Abstract: Refractory high entropy alloys (RHEAs) have drawn much attention for their potential applications in advanced reactors. While improved irradiation resistance to void swelling and helium bubble formation has been frequently reported, experimental investigation regarding their early-stage irradiation damage remains insufficient, which hinders the understanding of the behavior of point defects and small clusters. Here we select two typical RHEAs with desired mechanical properties, VTaTi and HfNbZrTi, as well as a conventional V-4Cr-4Ti alloy, and compare their irradiation-induced defect production and hardening under a low-dose irradiation to ∼0.1 dpa. Significant hardening is observed in V-4Cr-4Ti due to the pinning of deformation-induced dislocations by the high density of irradiation-induced loops. In contrast, the hardening in VTaTi is much weaker, corresponding well to the greatly reduced defect density. Strikingly, in HfNbZrTi, visible defect clusters are not observed with a Cs-corrected transmission electron microscope in the whole irradiation range, and no hardening effect is detected. Such strong suppression of irradiation damage is attributed to the large lattice distortion based on the ab initio calculations and the local chemical fluctuations based on the atomic-scale elemental mappings, which together hinder the mobility of interstitials. Furthermore, minor irradiation softening is evidenced by cross-sectional nanoindentation tests in HfNbZrTi, which is considered to be related to the evolution of short-range orders and interstitial impurities after irradiation.

Key words: Ion irradiation, High entropy alloy, Irradiation hardening, Irradiation defect, Lattice distortion, Local chemical fluctuation