The performance of structural materials can strongly influence the blanket design. Especially, the opera­tion temperature window and expected lifetime are the key parameters. Increase in the upper operation temperature limit can enhance the blanket operation temperature and thus plant efficiency. Therefore, enhancing the high-temperature strength is the key issue for improving the performance of the blanket and thus the attractiveness of the fusion power sys­tems. For this purpose, efforts have been made to develop advanced vanadium alloys with potential use at higher temperature.

One of the relatively simple ways to enhance the strength of the alloy is to change the thermal and mechanical treatment of the alloys. Especially, for­mation of a high density of precipitates can strengthen the alloy. Figure 21 shows microstructure and hardness of V-4Cr-4Ti as a function of the temperature of reheating for 1 h after annealing at 1373 K for 1 h. The annealing at 1373 K dissolves most of the thin precipitates and the reheating can form new precipitates. By choosing an appropriate reheating temperature (873-973 K), the materials can be strengthened by the high density offine precipitates. However, the strengthening by this treatment will be lost at >973 K because of the coarsening of the pre­cipitates. To prevent the coarsening, cold work was applied to the specimens. Figure 22 shows the mini­mum creep rate for standard V-4Cr-4Ti and solution annealed, aged, and cold-worked V-4Cr-4Ti. Sup­pression of the creep rate occurred at 1073 K but only with relatively high stresses.45 Microstructural analysis showed that the suppressive role of cold — work-induced dislocations was lost during the creep deformation by the change in the nature of the dis­locations from sessile й(100) type to gliding й/2(111) type.46 Further efforts are being made, for example, to cold-work followed by aging (strain-aging-induced strengthening).

High-temperature strength of V-Cr-Ti alloys can be enhanced by increasing the Cr level. However, high Cr alloys have low ductility and fabricability issues. Recent detailed survey in V-xCr-4Ti alloys showed that the strength at high temperature increases with a small change in the DBTT with the Cr level at ^7%.47

High-strength vanadium alloys were made by addi­tion of Y, O, and N to vanadium followed by mechani­cal alloying (MA) and hot isostatic pressing (HIP). The addition of Y, O, and N was intended to enhance mechanical properties by dispersion of Y2O3 and YN and scavenging O and N from the matrix. Alloys pro­duced by optimization of the processes had small grains and homogeneously dispersed particles and showed higher tensile strength than those of NIFS-HEATs with moderate uniform elongation, both at room tem­perature and 1073 K as shown in Figure 23 48 Fine

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grain and oxide dispersion increased high-temperature strength and inhibited formation of interstitial loops in the matrix by neutron irradiation because of the enhanced defect sinks. Thus, mechanically alloyed vanadium alloys have the potential to extend both low — and high-temperature operation limits.

Other efforts to improve high-temperature strength of vanadium alloys include strengthening by internal oxidation.49

4.12.8 Critical Issues

With the recent progress in the fabrication technology, the number of critical issues for the development of vanadium alloys for fusion reactors has been reduced. The remaining critical issues are thermal and irradia­tion creep, transmutant helium effects on high temper­ature mechanical properties, and radiation effects on fracture properties. The effect of helium, particularly, is still uncertain and can be evaluated precisely only with the use of 14MeV neutrons. This fact highly motivates the construction of a 14 MeV neutron source.

With the progress of the properties of vanadium alloys, the blanket concepts using the alloy become more attractive. Extension of the operation tem­perature window and lifetime of vanadium alloys contribute to the improvement of the quality of the blanket. Therefore, exploration of advanced vana­dium alloys from the current reference alloy is a valuable challenge for enhancing the expected per­formance, and then attractiveness, of fusion reactors.