Helium Bubbles in Fe: Equilibrium Configurations and Modification by Radiation
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Helium Bubbles in Fe: Equilibrium Configurations and Modification by Radiation
Xiao Gai, Roger Smith and Steven Kenny Mathematical Sciences Department, Loughborough University, Leicestershire, LE11 3TU, UK
ABSTRACT We have examined the properties of helium bubbles in Fe using two different Fe-He potentials. The atomic configurations and formation energies of different He-vacancy complexes are determined and their stability in the region of nearby collision cascades is investigated. The results show that the optimal He to Fe vacancy ratio increases from about 1:1 for approximately 5 vacancies up to about 4:1 for 36 vacancies. Collision cascades initiated near the complex show that Fe vacancies produced by the cascades readily become part of the He-vacancy complexes. The energy barrier for an isolated He interstitial to diffuse was found to be 0.06 eV. Thus a possible mechanism for He bubble growth would be the addition of vacancies during a radiation event followed by the subsequent accumulation of mobile He interstitials produced by the corresponding nuclear reaction. INTRODUCTION Reduced-activation ferritic/martensitic steels are candidate materials for use in nuclear reactors [1]. The presence of transmutation-created helium plays an important role in the microstructural evolution of these steels under neutron irradiation. Helium has a large effect on cascade damage. Interstitial helium atoms increase the production of Frenkel pairs whilst substitutionals tend to decrease this production [2]. Small helium-vacancy (HenVm) clusters may play an important role in the nucleation of He bubbles. However, the atomistic properties of He in metals are difficult to identify experimentally. Thus atomistic simulations such as molecular dynamics (MD) provide useful tools to study the formation and the stability of these clusters. Here, we present the results of a study on the formation of small helium-vacancy clusters in bcc iron and their interaction with nearby collision cascades, which will provide insight into the growth of the bubble. The results were obtained from our in-house MD code, LBOMD. COMPUTATIONAL METHOD The formation energies of the helium-vacancy clusters HenVm are evaluated using two different Fe-He potentials. The first Fe-He potential is a three-body potential by Stoller et al. [3], the other Fe-He potential is a many-body potential by Gao et al. [4]. The first potential is combined with the 1997 Ackland et al. potential [5] for the Fe-Fe interactions whilst the latter
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one uses the Ackland and Mendelev potential (AM-potential) for the Fe-Fe interactions [6]. Both of the potentials use the Aziz helium potential for the He-He interactions [7]. The formation energy of an He – vacancy complex is defined as the difference in total energy between a crystal containing a defect and a perfect bcc crystal of the same number of Fe atoms with the corresponding number of helium atoms in the fcc structure. So for n He atoms in a system with m vacancies,
where is the fo
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