Inert gas bubbles in bcc Fe

Xiao Gai,Roger Smith,S.D Kenny

doi:10.1016/j.jnucmat.2015.11.057

Xiao Gai, Roger Smith + Show 1 more

Open Access

https://doi.org/10.1016/j.jnucmat.2015.11.057

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Abstract

The properties of inert gas bubbles in bcc Fe is examined using a combination of static energy minimisation, molecular dynamics and barrier searching methods with empirical potentials. Static energy minimisation techniques indicate that for small Ar and Xe bubbles, the preferred gas to vacancy ratio at 0 K is about 1:1 for Ar and varies between 0.5:1 and 0.9:1 for Xe. In contrast to interstitial He atoms and small He interstitial clusters, which are highly mobile in the lattice, Ar and Xe atoms prefer to occupy substitutional sites and any interstitials present in the lattice soon displace Fe atoms and become substitutional. If a pre-existing bubble is present then there is a capture radius around a bubble which extends up to the 6th neighbour position. Collision cascades can also enlarge an existing bubble by the capture of vacancies. Ar and Xe can diffuse through the lattice through vacancy driven mechanisms but with relatively high energy barriers of 1.8 and 2.0 eV respectively. This indicates that Ar and Xe bubbles are much harder to form than bubbles of He and that such gases produced in a nuclear reaction would more likely be dispersed at substitutional sites without the help of increased temperature or radiation-driven mechanisms.

Highlights

Ferritic and martensitic steels are candidate materials for use in nuclear reactors [1,2]
Small bubbles up to around 1.5 nm in diameter can form at room temperature from such seed points but larger bubbles are more difficult to form by diffusion alone due to the induced strain in the bcc lattice which increases the energy barriers for diffusion towards the bubbles whilst reducing them in a direction away from the bubbles
Since He is not the only inert gas to occur as a result of nuclear reactions and in addition Ar and Xe are often used in ion bombardment experiments, it is important to understand if similar mechanisms can result in bubble growth for these gases

Summary

Introduction

Ferritic and martensitic steels are candidate materials for use in nuclear reactors [1,2]. Since He is not the only inert gas to occur as a result of nuclear reactions and in addition Ar and Xe are often used in ion bombardment experiments, it is important to understand if similar mechanisms can result in bubble growth for these gases. Microstructural evolution of P92 ferritic/martensitic steel irradiated by Arþ ion beams at doses from 0.6 to 230 displacements per atom (dpa) at room temperature was investigated with a conventional transmission electron microscope technique by Jin et al [4]. 17.61 eV 20.43 eV atmosphere so that the argon was incorporated into the bulk materials during the preparation process Such bubbles have been observed, it is not clear under what circumstances they can form. The techniques used here are the same as in Ref. [3], namely empirical potentials, static energy minimisation, molecular dynamics (MD) and barrier searching

Methodology

Formation energies

Molecular dynamics simulations and energy barrier calculations

Vacancy driven diffusion mechanisms

Pre-existing inert gas bubbles

Conclusion