Surface segregation and preferential sputtering in CrMo alloys

Surface analysis by AES of eleven CrMo alloys subject to 2 kV argon ion bombardment shows preferential sputtering of Cr. The data indicate a sputter rate constant ratio of 1.71 ± 0.05, in agreement with the ratio of the sputter yields for the pure metals. Annealing of a 60% Mo alloy segregates Cr to the surface. Surprisingly, more segregation is observed at 1125 than at 925 K. It is proposed that the sputter-modified sub-surface homogenises with the bulk at 1125 K but not at 925 K. This hypothesis is confirmed by diffusion rate and layer calculations.