Deformation mechanism in nanocrystalline metals: experiments and atomistic simulations

Molecular dynamics deformation studies of nanocrystalline metals suggest a plasticity mechanism where dislocations are nucleated at grain boundaries (GB), travel through the grain being hindered by local GB stress intensities, to be finally absorbed in the surrounding GBs [1][2]. Recent constant strain rate molecular dynamics simulations of nanocrystalline Al demonstrated that dislocations can also exhibit cross-slip via the Fleischer mechanism [3][4]. The GB structure strongly influences when and where crossslip occurs, allowing the dislocation to avoid local stress concentrations that otherwise can act as pinning sites for dislocation propagation. These simulation studies are performed on pure samples whereas it is well known that impurities influence the deformation behaviour [5]. Using a local chemical potential approach that optimizes the charge on only those atoms expected to be ionic, the influence of dilute oxygen in grain boundaries can be studied [6]. On the other hand, there are many experimental observations of stress driven grain coarsening, something that is seldom observed in simulations performed on a nanocrystalline 3-dimensional GB network [7]. Often coarsening is suggested to be related to coupled GB migration which is expected to be influenced by the role of impurities [6]. In this talk the latest suggestions from molecular dynamics are discussed in terms of the observed deformation mechanism, the role of impurities and last but not least, the effect of the high strain rates applied. Furthermore the simulation results are assessed in terms of experiments performed on nanocrystalline metals.