Molecular-dynamics methods were used to model diffusion in a 5 [100] Al tilt boundary and in the bulk (tables 7 and 8). The diffusion coefficient D and activation energy Q for atoms in the boundary and in bulk were calculated for several different Al empirical interatomic pair potentials. These included a Morse potential, spline potentials fitted to bulk experimental data (elastic constants, phonon spectra, etc.), and a pseudopotential. Reasonable agreement was obtained with experimental diffusion values for Al, although activation energies were low. There was also a wide variation in results from one potential to another because the atomic motion was sensitive to the shape of the primary well or minimum of the interatomic potential. Rescaling the data with rough estimates of the different bulk melting temperatures, that each potential predicted, reduced the discrepancy between potentials. This was shown to be true for virtually any pair potential by calculating diffusion rates for Morse potentials with changing potential-well depth, position, and width. The variations between potentials were also explained in a quantitative sense by a simple calculation of potential-energy barrier height for vacancy migration in a bulk model. A method was given for using a linear relationship between barrier height and melting temperature to predict diffusion coefficients and general transport properties in grain boundaries and bulk for any pair potential in any face-centered cubic metal.

Effect of Interatomic Potential on Simulated Grain-Boundary and Bulk Diffusion: a Molecular-Dynamics Study. Plimpton, S.J., Wolf, E.D.: Physical Review B, 1990, 41[5], 2712-21

Table 7

Grain-boundary diffusion parameters and migration energy for Al

Table 8

Bulk diffusion parameters and migration energy for Al