GALCIT Colloquium

Nanocrystalline metals offer potential for desirable combinations of high strength and ductility. Deformation mechanisms in these materials are complex and multifaceted, influenced by grain boundary network and structure.  Nanoscale deformation processes such as partial dislocation nucleation, dislocation absorption/desorption, and grain boundary shuffling/sliding processes are mediated at material interfaces. Grain boundaries and their junctions fundamentally control nanocrystalline material inelastic deformation mechanisms. Atomistic modeling methods are useful to explore interface-mediated phenomena within nanostructured materials and point to a basic understanding of material deformation. We apply nonlocal kinematic measures from continuum mechanics as metrics to assist in interpreting results of atomistic simulations with regarding to grain boundary atom shuffling versus partial dislocation nucleation as a function of grain size.  Continuum metrics computed from simulations include deformation gradient and Green strain, as well as dual vectors for rotation and vorticity.  Useful insight into the origins of plastic deformation in nanocrystalline metals is gained by their application, as well as sequencing of operative deformation mechanisms during the process of inelastic deformation, enabling understanding of the contributions of competing mechanisms.  Both bicrystals and polycrystals are considered.

The influence of multiaxial stress states on the elasto-plastic deformation behavior of nanocrystalline metals is explored using molecular dynamics simulations. The inelastic yield transition is expressed in terms of an avalanche in stress-driven defect migration events. Avalanching is resolved from the atomic dynamics under non-equilibrium deformation using a novel method based on ensemble thermometry. The deformation response of a Cu ensemble of 5 nm mean grain size was computed at a temperature of 10K under both stress-controlled as well as strain-controlled biaxial loading. Initial yield in nanocrystalline Cu at the grain size considered is weakly anisotropic and shows tension-compression strength asymmetry. Additionally, transient non-associativity of the inelastic strain increment with a von Mises yield surface is observed in the regime of initial yield. Physical origins of tension-compression strength asymmetry are investigated, with both the crystalline phase and the interfacial regions contributing significantly to the inelastic strain.  A systematic study based on molecular dynamics simulations is performed to quantify the deformation behavior of fcc Cu nanocrystals with mean grain size in the range of 5-20 nm under uniaxial tension and compression, as well as shear with superimposed hydrostatic stress.  Results indicate that a compressive hydrostatic stress inhibits stress-induced shear transformations in the interfacial regions and is associated with higher effective flow stress of the microstructure at the grain sizes considered, while tensile hydrostatic stress has the opposite effect.  The influence of initial high effectives stresses induced in the vicinity of grain boundaries is considered and analyzed.