Microstructural evolution adjacent to grain boundaries under cascade damage conditions and helium production

It is well established that clusters of both vacancies and self-interstitial atoms (SIAs) are produced in displacement cascades. Ample evidence has also been presented showing that SIAs produced in the form of small dislocation loops may be highly glissile. Such loops may glide to and may be absorbed by extended sinks such as dislocations and grain boundaries (GBs). The loss of SIAs by this process causes a vacancy supersaturation representing an efficient driving force for void swelling, in particular in regions adjacent to GBs. Enhanced swelling in regions adjacent to GBs has been observed in several metals subject to irradiation by both fast fission neutrons and 600 MeV protons. In the latter case, however, the width of the region of enhanced swelling is smaller and the amount of swelling is significantly lower than in the former case. Recently, enhanced swelling near GBs as induced by the cascade damage of fast neutrons has been discussed in terms of SIA loop escape to GBs assuming that the range of glissile loops and, via this, the width of the peak zone, is controlled by the visible void structure. In the present paper, this model is applied to irradiation with 600 MeV protons where the cascade damage is accompanied by a high helium production rate. It is shown that, in this case, the width of the peak zone is controlled by the (mostly invisible) bubble structure rather than by the (visible) void structure. The reduced swelling relative to that under neutron irradiation is attributed to the screening of the GBs with respect to loop capture by the bubble structure in the void denuded zone.