Oxidation performance of VCrTi alloys

Vanadium-base alloys are being considered as candidates for the first wall in advanced V–Li blanket concepts in fusion reactor systems. However, a primary deterrent to the use of these alloys at elevated temperatures is their relatively high affinity for interstitial impurities, i.e. O, N, H, and C. We conducted a systematic study to determine the effects of time, temperature, and oxygen partial pressure (pO2) in the exposure environment on O uptake, scaling kinetics, and scale microstructure in V-(4–5) wt.% Cr-(4–5) wt.% Ti alloys. Oxidation experiments were conducted on the alloys at pO2 in the range of 5×10−6–760 torr (6.6×10−4–1×105 Pa) at several temperatures in the range of 350–700°C. Models that describe the oxidation kinetics, oxide type and thickness, alloy grain size, and depth of O diffusion in the substrate of the two alloys were determined and compared. Weight change data were correlated with time by a parabolic relationship. The parabolic rate constant was calculated for various exposure conditions and the temperature dependence of the constant was described by an Arrhenius relationship. The results showed that the activation energy for the oxidation process is fairly constant at pO2 levels in the range of 5×10−6–0.1 torr. The activation energy calculated from data obtained in the air tests was significantly lower, whereas that obtained in pure-O tests (at 760 torr) was substantially higher than the energy obtained under low-pO2 conditions. The oxide VO2 was the predominant phase that formed in both alloys when exposed to pO2 levels of 6.6×10−4 to 0.1 torr. V2O5 was the primary phase in specimens exposed to air and to pure O2 at 760 torr. The implications of the increased O concentration are increased strength and decreased ductility of the alloy. However, the strength of the alloy was not a strong function of the O concentration of the alloy, but an increase in O concentration did cause a substantial decrease in ductility.