The role played by two parallel free surfaces in the deformation mechanism of nanocrystalline metals: A molecular dynamics simulation

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Abstract
We report molecular dynamics simulations which investigate the influence of two parallel free surfaces in the deformation mechanisms of model nanocrystalline fee materials. The purpose of this simulation is to study which phenomena observed in in-situ tensile experiments performed in the electron microscope can be expected to be intrinsic properties of the deformation process and which phenomena result from the thin-film geometry necessary to obtain optimal (high-resolution) transmission electron microscopy contrast and resolution. It is found that, in nanocrystalline samples with thin-film thickness comparable with that used in experiment, the extent of the surface's influence is of the order of the grain size. For a sample with a mean grain size of 5 nm, this is evidenced by an increase in intergranular deformation (sliding) at the surface and, for a sample with a mean grain size of 12nm, in addition to increased sliding at the surface, there is a significant increase in dislocation activity that extends throughout the sample.