Helium damage in metal-tritium systems

The time evolution of Debye-Scherrer lines was investigated in metal-tritium-helium systems by neutron-scattering techniques. Polycrystalline samples of ${\mathrm{TaT}}_{\mathit{x}}$, ${\mathrm{YT}}_{\mathit{x}}$, and ${\mathrm{ScT}}_{\mathit{x}}$ were measured over a period of three years. The results show that helium damage is governed by the behavior of self-interstitial atoms and dislocation loops created by helium clustering and bubble formation. The self-interstitials and loops induce a lattice expansion in the early stages of helium formation. For higher helium concentrations the self-interstitials and loops produced are completely incorporated into an evolving dislocation network. In hexagonal rare-earth systems a preferential condensation of loops into a dislocation network lying in basal planes is observed. Additional small-angle-scattering experiments show that platelike helium cavities are formed in the hexagonal systems.