NANOCRYSTALS

Clearly, atomistic simulations are most useful to characterize the structure of grain boundaries and unit processes of dislocation emission, ledge formation, absorption and transmission. But, the large length and time scales of polycrystalline responses preclude application of atomistics alone and necessitate a strategy for bridging scales based on continuum models. However, conventional continuum crystal plasticity is inadequate for this purpose for a number of reasons, most notably in its inability to distinguish the effects of GB character on interfacial sliding and dislocation nucleation/absorption processes. GBs are treated as geometric boundaries for purposes of kinematics compatibility in conventional theory. Moreover, although continuum micromechanics-based approaches have been developed to incorporate GB boundary surface effects in NC metals (Qu 1993), they are still not able to factor in dislocation sources in nucleation-dominated regimes and to predict appropriate concentrations of stress at grain boundary ledges and triple junctions. Moving towards an appropriate theory of cooperative response of nanocrystalline materials requires a combination of three modeling elements: molecular statics/dynamics, enhanced continuum crystal plasticity, and self-consistent micromechanics. Such a theory should be able to model kinetics of dislocation nucleation and motion properly, as well as coarsening and shear banding phenomena. The latter is a challenge that requires the notion of cooperative slip localization to be introduced over many grains. The objective of the proposed ANR project is to develop a framework that can link scales of atomic level GB structure with emission of dislocations, GB-dislocation interactions, and GB sliding processes, informing the structure of enhanced crystal plasticity modeling (based on generalized continua like Cosserat media or strain gradient media) and self-consistent modeling methodology of anisotropic elastic-plastic crystals that can handle both bulk dislocation activity and GB processes. The theory will be founded on consideration of the surface area to volume ratio in polycrystals, along with accurate accounting for surface energies and activation energy estimates for various nucleation sources, which affect the change to grain boundary-mediated deformation processes at grain sizes below several hundred nm. Grain size distribution effects will be considered as well. The multi-scale numerical modeling framework will be enriched by experimental methods in order to apply and validate the different findings and possibly overcome scale limitations of atomistic modeling. ...