Free surface impact on radiation damage in pure nickel by in-situ self-ion irradiation: can it be avoided?

Abstract A major issue of in-situ radiation damage studies is related to the influence of free surfaces. Free surfaces in thin foils are indeed strong sinks for radiation-induced defects. Nevertheless, in-situ irradiation is a powerful tool to study real-time microstructural evolution and obtain insight into dynamic mechanisms of radiation damage. Thus, a detailed evaluation of surface effects is essential to validate existing results and provide guideline for future comparative experiments. In this work, nickel is chosen as model material to conduct systematic studies on surface effects due to the high mobility of its self-interstitials. Ultra-high purity Ni thin foils are in-situ irradiated by 2 MeV Ni2+ ions at high temperatures (400-700°C). Microstructural evolution analysis and detailed characterization of dislocation loops are performed in function of specimen thickness. The present work shows: (i) a drastic influence of thickness on the microstructural evolution and irradiated microstructure with the existence of a critical thickness depending on temperature; (ii) a good prediction of an adequate irradiation thickness with a vacancy concentration calculation model; (iii) an impact of free surfaces on the fine distribution of loop Burgers vectors; (iv) a first determination of migration energy of vacancies in Ni considering the temperature dependence of the loop-depleted zones; (v) a production bias model showing that a loss of 10% interstitials favors the growth of vacancy loops observed for the first time in Ni at 510°C.