**2. Experimental Procedure**

In total, 15 types of V–(4–8)Cr–(0–4)Ti ternary alloys were fabricated by arc melting. Table 1 shows the chemical composition of the V–Cr–Ti alloys that were used in this study. Two impurity levels of each alloy were prepared to investigate the effect of interstitial impurity for irradiation hardening during He-ion irradiation. The highly purified alloy was marked as "(h)". The V–Cr–Ti alloys from conventional fabrication contain ~500 ppm of C+N+O interstitial gas impurity. In contrast, highly purified V–Cr–Ti alloys, which are marked as "(h)", contain approximately half the interstitial gas impurity in conventional alloys by using highly purified vanadium ingots in fabrication [11]. Thin specimen plates of 10 mm × 2 mm × 0.2 mm were cut out and annealed for 2 h at 1000 ◦C in a vacuum (∼<sup>2</sup> <sup>×</sup> <sup>10</sup>−<sup>4</sup> Pa). The samples were irradiated with 2-MeV <sup>4</sup>He ions using a tandem accelerator at the Wakasa Wan Energy Research Center. Sectional shapes of the <sup>4</sup>He2+ beams existed in a 9-mm-diameter circle or ellipse with a major axis of 10 mm and a minor axis of 6 mm. These beams were scanned to irradiate the samples uniformly. The horizontal and vertical widths of the scanned beams were 13 mm × 13 mm for the former beam and 13 mm <sup>×</sup> 17 mm for the latter beam. Time-averaged current densities were 0.4 <sup>µ</sup>A/cm<sup>2</sup> and 0.9 µA/cm<sup>2</sup> , respectively. During the irradiation, the sample stage was heated on a Mo holder with a ceramic heater. The temperature was maintained within ± 5 ◦C during ion irradiation. Specimens were irradiated at 500 ◦C and 700 ◦C up to doses of 0.5 dpa at a peak position and 3.4 µm depth.

**Table 1.** Chemical composition of V–Cr–Ti alloys. Marks (h) indicate the type of highly purified alloy from the original alloy.


Figure 1 shows the damage profile and He ion range in vanadium as calculated by using the SRIM-code (the stopping and range of ions in matter). After He ion irradiation, a nano-indentation test was examined at room temperature by using a Elionix ENT-1100a (Elionix Inc., Tokyo, Japan) nano-indenter with a Berkovich diamond indenter tip and a direction of indentation parallel to the ion beam axis, which is normal to the irradiated surface. The nano-indenter test was carried out with an indentation depth of 500 nm. The indentation depth was determined from the effective depth where the plastic and elastically deformed area was expanded during the nano-indenter test that corresponds to five times the length of the nano-indenter depth. To avoid uncertainty about the specimen surface condition, a scanning electron microscopy (SEM) observation with electron backscattered diffraction measurement was carried out on the indentation surface after the indentation test to ascertain that the indentation test was not on/near grain boundary on the specimen

surface. For each sample, 12 tests were conducted, and 10 of the measured data points were included in the analysis, rejecting the maximum and minimum measured values. In this study He ions were used as the projectile because the penetration depth is sufficient to assess mechanical properties as irradiation hardening through nano-indentation hardness measurements [13]; it should be noted that the first wall and divertor of a fusion reactor are expected to be subject to high fluxes of moderate to low-energy helium ions created as fusion reaction products and as a result He bubbles may form around the He ion penetration range which was 3.6 µm in this study.

**Figure 1.** Calculation results of ion range profile of implanted He ion and vacancy concentration profile in V by 2-MeV He<sup>+</sup> ion irradiation.
