**5. Conclusions**

To survey an optimum composition to reduce the radioactivity in V–Cr–Ti alloys, 15 types of V–(4–8)Cr–(0–4)Ti alloys were fabricated. These alloys were subjected to 2-MeV He-ion irradiation using a tandem accelerator at the Wakasa Bay Energy Research Center. Specimens were irradiated at 500 ◦C and 700 ◦C up to doses of 0.5 dpa at peak positions in 3.6 µm depth. To investigate the effect of Cr and Ti addition and gas impurities for irradiation hardening behavior in the V–Cr–Ti alloys, nano-indentation tests were examined at room temperature. Cr and Ti addition to V–Cr–Ti alloys for solid–solution hardening is low in the unirradiated V–(4–8)Cr–(0–4)Ti alloys. Irradiation hardening could be observed among all V–Cr–Ti alloys irradiated at 500 ◦C and 700 ◦C. The V–4Cr–1Ti alloy irradiated at 500 ◦C shows the highest irradiation hardening among all V–Cr–Ti alloys and the irradiation hardening was 2.8 times larger than the hardness of unirradiated V–4Cr–1Ti alloy. The gas impurity increased the irradiation hardening in V–4Cr–xTi alloys. The Cr addition dependence on irradiation hardening to V–yCr–1Ti was not remarkable. The effect of interstitial gas impurity for irradiation hardening of V–yCr–Ti showed that the conventional fabricated alloys had a larger irradiation hardening increase than the highly purified alloys. The significant irradiation hardening in V–4Cr–1Ti is caused by the formation of Ti(CON) precipitate from He-ion irradiation. Because the thermal creep strength of the V–3Ti is highest among all V–Ti system alloys and small precipitates formed in thermal-creep deformed V–4Cr–4Ti alloys in previous studies, the formation of Ti(CON) precipitate results from hardening during irradiation. Consequently, to suppress irradiation hardening and void swelling, the optimum composition of V–Cr–Ti alloys for structural materials of fusion reactor engineering is proposed to be a highly purified V–(6–8)Cr–2Ti alloy.

**Author Contributions:** Conceptualization, K.-i.F.; TEM observation, K.i.F. and T.N.; Sample preparation, T.N., S.M., and K.F.; Ion irradiation, R.I. and S.M.; Nano indentation, S.M., K.F. and Y.K.; Manuscript writing, K.F. All authors have read and agreed to the published version of the manuscript.

**Funding:** This study was partly supported by the NIFS Budget, Code NIFS17KEMF098. This work was supported by a JSPS Grant-in-Aid for Scientific Research (A) 20H00144.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement :** Not applicable.

**Data Availability Statement:** Data available in a publicly accessible repository.

**Acknowledgments:** The authors are grateful to the technical staffs of the tandem accelerator at the Wakasa-Wan Energy Research center for supplying high-quality ion beams.

**Conflicts of Interest:** The authors declare no conflict of interest.
