Abstract
The properties of helium bubbles in a body-centred cubic (bcc) Fe lattice have been examined. The atomic configurations and formation energies of different He–vacancy complexes were determined. The 0K results show that the most energetically favourable He to Fe vacancy ratio increases from about 1:1 for approximately 5 vacancies up to about 4:1 for 36 vacancies. The formation mechanisms for small He clusters have also been considered. Isolated interstitials and small clusters can diffuse quickly through the lattice. MD simulations of randomly placed interstitial He atoms at 500K showed clustering over the time scale of nanoseconds with He clusters containing up to 4 atoms being mobile over this time scale. He clusters containing 4 or 5 atoms were shown to eject an Fe dumbbell interstitial which could then detach from the He cluster and diffuse with the remaining He–vacancy complex being effectively immobile. Collision cascades initiated near larger bubbles showed that Fe vacancies produced by the cascades readily become part of the He–vacancy complexes. Energy barriers for He to join an existing bubble as a function of the He–vacancy ratio are also calculated. These can be larger than the diffusion barrier in the pristine lattice, but are lower when the bubbles contain excess vacancies, thus indicating that bubble growth may be kinetically constrained.
Highlights
Reduced-activation ferritic/martensitic steels are candidate materials for use in nuclear reactors [1,2]
Helium effects on microstructural evolution in tempered martensitic steels were characterised using a novel in situ He-implanter technique by Yamamoto et al [6] combined with neutron irradiation at 500 °C
Single helium atoms prefer to be substitutional rather than interstitial but the energy barrier for this to occur is large. An isolated He atom injected into a bcc Fe lattice would be expected to be located at a tetrahedral site
Summary
Reduced-activation ferritic/martensitic steels are candidate materials for use in nuclear reactors [1,2]. Work by Henry et al [8] demonstrated the effect of helium on the fracture properties of a 9Cr martensitic steel The size of He bubbles observed in the TEM micrograph was of the order of 0.5 nm in diameter and this small size was attributed to the influence of Cr inhibiting bubble growth. Yang et al [22] considered the nucleation of He bubbles at 800 K in bcc Fe using a methodology similar to that described here They were able to conclude that He would diffuse with a low energy barrier of 0.06 eV rapidly forming small clusters that in turn could diffuse and that He4 clusters could emit an Fe interstitial and bind with a vacancy. To describe the He–He interactions, we use the Aziz helium potential [26]
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