BSTRACTNanocrystalline materials are radiation-tolerant materials’ candidates due to their high defect sink density. Here

nanocrystalline iron films were irradiated with 10 keV helium ions

in situ

in a transmission electron microscope at elevated temperatures. Grain-size-dependent bubble density changes and denuded zone occurrence were observed at 700 K

but not at 573 K. This transition

attributed to increased helium–vacancy migration at elevated temperatures

suggests that nanocrystalline microstructures are more resistant to swelling at 700 K due to decreased bubble density. Finally

denuded zone formation had no correlation with grain size and misorientation angle under the conditions studied.