**1. Introduction**

Vanadium alloys are attractive blanket structural materials in fusion power systems because of their low induced activation characteristics, high-temperature strength, good compatibility with a liquid lithium environment and high thermal stress [1–3]. Critical issues of vanadium alloys, such as corrosion and oxidation have been resolved by Cr and Ti addition to the vanadium matrix, and recent efforts have focused on developing the V–4Cr–4Ti alloy as a candidate alloy for Li-blanket systems in fusion reactors [4,5]. The susceptibility of V–4Cr–4Ti alloys to low-temperature embrittlement during neutron irradiation may limit their application in low-temperature (<400 ◦C) regimes [6]. To improve this drawback, highly purified V–4Cr–4Ti alloys, such as NIFS-HEAT-1 and -2, have been developed by the National Institute for Fusion Science (NIFS) [7–10]. The NIFS-HEAT-2 has shown significantly lower radioactivity for full remote recycle over 25 years of cooling after being used in the first wall of a fusion commercial reactor. A reduction in the cooling period provides economical and safety benefits to reduce the amount of radiation waste material [11].

Nagasaka suggested that the cooling period of V–Cr–Ti alloy for full remote recycling can be reduced within 10 years by reducing the Ti addition from a V–4Cr–4Ti alloy [12]. The reduction in Ti addition reduces the cooling time and results in swelling and a loss of mechanical strength under neutron irradiation.

To optimize the Ti and Cr addition to V–Cr–Ti alloys to balance the irradiation hardening and swelling behavior, a set of V–Cr–Ti alloys with a lower Ti content and higher Cr content was produced from V–4Cr–4Ti alloys. In this study, nano-indenter tests were

**Citation:** Fukumoto, K.-i.; Kitamura, Y.; Miura, S.; Fujita, K.; Ishigami, R.; Nagasaka, T. Irradiation Hardening Behavior of He-Irradiated V–Cr–Ti Alloys with Low Ti Addition. *Quantum Beam Sci.* **2021**, *5*, 1. https://doi.org/10.3390/qubs5010001

Received: 29 November 2020 Accepted: 29 December 2020 Published: 31 December 2020

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performed to investigate irradiation hardening in a new set of V–Cr–Ti alloys after Heion irradiation to survey the optimum composition to compensate between radioactivity reduction and irradiation hardening behavior in V–Cr–Ti alloys.
