  The mobility of vacancies in alloys may limit dislocation climb. Using a
combined density functional theory and kinetic Monte Carlo approach we
investigate vacancy diffusion in Ni-Re, Ni-W, and Ni-Ta binary alloys up to 10
at.% solute concentration. We introduce an interaction model that takes into
account the chemical environment close to the diffusing atom to capture the
effect of solute-host and solute-solute interactions on the diffusion barriers.
In contrast to an ideal solid solution it is not only the diffusion barrier of
the solute atom that influences the vacancy mobility, but primarily the change
in the host diffusion barriers due to the presence of solute atoms. This is
evidenced by the fact that the observed vacancy slowdown as a function of
solute concentration is larger in Ni-W than in Ni-Re, even though Re is a
slower diffuser than W. To model diffusion in complex, non-dilute alloys an
explicit treatment of interaction energies is thus unavoidable. In the context
of Ni-based superalloys two conclusions can be drawn from our kinetic Monte
Carlo simulations: the observed slowdown in vacancy mobility is not sufficient
to be the sole cause for the so-called Re-effect; and assuming a direct
correlation between vacancy mobility, dislocation climb, and creep strength the
experimentally observed similar effect of W and Re in enhancing creep strength
can be confirmed.
