Unexpected variations in the kinetics of solid solution alloys due to local interactions

Diffusion of atoms in solids is one of the most fundamental kinetic processes that ultimately governs many materials properties. Here, we report on a combined first-principles and kinetic Monte Carlo study of macroscopic diffusion properties of disordered Ti-Ta alloys over the entire composition range. Using simple cluster expansion model Hamiltonians parametrized on density functional theory data, we compute transport properties explicitly including local interactions between the two atomic species and compare them with the non-interacting diffusion model for disordered, random alloys. Surprisingly, we find that although these alloys thermodynamically behave as nearly random solid solutions, their kinetic properties deviate significantly from the behavior predicted by diffusion models for non-interacting systems. We attribute these differences in transport properties to the local interactions that create a rather corrugated potential energy landscape and consequently give rise to energetically non-degenerate end-states of diffusion processes which cannot be realized in a non-interacting disordered or other simpler diffusion models. The findings emphasize the limitations of the widely known non-interacting disordered diffusion model for such systems. Furthermore, we explain that changes in mobility in these alloys is predominantly due to changes in the correlation factor caused by the local interactions. Our work thus highlights the importance of explicitly including local interactions when assessing the transport properties of thermodynamically nearly disordered alloys.