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Workshop on Modelling of Radiation Damage and its Effects on Materials, at Oxford University in September 2013

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  • Atomistic Studies on Hydrogen and Helium in Fe

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    EAMDFT

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    2 Molecular Hydrogen and Perfect Bulk Iron

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    2 Hydrogen in T-site

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    -10 -8 -6 -4 -2 0 2E - EF (eV)

    Hydrogen at MonovacancyVacancies act as strong traps for hydrogen, however, dissociation of a hydrogen-vacancy pair is more likely than coherent migration

    Bulk diusion is very low barrier, and it may proceed via two competing paths

    We must account for zero-point energy (ZPE) corrections

    Dierent environments result in changes to the electronic structure of hydrogen, with delocaliza-tion in space accompanied by delocalization in energy

    Diusion and Electronic Structure of Hydrogen in -Fe

    Saturation of the surface may be followed by molecule formation within the void

    Models based on surface absorption sites are being developed to predict when saturation occurs

    Due to the tendency to hybridize with Fe, atomic H sticks to the surface of vacancy clusters; molecules are unstable

    Saturation

    Hydrogen

    Hydrogen retention and embrittlement are relevant issues for todays ssion reactors and tomorrows fusion industry

    Atomistic simulations -- density functional theory, molecular dynam-ics, and Monte Carlo -- can be used to give insight into fundamental lattice defect-impurity interactions

    This study focuses on the interplay between hydrogen and vacancies in body-centered cubic iron, keeping these questions in mind:

    How is the diusion of hydrogen aected by vacancies, and vice versa?

    How is the form and behavior of a hydrogen-vacancy cluster aected by concentration? size?

    Hydrogen + Helium

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    Hydrogen atoms

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    When hydrogen and helium are both present, additional damage to the material can occur; understanding this synergistic interaction will be vital for fusion environments

    Although H has only minimal self-attraction in the bulk, He tends to form clusters

    Hydrogen is trapped at these clusters, perhaps attracted to the free volume between He and the Fe

    Displacement of Fe surrounding cluster (units of a0, MDMC

    result)

    Hydrogen may assist in helium-induced Frenkel pair ejection

    A substitutional He atom can trap up to 8 hydrogen atoms (as opposed to the 6 that a monovacancy can trap), even though the direct attraction between H and He is negligible

    For more details, see our paper:

    Interplay between hydrogen and vacancies in -Fe Physical Review B 87, 174103 (2013)

    or contact us at: [email protected] [email protected]

    We use an iterative method to search for low energy states of defect clusters, consisting of perturbation, minimization, and Monte Carlo selection criteria

    Minimization may be handled using dierent methods:

    Empirical interatomic potentials, molecular dynamics, LAMMPS

    Density functional theory, SIESTA

    Low Energy State Searches

    Erin Hayward and Chu Chun FuCEA-Saclay, DEN/DMN, Service de Recherches de Mtallurgie Physique