exposed to helium plasma in MAGPIE across fluences from 1x10 22 to 1.2x10 24 Hem -2. Samples exposed to fluences up to 1x10 23 Hem -2 showed no signs of damage. Fluences of 5x10 23 Hem -2 and higher showed significant helium-cluster formation within the first 30 nm, with positron lifetimes in the vicinity 0.5-0.6 ns. The sample temperature was significantly higher for these higher fluence exposures (~400 K) due to plasma heating. This higher temperature likely enhanced bubble formation by significantly increasing the rate interstitial helium clusters generate vacancies, which is we suspect is the rate-limiting step for helium-vacancy cluster/bubble nucleation in the absence of pre-existing defects.

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Section: Introductionmentioning

confidence: 99%

Effect of W self-implantation and He plasma exposure on early-stage defect and bubble formation in tungsten

et al. 2018

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“…Tungsten exposed to plasma under appropriate conditions results in structural defects and irregular microstructure such as fuzz and blistering [9,10]. Since such defects form on length and time scales that are much too long for atomistic simulations, coarse-grained simulations such as kinetic Monte Carlo [11] and continuumbased simulations such as cluster dynamics [12,13] will be important to future studies of helium transport in fusion-relevant materials. An important step in this process is the interaction of helium with grain boundaries, including the rates of transport of helium clusters that interact with grain boundaries.…”

Section: Introductionmentioning

confidence: 99%

Helium in-plane migration behavior on 〈1 0 0〉 symmetric tilt grain boundaries in tungsten

Yang¹,

Hammond²

2018

J. Phys.: Condens. Matter

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We present the results of an atomistic modeling study of small helium cluster migration in the plane of symmetric tilt grain boundaries. The relevant migration pathways and energies were determined by way of temperature accelerated dynamics and the nudged elastic band method. We find that small helium clusters show much higher migration energies when bound to the grain boundary than in the bulk for all types of grain boundaries, indicating strongly-impeded helium transport behavior. Larger helium clusters (up to three helium atoms) tend to have higher migration energies compared with smaller clusters. Longer-distance migrations also tend to have higher migration energies, but helium cluster migration is highly affected by the structure of the grain boundary. The binding energy of the grain boundaries studied is high enough that helium clusters would be unlikely to leave the grain boundary plane. However, vacancy migration energies are relatively low compared to the bulk, and are also much lower than helium cluster migration energies on the grain boundary plane. This suggests that helium cluster migration on the grain boundary is actually governed by the rate of vacancy migration: in the bulk, helium clusters are mobile, but they become bound to and immobilized by grain boundaries, forming bubbles. Bubbles, however, are likely more mobile on the grain boundary than they are in the bulk due to the increased rate of vacancy migration on the grain boundary. We expect similar migration behavior for other types of grain boundaries because of the increased excess volume found near all grain boundaries.

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“…Since a change in the chemical potentials affects only the cluster concentrations in Eq. 3, KineCluE is a crucial step in developing a general and efficient framework to provide accurate atomic-based kinetic properties for higher-scale models such as object kinetic Monte Carlo [58][59][60][61][62][63][64], cluster dynamics [65][66][67][68] or phasefield models [69][70][71].…”

Section: Cluster Transport Coefficientsmentioning

confidence: 99%

KineCluE: A kinetic cluster expansion code to compute transport coefficients beyond the dilute limit

Schuler

Messina

Nastar

2020

Computational Materials Science

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This paper introduces the KineCluE code that implements the self-consistent mean-field theory for clusters of finite size. Transport coefficients are obtained as a sum over cluster contributions (in a cluster expansion formalism), each being individually computed with KineCluE. This method allows for the calculation of these coefficients beyond the infinitely dilute limit, and is an important step in bridging the gap between dilute and concentrated approaches. Inside a finite volume of space containing the components of a given cluster, all kinetic trajectories are accounted for in an exact manner. The code, written in Python, adapts to a wide variety of systems, with various crystallographic structures (possibly under strain), defects and solute amount and types, and various jump mechanisms, including collective ones. The code also features a set of useful tools, such as the sensitivity study routine that allows for the identification of the most important jump frequencies to get accurate transport coefficients with minimum computational cost. PROGRAM SUMMARYProgram Title: KineCluE (KINEtic CLUster Expansion) Licensing provisions: LGPL Programming language: Python 3.6 Nature of problem: Providing a general method for computing transport coefficients from atomic jump frequencies, taking into account kinetic correlations.Solution method: The program relies on the selfconsistent mean field (SCMF) theory. The system is described in terms of lattice sites, defects and jump mechanisms. The first part of the code translates the diffusion problem for such system into an analytical linear eigenvalue problem. The second part of the code assigns numerical values to each analytical variable and then solves the linear problem.

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Kinetic Monte Carlo Simulations of Helium Cluster Nucleation in Tungsten with Preexisting Vacancies

Cited by 22 publications

References 44 publications

Effect of W self-implantation and He plasma exposure on early-stage defect and bubble formation in tungsten

Effect of W self-implantation and He plasma exposure on early-stage defect and bubble formation in tungsten

Helium in-plane migration behavior on 〈1 0 0〉 symmetric tilt grain boundaries in tungsten

KineCluE: A kinetic cluster expansion code to compute transport coefficients beyond the dilute limit

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