Electronic Origin of Fast Sulfur Diffusion in 3d Transition Metals

The microscopic origins of abnormally fast diffusion of sulfur in nickel have been investigated. Transition state theory of vacancy mediated diffusion of substitutional impurities with parameters calculated from first-principle density-functional theory (DFT) was used to determinate of the diffusion coefficients of S and Al impurities in fcc Ni. Sulfur diffusion coefficient was found to be two orders of magnitude higher than for aluminum in good agreement with experimental data. We found that sulfur has a very low barrier for jump toward vacancy and also significantly decreases migration barriers for neighboring nickel atoms. We discuss the microscopic factors contributing to the dramatic difference in S and Al diffusion coefficients and show that electronic structure and chemical bonding play crucial role in enhanced diffusion of S. We also found that S considerably increases Ni self-diffusion rate. The implications of S effect on the stability of thermally grown oxides in superalloys are discussed.