Density functional study of self-diffusion along an isolated screw dislocation in fcc Ni

Abstract

Vacancy-mediated diffusion along dislocations, often referred to as pipe diffusion, can contribute to creep deformation of metals in many engineering applications. This process is studied along an $\frac{a}{2}\ensuremath{\langle}1\overline{1}0\ensuremath{\rangle}$ screw dislocation in fcc Ni using a density functional theory approach. An accurate geometrical configuration of the screw dislocation core, dissociated into Shockley partial dislocations and separated by a stacking fault, was previously derived using a lattice Green's function technique. Activation energies and jump frequencies are calculated for atom-vacancy exchanges that contribute to diffusion around and along one of the partial cores. This analysis reveals the significant role of the sites within the compressive component of the dislocation, the dominant contribution of the hops around the screw geometry rather than directly along the dislocation line, and the importance of including the stacking fault sites. Kinetic Monte Carlo simulations use these energies and frequencies to generate diffusion coefficients that account for correlation effects. Near 80% of the melting temperature ${T}_{m}$, these pipe diffusivities are an order of magnitude higher than those found in fcc regions, and they are eight orders higher at room temperature. Calculations are compared to experimental results and the differences are discussed. While pipe diffusion is unlikely to contribute to isotropic mass flux at low dislocation densities, it will accelerate dislocation mechanisms controlling creep and climb.