A key aspect in predictively modeling the response of materials exposed to many radiation environments is understanding the role of light transmutation products. He in particular can result in the swelling and precipitation of bubbles, both of which can substantially deteriorate the mechanical properties [1]. In this study, in situ TEM characterization of nanocrystalline Fe samples implanted with 10 keV He+ is performed to understand and quantify the mechanisms underlying He diffusion and cavity nucleation under a wide temperature range

Nanocrystalline free-standing Fe thin films were produced by pulse laser deposition and annealed in situ at 550 C in order to stabilize the grain structure for the subsequent experiments. Two types of in situ experiments were performed to study defect evolution and cavity formation:(1) He implantation at room temperature followed by annealing to 600 C, and (2) He implantation at elevated temperatures up to 600 C. In situ He implantations were carried out utilizing a JEOL 2100 TEM and a 10 kV Colutron ion accelerator that are part of the In situ Ion Irradiation TEM facility at Sandia [2]. The implantation occurred in a hummingbird heating stage tilted 40 towards the He beam operated at an average flux of approximately 10 14 He+/cm 2 s to a total concentration of~ 4% He at the end of range.