Effects of grain boundary structure in discrete simulations of multiscale dislocation dynamics

Abstract

This research develops a multiscale approach to study the defect-driven evolution of the microstructure in polycrystalline FCC metals, by using atomic-informed models to reproduce multi-grain, microscale simulations with discrete dislocation dynamics (DD). The dynamically evolving dislocation and grain boundary (GB) microstructure dictate the non-linear ‘plastic’ response of polycrystalline metals during deformation. The processes of intercrystalline plastic coupling are generally a consequence of multi-dislocation pile up interactions with GBs, however such effects are beyond feasible limits of conventional atomic simulations or experimental studies. More accurate computational models of the discrete micro-macro scale dislocation and GB dynamics could be applied to elucidate the fundamental mechanisms and reduce the dependence on costly experimental testing. This work expands upon the physics-based single crystal DD framework by using molecular dynamics simulations to develop structure-property models that compensate for the variable characteristics of GB interfaces. This research involved studies with a broad set of equilibrium, non-equilibrium and metastable atomic bicrystals. The results showed that the intercrystalline strength was strongly influenced by the ‘GB shape’ characteristics of the structural free volume, in addition to the intrinsic energy. Defect interaction studies verified the realistic conditions for the observation of structural multiplicity, and showed the significance of intermediate ‘transitory’ GB states. Observation of the slip transmission process showed that the GB dislocations acted as pinning sites, so that the intercrystalline plasticity resembled Orowan-type obstacle bypassing. A novel multi-orientation approach was developed to provide a control routine for modelling atomistic dislocation dynamics that enabled a self-consistent multiscale comparison with DD simulations. Insights regarding dislocation core construction and Burgers vector analysis played an integral role in the development of atomically accurate DD representations of the GB dislocation structures. A world-first 3D polycrystal DD approach was implemented to provide a discrete representation of the dislocation structure of high angle GBs, through the culmination of the atomistic insights and many years of concerted code development. This robust, versatile implementation enables tailor-made polycrystal simulation specimens with arbitrary shape, grain orientation and size to be generated from common .stl files. The unique, comprehensive modelling framework provides a realistic reproduction of key intercrystalline slip mechanisms including GB sliding, dissociation and absorption, dislocation transmission and reflection. As such, the present work provides a novel contribution of relevance, particularly, to the developing sector of GB engineered nanocrystalline materials.