**3. Results**

Figure 2 shows the results from the nano-indenter test for unirradiated V–4Cr–xTi alloys and He-irradiated V–4Cr–xTi alloys at 500 ◦C. In the unirradiated V–4Cr–xTi alloys, the effects of Ti addition and interstitial gas impurity on the nano-indentation hardness were not apparent. The hardness of the V–4Cr–xTi and V–4Cr–xTi(h) alloys did not change at ~1800 N/mm<sup>2</sup> . The He-irradiated V–4Cr–xTi alloys showed a large irradiation hardening. V–4Cr–1Ti showed the largest irradiation hardening of the alloys and the hardness reached 5100 N/mm<sup>2</sup> , which was approximately 2.8 times larger than the hardness of unirradiated V-4Cr-1Ti alloys. For the V–4Cr–xTi with 2% to 4% Ti addition, the irradiation hardening decreased and did not change as much with increasing Ti addition. Therefore 1% Ti addition is most effective to produce irradiation hardening in V–4Cr–xTi (x = 0 to 4). From 0.1% to 1%Ti addition in V–4Cr–xTi alloys, the alloys that contained more interstitial gas impurity showed a larger increase in irradiation hardening. V–4Cr–xTi alloys that contained more than 2% Ti addition did not change the irradiation hardening increase even though the amount of gas interstitial impurity increased. This result suggests that the interstitial gas impurity does not contribute to irradiation hardening in V–4Cr–xTi alloys with more than 2% Ti addition.

Figure 3 shows the irradiation hardening increase of V–4Cr–xTi alloys with the conventional impurity level irradiated at 500 ◦C and 700 ◦C. The He-irradiated V–4Cr–xTi alloys at 700 ◦C showed irradiation hardening but the amount of irradiation hardening increase at 700 ◦C was smaller than that at 500 ◦C. This result indicates that damaged microstructures that formed at 700 ◦C irradiation may be coarser than those formed at 500 ◦C irradiation, and irradiation hardening at 700 ◦C irradiation is smaller than that at 500 ◦C irradiation. These microstructural features depending on irradiation temperature and damage level have been reported elsewhere [5,14,15].

**Figure 2.** Ti content dependence of nano-indentation hardening for unirradiated V–4Cr–xTi alloys and He-irradiated V–4Cr–xTi alloys at 500 ◦C.

**Figure 3.** Irradiation hardening increase of V–4Cr–xTi alloys with conventional impurity level irradiated at 500 ◦C and 700 ◦C as a function of Ti content.

Figure 4 shows the nano-indenter test results for unirradiated and He-irradiated V– yCr–1Ti alloys with a conventional impurity level irradiated at 500 ◦C and 700 ◦C. In the unirradiated V–yCr–1Ti alloys, the nano-indentation hardness of the V–yCr–1Ti alloys increased slightly with an increase in Cr addition, which was caused by solution hardening because of Cr addition in V–Cr–Ti alloys. The interstitial gas impurity in the V–yCr–1Ti alloys did not affect the hardening behavior much. Irradiation hardening of He-irradiated V–yCr–1Ti was apparent in all alloys. Significant irradiation hardening occurred among all alloys, but the irradiation hardening decreased with an increase in Cr addition. The significant reduction in irradiation hardening in the V–8Cr–1Ti alloys was apparent. The effect of interstitial gas impurity on the irradiation-hardening behavior of V–yCr–1Ti showed that the conventional fabricated alloys had a larger irradiation hardening increase than the highly purified alloys. The irradiation hardening at 500 ◦C irradiation was larger than that at 700 ◦C irradiation in all V–yCr–1Ti alloys. At 500 ◦C irradiation, the irradiation hardening decreased with an increase in Cr addition. The irradiation hardening of the V–yCr–1Ti alloys that were irradiated at 700 ◦C increased with an increase in Cr addition.

From the result of heat treatment of V-4Cr-4Ti alloys at high temperatures, it has been reported that fine precipitate forms above 700 ◦C of annealing temperature [16]. Hence, it is concluded that the hardness increase at 500 ◦C in this study is not solely caused by the precipitation induced by the irradiation temperature at least.

**Figure 4.** Cr content dependence of nano-indentation hardening for unirradiated V–yCr–1Ti alloys and He-irradiated V–yCr–1Ti alloys at 500 ◦C and 700 ◦C.
