MICROSTRUCTURAL EVOLUTION ADJACENT TO GRAIN-BOUNDARIES UNDER CASCADE DAMAGE CONDITIONS AND HELIUM PRODUCTION

It is well established that clusters of both vacancies and self-inters titial atoms (SIAs) are produced in displacement cascades. Ample evide nce has also been presented showing that SIAs produced in the form of small dislocation loops may be highly glissile. Such loops may glide t o and may be absorbed by extended sinks such as dislocations and grain boundaries (GBs). The loss of SIAs by this process causes a vacancy s upersaturation representing an efficient driving force for void swelli ng, in particular in regions adjacent to GBs. Enhanced swelling in reg ions adjacent to GBs has been observed in several metals subject to ir radiation by both fast fission neutrons and 600 MeV protons. In the la tter case, however, the width of the region of enhanced swelling is sm aller and the amount of swelling is significantly lower than in the fo rmer case. Recently, enhanced swelling near GBs as induced by the casc ade damage of fast neutrons has been discussed in terms of SIA loop es cape to GBs assuming that the range of glissile loops and, via this, t he 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 p roduction rate. It is shown that, in this case, the width of the peak zone is controlled by the (mostly invisible) bubble structure rather t han by the (visible) void structure. The reduced swelling relative to that under neutron irradiation is attributed to the screening of the G Bs with respect to loop capture by the bubble structure in the void de nuded zone.