Atomic-scale study of diffusion in A15 Nb3Sn

The point defect and diffusion properties of A15 Nb3Sn are investigated using ab initio density functional theory calculations and statistical thermodynamics. The defect structure is found to be of antisite type, with small amounts of Nb vacancies, and Sn vacancies showing a trend towards instability. Diffusion occurs mainly on the Nb-sublattice (restricted to intrachain jumps for both species), Sn-sublattice exchanges being unlikely for both species. In addition, ordering (disordering) is found to occur via Sn (Nb) jumps. The calculated Nb and Sn tracer diffusion coefficients exhibit a low sensitivity to the alloy composition around stoichiometry at 1000 K, with D-Nb(*)> D-Sn(*) provided the correlation between atomic jumps is taken into account. Agreement with interdiffusion measurements is reached with reasonably low values for the geometrical correlation factor.