Kinetic Monte Carlo simulations of vacancy diffusion in non-dilute Ni-X (X=Re,W,Ta) alloys

TL;DR

This study uses kinetic Monte Carlo simulations combined with density functional theory to analyze vacancy diffusion in non-dilute Ni-X alloys, revealing the importance of host diffusion barriers and interactions on vacancy mobility.

Contribution

It introduces an interaction model accounting for local chemical environments, highlighting the impact of solute-host interactions on vacancy diffusion in complex alloys.

Findings

Vacancy slowdown is more influenced by host diffusion barriers than solute barriers.

The slowdown effect varies between Ni-W and Ni-Re alloys despite Re diffusing slower.

Enhanced creep strength correlates with vacancy mobility reduction, not solely solute effects.

Abstract

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.