Abstract
Transmission electron microscopy (TEM) was used to study the effects of ion-irradiation at grain boundaries in a single phase austenitic alloy (Fe,15%Cr,15%Ni) with particular reference to the nucleation and growth of helium gas bubbles. The Harwell Dual Beam Facility was used to implant helium-ions at high temperatures (450 to 600°C). Simultaneous implantation of chromium-ions was used to study the synergistic effects of helium implantation and displacement damage on the development of the radiation induced microstructures. TEM showed that all interfaces except coherent twin interfaces exhibit preferential nucleation of helium bubbles. The density of bubbles at grain boundaries increased with gas implantation rate, decreased slightly with temperature but was independent of extra displacement damage during dual-beam irradiation. The presence of resolvable arrays of grain boundary dislocations at interfaces enhanced the density of helium bubbles at all temperatures studied. A nucleation model was developed to account for the observed density of bubbles at grain boundaries and at grain boundary dislocations. The model was used to show that helium diffusion at grain boundaries and along grain boundary dislocations is slow relative to helium-interstitial migration within grains. Two forms of heterogeneous dislocation loop nucleation were identified at grain boundaries. First, interstitial dislocation loops were observed at coherent twin interfaces. The loop shape was dependent on irradiation conditions and was interpreted in terms of the relative rates of diffusion of interstitial atoms at the twin interface and along the dislocation line. The second form of heterogeneous loop nucleation occured in the vicinity of some grain boundaries and is consistent with the de-channelling of ions at crystal interfaces.