Effect of Specimen Surface on Dislocation Loop Nucleation under Ion Irradiation

Dislocation loop nucleation mechanism in Type 304 stainless steel under ion irradiation, stressing the role of the specimen surface, is investigated. Specimens (0.1 mm thick) are subjected to 300 keV He+ irradiation in the following two ways. In thin foil irradiation, the specimens are electro-polished for TEM observation before being irradiated. In thick foil irradiation, after ion irradiation the specimens are electro-polished from the unirradiated rear surface. Irradiations are performed at the dose rates between 10−5 and 10−4 dpa/s and at temperatures between 300 and 500°C, the total dose being 0.1 dpa. In the thin foil case, the dislocation loop density is proportional to the dose rate, and the apparent activation energy of the dislocation loop nucleation is determined as 1.4×10−19 J (0.9 eV). In the thick foil case, the density is proportional to the 0.7 power of the dose rate, and the apparent activation energy is 8×10−20 J (0.5 eV). These experimental observations are analysed using a rate-theory based model in which a di-interstitial as nucleus of the dislocation loop and point defect flow to the surface were considered. Experimental observations are consistent with the theoretical predictions and good agreement is obtained when the migration energy of interstitials and the binding energy of the impurity-interstitial complex are estimated to be 6×10−20 and 8×10−20 J (0.4 and 0.5 eV), respectively.