The effect of bulk carbon impurity on the radiation-induced surface composition modification of V-Cr alloys at elevated temperature

The effect of 1 to 3 keV argon ion sputtering on the surface composition of a V/Cr alloy containing 31.6 at% Cr and carbon as the dominant impurity has been investigated at 300 and 800 K. The results obtained closely parallel those reported previously for V/Cr alloys which contained bulk oxygen impurity. Sputtering at 300 K depletes the surface in Cr whereas at 800 K sputtering enhances the surface Cr concentration. Annealing at 800 K in the absence of radiation causes the surface to become enriched in both vanadium and carbon, which co-segregate. The mechanism for Cr enrichment is thus co-segregation of V and C, only possible at high temperature, and a consequent depletion in the surface in vanadium on sputtering. Sequential cycles of 800 K annealing without sputtering and 300 K sputtering, produce the same result, Cr surface enrichment which occurs during the 300 K sputtering segment of the cycle. This shows that impurity co-segregation with V and sputter-depletion of V rather than radiation induced segregation of Cr is the dominant mechanism of surface composition modification.