3.3.1 Interstitial and substitutional solubility

This chapter talks about the atomistic properties, namely, interstitial solubility and substitutional solubility of helium. In contrast to the forced introduction of helium into the solids athermally, the solubility of helium is defined as the concentration of the helium atoms in a solid which is in thermodynamic equilibrium with a surrounding gas phase of pressure and temperature. Minimization of the energy of the system of metal atoms and helium atoms on lattice and/or defect sites yields the position of the atoms and the corresponding energies of formation, binding, relaxation and migration. Calculated formation energies of He in various interstitial positions in bcc metals (V, Fe, Mo, Ta, W) and fcc metals (Ni, Cu, Pd, Au) are tabulated. It is very difficult to state general criteria for the reliability of such calculations of formation energies since the sensitivity of the different properties to the chosen potentials can be quite different. This is exemplified in a table showing that e.g. vastly different binding energies for di-interstitial He are predicted whereas the binding energies to a vacancy are rather consistent and certainly do not reflect the enormous differences in the Al-Al potentials employed.