Effects of Fast-Neutron Irradiation on Tensile Properties and Swelling Behavior of Vanadium Alloys

Abstract

Fast-neutron-induced void formation in commercial pure vanadium is dramatically reduced by additions of titanium and/or chromium. As little as 1 percent by weight titanium decreases the void number density and void volume fraction by factors of 30 and 100, respectively, compared with pure vanadium after exposure at 525 C (977 F) to a fluence of 1.1 × 1022 n/cm2, En > 0.1 MeV. Additions of 3 percent by weight titanium or greater suppress void formation entirely under the same exposure conditions. Alloys of 10 and 15 percent by weight chromium have also been found beneficial in limiting swelling. Internal-friction measurements show that the reduction in void formation in the vanadium-titanium alloys may be partly related to the removal of interstitial impurities, oxygen, and nitrogen by the titanium to form a coherent precipitate of Ti3O2 or a complex oxynitride. Excess oxygen in the alloy after consumption of the available titanium as a precipitate results in increased void formation and also loss of precipitate coherency. The tensile properties of V-20Ti alloy are only slightly affected by in-reactor exposures as high as 5.4 × 1022 n/cm2, En > 0.1 MeV. Initial hardening is observed at low fluences and irradiation temperatures that are probably due to point-defect clusters or precipitates nucleating on interstitial impurities. However, the hardening effect saturates after low neutron exposures and is found to be independent of both fluence and temperature. Specimens of V-20Ti irradiated at ∼600 C (1112 F) to the highest fluence of ∼ 6 × 1022 n/cm2, En > 0.1 MeV showed a few voids and a dislocation network. However, under these conditions, swelling is negligible and is not sufficiently severe to cause any noticeable effect on the tensile properties.