EFFECT OF SPECIMEN SURFACE ON DISLOCATION LOOP NUCLEATION UNDER ION IRRADIATION

Dislocation loop nucleation mechanism in Type 304 stainless steel unde r ion irradiation, stressing the role of the specimen surface, is inve stigated. Specimens (0.1 mm thick) are subjected to 300 keV He+ irradi ation in the following two ways. In thin foil irradiation, the specime ns are electro-polished for TEM observation before being irradiated. I n thick foil irradiation, after ion irradiation the specimens are elec tro-polished from the unirradiated rear surface. Irradiations are perf ormed at the dose rates between 10(-5) and 10(-4) dpa/s and at tempera tures between 300 and 500-degrees-C, the total dose being 0.1 dpa. In the thin foil case, the dislocation loop density is proportional to th e dose rate, and the apparent activation energy of the dislocation loo p nucleation is determined as 1.4 x 10(-19) J (0.9 eV). In the thick f oil case, the density is proportional to the 0.7 power of the dose rat e, and the apparent activation energy is 8 x 10(-20) J (0.5 eV). These experimental observations are analysed using a rate-theory based mode l in which a di-interstitial as nucleus of the dislocation loop and po int defect flow to the surface were considered. Experimental observati ons are consistent with the theoretical predictions and good agreement is obtained when the migration energy of interstitials and the bindin g energy of the impurity-interstitial complex are estimated to be 6 x 10(-20) and 8 x 10(-20) J (0.4 and 0.5 eV), respectively.