  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.
