A fair amount of data is available for radiation response of vanadium alloys partly because they were candidates of cladding materials of LMFBR. For example, void swelling is known to be quite small if the alloy contains Ti. However, data are limited for V-4Cr-4Ti because this composition was decided as the reference one for fusion only recently. For this alloy, the feasibility issues of radia­tion effects are considered to be loss of ductility at lower temperature, embrittlement enhanced by trans­mutant helium at high temperature, and irradiation creep at intermediate to high temperature.

The mechanism of the loss of uniform elongation of vanadium alloys at relatively low temperature (<673 K) and low dose (~0.1 dpa) has been a long­term research subject. Microstructural observation after tensile tests showed that radiation-induced defect clusters were lost in layer structures and the defect-free zones were accompanied by dislocation channels as shown in Figure 16.37 This fact implies flow localization during deformation. Although the mechanism of the flow localization needs further inves­tigation, it is inferred that interaction ofdislocations with
radiation-induced defect clusters, precipitates, or com­plexes of the two species is responsible. If the precipi­tates, most likely Ti-CON, play the role in this process, reduction of impurities in the matrix can improve the properties. Figure 17 compares the uniform elongation after irradiation for V-(4-5)Cr-(4-5)Ti alloys and those with doping of Al, Si, and Y. The significant increase in uniform elongation by the addition of Al, Si, and Y, which are known as getters of interstitial impurities such as O, N, and C in the matrix, suggests that the reduction of the interstitial impurities in solution enhances the radiation resistance.38 The effects of inter­stitial impurities on the formation of dislocation loops and precipitates were investigated by ion irradiations. Figure 18 shows temperature dependence of the densities of loops and precipitates.39 The loop density was not influenced by O level, but the precipitate density increased with O level below 973 K.

Helium embrittlement is a critical issue, which is thought to determine the upper temperature limit for vanadium alloys. Past experimental evaluations of the helium effects involved uncertainties because controlled generation of helium during irradiation in a similar manner to that in fusion condition has been quite difficult. As a result, the past evaluation of the helium effects varied from weak to very strong.3 The Dynamic Helium Charging Experiment (DHCE) using fission reactors40 is one of the few potential neutron irradiation experiments with controlled variation of He/dpa ratio including typical fusion

conditions. DHCE is highly anticipated as a potential method to extend our understanding of the helium effects. However, for conclusive evaluation, a 14MeV neutron source is certainly necessary.

The irradiation creep tests have made progress recently, partly because of the progress in fabricating high quality pressurized creep tube specimens with reduced impurity levels. Figure 19 shows the nor­malized creep strain as a function of applied stress by irradiation in HFIR and JOYO in Li and Na

US-832665 698 K-Li (HFIR) NIFS-HEAT-2 698 K-Li (HFIR) NIFS-HEAT-2 731 K-Na (JOYO) о 2p

50 100 150 200

Applied stress (MPa)

Figure 19 Creep strain as a function of applied stress for V-4Cr-4Ti (US-832665 and NIFS-HEAT-2) irradiated in Li (HFIR) and Na (JOYO) environments. The creep strain was normalized as that at two displacements per atom. Reproduced from Fukumoto, K.; Narui, M.; Matsui, H.; etal. J. Nucl. Mater. 2009, 386-388, 575-578.

environments. The data also compare the perfor­mances of US and Japanese reference alloys.41 It was found that the creep strain rate exhibited a linear relationship with the effective stress up to 150 MPa at ^700 K and the differences with the environments and the heats are small.