Positron studies of interaction between yttrium atoms and vacancies in bcc iron with relevance for ODS nanoparticles formation

The very high calculated binding energy of vacancy (V)–Ysub (1.45 eV) in Fe makes it be one possible earliest formation stage of (Y, Ti, O) nanoclusters in ODS alloy. Our direct slow positrons annihilation observations are used to valid the interaction between V and Y. The pure bcc iron samples have been implanted by 1.2 MeV Y ions at three fluences from 1 × 1014 to 3 × 1015 cm−2. Vacancy clusters are observed for all these three fluences. Their size and concentration decrease with Y concentration measured by using SIMS. Two hypotheses are proposed to explain the results, including the formation of complexes Vm–Yn and/or of precipitates Ym–Xn (X = Y, O, etc.). In addition, vacancy clusters are detected deeper than predicted by SRIM calculation due to, at least for a part, channelling which is confirmed by Marlowe calculation and SIMS measurements.