Microstructural evolution under irradiation in V-Ti-Ta-(Hf) refractory high-entropy alloys: Temperature effects revealed by atom probe tomography

V-Ti-Ta-based refractory high-entropy alloys exhibit promising irradiation resistance and are considered strong candidates for structural materials in next-generation nuclear power systems. However, comprehensive studies on their compositional design and temperature-dependent irradiation behavior remain limited. In this study, irradiation-induced hardening and microstructural evolution of VTiTa and VTiTaHf alloys were systematically investigated under ion irradiation at room temperature (RT) and 773 K using nanoindentation and atom probe tomography. In the VTiTa alloy, RT irradiation led to the formation of V-enriched particles and dislocations, whose number density and size were closely correlated with local compositional variations. At 773 K, these V-enriched particles disappeared, the dislocation density decreased, and their average size increased. The addition of Hf further suppressed the formation of V-enriched particles and dislocations under RT irradiation. Moreover, an atomic-scale homogeneous elemental distribution was maintained in the VTiTaHf alloy at 773 K. These microstructural changes were consistent with the observed irradiation-induced hardening behavior, where the VTiTaHf alloy demonstrated superior radiation resistance at elevated temperatures. These findings suggest that Hf incorporation not only enhances the irradiation tolerance of the alloy at high temperatures but also contributes to reducing the radioactive decay period, which is a critical requirement for application in nuclear environments.