Radiation-induced segregation in ion-irradiated stainless steels - the origin of compositional fluctuations in irradiated alloys

The role of helium in producing intragranular compositional fluctuations in irradiated stainless steels has been inventigated. The alloy under investigation was a solution treated, niobium stabilised autenitic steel (FV548) which had been irradiated to 5 dpa with either 46.5 MeV Ni6+ or 22 MeV C2+ ions both with and without helium pre-injection. Ion irradiations to 5 dpa produced compositional fluctuations only when the samples had been pre-injected with 10 ppm helium. Without the helium implantation no fluctuations could be detected. These observations are consistent with a mechanism described previously to account for the production of fluctations in neutron irradiated steels. It is believed that the presence of clusters of small gas bubbles is responsible for the fluctuations as a result of radiation induced segregation. Examples of strong segregation to dislocations are also presented and discussed briefly. (orig.)