| Summary: | High-entropy alloys (HEAs) have shown promising potential applications in advanced reactors due to the outstanding mechanical properties and irradiation tolerance at elevated temperatures. In this work, the novel low-activation Ti<sub>2</sub>ZrHf<sub>x</sub>V<sub>0.5</sub>Ta<sub>0.2</sub> HEAs were designed and prepared to explore high-performance HEAs under irradiation. The microstructures and mechanical properties of the Ti<sub>2</sub>ZrHf<sub>x</sub>V<sub>0.5</sub>Ta<sub>0.2</sub> HEAs before and after irradiation were investigated. The results showed that the unirradiated Ti<sub>2</sub>ZrHf<sub>x</sub>V<sub>0.5</sub>Ta<sub>0.2</sub> HEAs displayed a single-phase BCC structure. The yield strength of the Ti<sub>2</sub>ZrHf<sub>x</sub>V<sub>0.5</sub>Ta<sub>0.2</sub> HEAs increased gradually with the increase of Hf content without decreasing the plasticity at room and elevated temperatures. After irradiation, no obvious radiation-induced segregations or precipitations were found in the transmission electron microscope results of the representative Ti<sub>2</sub>ZrHfV<sub>0.5</sub>Ta<sub>0.2</sub> HEA. The size and number density of the He bubbles in the Ti<sub>2</sub>ZrHfV<sub>0.5</sub>Ta<sub>0.2</sub> HEA increased with the improvement of fluence at 1023 K. At the fluences of 1 × 10<sup>16</sup> and 3 × 10<sup>16</sup> ions/cm<sup>2</sup>, the irradiation hardening fractions of the Ti<sub>2</sub>ZrHfV<sub>0.5</sub>Ta<sub>0.2</sub> HEA were 17.7% and 34.1%, respectively, which were lower than those of most reported conventional low-activation materials at similar He ion irradiation fluences. The Ti<sub>2</sub>ZrHfV<sub>0.5</sub>Ta<sub>0.2</sub> HEA showed good comprehensive mechanical properties, structural stability, and irradiation hardening resistance at elevated temperatures, making it a promising structural material candidate for advanced nuclear energy systems.
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