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Atomistic Studies on Hydrogen and Helium in Fe 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 O1 T1 T2 T4 O1’ Δ E (eV) EAM DFT -2 -1 0 1 2 Molecular Hydrogen and Perfect Bulk Iron -2 -1 0 1 2 Hydrogen in T-site -2 -1 0 1 2 -10 -8 -6 -4 -2 0 2 E - E F (eV) Hydrogen at Monovacancy Vacancies act as strong traps for hydrogen, however, dissociation of a hydrogen-vacancy pair is more likely than coherent migration Bulk diffusion is very low barrier, and it may proceed via two competing paths We must account for zero-point energy (ZPE) corrections Different environments result in changes to the electronic structure of hydrogen, with delocaliza- tion in space accompanied by delocalization in energy Diffusion and Electronic Structure of Hydrogen in α-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 today’s fission reactors and tomorrow’s 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 diffusion of hydrogen affected by vacancies, and vice versa? How is the form and behavior of a hydrogen-vacancy cluster affected by concentration? size? Hydrogen + Helium 0 5 10 15 20 25 0 1 2 3 4 5 6 7 8 Hydrogen atoms Helium atoms 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 α-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 different methods: Empirical interatomic potentials, molecular dynamics, LAMMPS Density functional theory, SIESTA Low Energy State Searches Erin Hayward and Chu Chun Fu CEA-Saclay, DEN/DMN, Service de Recherches de Métallurgie Physique

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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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Page 1: Oxford Poster

Atomistic Studies on Hydrogen and Helium in Fe

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

O1 T1 T2 T4 O1’

ΔE

(eV)

EAMDFT

-2

-1

0

1

2 Molecular Hydrogen and Perfect Bulk Iron

-2

-1

0

1

2 Hydrogen in T-site

-2

-1

0

1

2

-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 di�usion is very low barrier, and it may proceed via two competing paths

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

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

Di�usion and Electronic Structure of Hydrogen in α-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 today’s �ssion reactors and tomorrow’s 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 di�usion of hydrogen a�ected by vacancies, and vice versa?

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

Hydrogen + Helium

0 5 10 15 20 25

0

1

2

3

4

5

6

7

8

Hydrogen atoms

Hel

ium

ato

ms

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 α-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 di�erent 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 Métallurgie Physique