Grain-boundary diffusion creep in nanocrystalline palladium by molecular-dynamics simulation

Abstract

Molecular-dynamics (MD) simulations of fully three-dimensional (3D), model nanocrystalline face-centered cubic metal microstructures are used to study grain-boundary (GB) diffusion creep, one mechanism considered to contribute to the deformation of nanocrystalline materials. To overcome the well-known limitations associated with the relatively short time interval used in our MD simulation (typically <10 −8 s), our simulations are performed at elevated temperatures where the distinct effects of GB diffusion are clearly identifiable. In order to prevent grain growth and thus to enable steady-state diffusion creep to be observed, our input microstructures were tailored to (1) have a uniform grain shape and a uniform grain size of nm dimensions and (2) contain only high-energy GBs which are known to exhibit rather fast, liquid-like self-diffusion. Our simulations reveal that under relatively high tensile stresses these microstructures, indeed, exhibit steady-state diffusion creep that is homogeneous, with a strain rate that agrees quantitatively with that given by the Coble-creep formula. The grain-size scaling of the Coble creep is found to decrease from d −3 to d −2 when the grain diameter becomes of the order of the GB width. For the first time a direct observation of the grain-boundary sliding as an accommodation mechanism for the Coble creep, known as Lifshitz sliding, is reported.