Properties of vanadium-base alloys irradiated in the Dynamic Helium Charging Experiment

One property of vanadium-base alloys that is not well understood in terms of their potential use as fusion reactor structural materials, is the effect of simultaneous generation of helium and neutron damage. In the present Dynamic Helium Charging Experiment (DHCE), helium was produced uniformly in the specimen at linear rates of approximate to 0.4 to 4.2 appm helium/dpa by the decay of tritium during irradiation to 18-31 dpa at 425-600 degrees C in Li-filled capsules in a sodium-cooled fast reactor. This paper presents results of postirradiation examination and tests of microstructure and mechanical properties of V-5Ti, V-3Ti-1Si, V-8Cr-6Ti, and V-4Cr-4Ti (the latter alloy has been identified as the most promising candidate vanadium alloy). Effects of helium on tensile strength and ductility were insignificant after irradiation and testing at > 420 degrees C. However, postirradiation ductilities at < 250 degrees C were higher than those of the non-DHCE specimens (< 0.1 appm helium), whereas strengths were lower, indicating that different types of hardening centers are produced during DHCE and non-DHCE irradiation. Ductile-brittle transition behavior of the DHCE specimens was also determined from bend tests and fracture appearance of transmission electron microscopy (TEM) disks and broken tensile specimens. No brittle behavior was observed at temperatures > -150 degrees C in DHCE specimens. Predominantly brittle-cleavage fracture morphologies were observed only at -196 degrees C in some specimens that were irradiated to 31 dpa at 425 degrees C during the DHCE. For the helium generation rates in this experiment (approximate to 0.4-4.2 appm He/dpa), grain-boundary coalescence of helium microcavities was negligible and intergranular fracture was not observed.