HELIUM BUBBLES IN FENICR AFTER POSTIMPLANTATION ANNEALING

A FeNiCr sample was implanted at room temperature with 1200 appm heliu m and subsequently annealed at 973 K for times ranging from 15 min up to 2780 h. Formation and coarsening of the helium bubbles were investi gated by small angle neutron scattering (SANS) and transmission electr on microscopy (TEM). The helium bubbles were found in a bimodal size d istribution: small bubbles in the bulk and large bubbles close to the grain boundaries. The bulk bubbles contain helium under marked overpre ssure (as compared to the thermal equilibrium value) and their coarsen ing was identified to occur via migration and coalescence controlled b y surface diffusion with an activation energy of 1.9 +/- 0.4 eV. The m easured size distributions are in reasonable agreement with the corres ponding results obtained by computer simulation. The large bubbles dev elop in regions close to the grain boundaries. From the measured width of these regions the self diffusion coefficient could be estimated an d good agreement with literature values was found.