Large-scale atomistic simulations were used to study various mechanisms of plastic deformation in uncapped sub-micron thick polycrystalline films. It had been shown that diffusional mass transport along grain boundaries in thin films led to the formation of a novel defect which was identified as being a diffusion wedge. A crack-like stress field eventually developed, near to the grain-boundary/substrate junction, as tractions along the grain boundaries relaxed under the constraint that adhesion between the film and the substrate prohibited strain relaxation close to the interface. The occurrence of crack-like stress concentrations caused the nucleation of an unexpected class of dislocations near to the root of the grain boundary; on glide planes parallel to the film surface. Such dislocations were unexpected because there was no driving force for parallel glide in the overall biaxial stress field. It was demonstrated here that parallel glide dislocations dominated plasticity in polycrystalline sub-micron thin films when tractions along the grain boundaries were relaxed by diffusional creep. It was shown that partial dislocations played an important role in the plasticity of nanostructured thin films, and that the grain-boundary structure had a marked effect upon the dislocation density in neighboring grains. In order to permit the modelling of thicker films, a discrete dislocation model of diffusional creep was proposed for the investigation of the effect of parallel glide upon the flow stress of sub-micron films. A deformation map was used to summarize the ranges of predominance of various strain relaxation mechanisms in ultra-thin films. It was shown that, as well as the classic threading dislocation regime, numerous novel mechanisms appeared when the film thickness approached the nanoscale.

Hierarchical Multi-Scale Modelling of Plasticity of Sub-Micron Thin Metal Films. M.J.Buehler, A.Hartmaier, H.Gao: Modelling and Simulation in Materials Science and Technology, 2004, 12[4], S391-413