Radiation damage in helium ion irradiated nanocrystalline Fe

Fe films with an average columnar grain size varying from 49 to 96 nm are deposited by magnetron sputtering technique. Sputtered films have predominant body centered cubic structure together with a small fraction of face centered cubic phase. Bulk Fe with an average grain size of 700 nm is also irradiated at the same condition for comparison. Helium bubbles are observed in Fe films and bulk Fe irradiated by 100 keV helium ions to a fluence of 6 × 1020 ions/m2 at room temperature. Smaller grains lead to lower density of He bubbles. Radiation hardening in Fe films is much less than that of bulk Fe, and is a combined consequence of He bubble induced hardening and radiation induced compressive stress in Fe films.