A variable-gap model for calculating free energies of helium bubbles in metals

We propose a variable-gap energy model for helium bubbles in metals, based on molecular dynamics (MD) calculations. The emphasis is put on the appropriate description of the helium-metal repulsion, which can be modelled as a variable-size gap between regions occupied by helium and metal atoms. Each contribution to the bubble energy is parametrized on MD calculations performed in iron. The model is shown to reproduce accurately the dissociation energies obtained by MD over a large range of helium-to-vacancy ratios. Improvements over previous models are shown on a few equilibrium properties: binding energies, solid to fluid transition, helium density in bubbles and validity of Laplace law. Beyond the iron case, such a model should be valid in other metals where helium behavior is similar.