Helium bubbles in FeNiCr after post-implantation annealing

Abstract A FeNiCr sample was implanted at room temperature with 1200 appm helium and subsequently annealed at 973 K for times ranging from 15 min up to 2780 h. Formation and coarsening of the helium bubbles were investigated by small angle neutron scattering (SANS) and transmission electron microscopy (TEM). The helium bubbles were found in a bimodal size distribution: small bubbles in the bulk and large bubbles close to the grain boundaries. The bulk bubbles contain helium under marked overpressure (as compared to the thermal equilibrium value) and their coarsening was identified to occur via migration and coalescence controlled by surface diffusion with an activation energy of 1.9 ± 0.4 eV . The measured size distributions are in reasonable agreement with the corresponding results obtained by computer simulation. The large bubbles develop in regions close to the grain boundaries. From the measured width of these regions the self diffusion coefficient could be estimated and good agreement with literature values was found.