3 - Atomistic Computer Simulation of Diffusion

Abstract

This chapter presents an overview of atomistic simulation methods currently available for diffusion modeling in materials, focusing on metals and metallic systems. The first step in any atomistic simulation is to establish a model that describes atomic interactions. Since diffusion processes involve the motion of atoms over considerable distances and require statistical averaging, diffusion simulations inevitably deal with relatively large ensembles of atoms. This explains why the overwhelming majority of such simulations are based on classical interatomic potentials, also called force fields. Interatomic potentials allow fast molecular dynamics (MD) and Monte Carlo simulations to be performed for systems containing up to millions of atoms. In addition, various examples of applications are discussed, two areas are emphasized where much progress is achieved due to computer simulations. The first area is diffusion in ordered intermetallic compounds. The atomic order in such compounds imposes strong selection rules on possible diffusion mechanisms by favoring mechanisms that either preserve the order or destroy it only locally and temporarily. The other area is diffusion in grain boundaries in which new collective mechanisms have been discovered that involve both vacancies and interstitials as equal partners. The common thread of these and other examples is the notion that atomistic computer simulations offer a powerful tool for gaining deeper insights into diffusion phenomena, and that this tool is applicable not only to simple systems but also to complex materials of technological importance.