Molecular dynamics simulations of the effects of vacancies on nickel self-diffusion, oxygen diffusion and oxidation initiation in nickel, using the ReaxFF reactive force field

A ReaxFF reactive force field was developed for the nickel-oxygen system. The quantum mechanical (QM) data used to derive the force field parameters included the equations of state of various phases of nickel and that of nickel oxide (NiO), the vacancy formation energy and the vacancy-mediated self-diffusion barrier in the face-centered cubic nickel. Furthermore, in order to study the interstitial diffusion of oxygen atoms in the nickel matrix, the oxygen insertion energies and the diffusion barriers were included in the training set. The force field was validated by performing molecular dynamics (MD) simulations of self-diffusion of nickel and the interstitial diffusion of oxygen. The predicted diffusivity and the activation energy achieved quantitative agreement with their respective published values. Furthermore, this force field enables study of the effects of vacancies on the diffusion of dissolved oxygen and the successive initiation of internal oxidation. A new oxygen diffusion mechanism is proposed in which the oxygen atom diffuses via the movement of the oxygen-vacancy pair. In addition, the MD simulation results suggest that the voids at the grain boundaries can induce local oxygen segregation due to the strong oxygen-vacancy binding effect, which is responsible for the formation of a nickel oxide particle in the void. These results demonstrate that the ReaxFF MD study can contribute to bridging the gap between the QM calculations and the experimental observations in the study of metal oxidation.