The effect of internal free surfaces on void swelling of irradiated pure iron containing subsurface trenches

Abstract

We studied the effects of internal free surfaces on the evolution of ion-induced void swelling in pure iron. The study was initially driven by the motivation to introduce a planar free-surface defect sink at depths that would remove the injected interstitial effect from ion irradiation, possibly enhancing swelling. Using the focused ion beam technique, deep trenches were created on a cross section of pure iron at various depths, so as to create bridges of thickness ranging from 0.88 _m to 1.70 _m.  Samples were then irradiated with 3.5 MeV Fe2+ ions at 475  C to a fluence corresponding to a peak displacement per atom dose of 150 dpa. The projected range of 3.5 MeV Fe2+ ions is about 1.2 _m so the chosen bridge thicknesses involved fractions of the ion range, thicknesses comparable to the mean ion range (peak of injected interstitial distribution), and thicknesses beyond the full range. It was found that introduction of such surfaces did not enhance swelling but actually decreased it, primarily because there were now two denuded zones with a combined stronger influence than that of the injected interstitial. The study suggests that such strong surface effects must be considered for ion irradiation studies of thin films or bridge-like structures.