PHASE FIELD MODEL OF INELASTIC DEFORMATIONS AND GRAIN BOUNDARIES

Abstract

Mechanical and functional behaviour of nanocrystalline (NC) materials (d<100nm) is remarkably different to their coarse-grained (CG) counterparts. Experimental and atomistic simulation studies have shown interesting transitions from dislocation-mediated plasticity to grain boundary-mediated plasticity when the grain size is decreased to the nanoscale. Other studies of NC shape memory alloys (SMAs) have demonstrated an interesting behaviour of suppressing the underlying martensitic transformations with the grain refinement. In some metals, the increased volume fraction of grain boundaries manifests the transition from perfect to partial dislocation slip eventually leading to deformation twins that coevolve with grain boundaries. In this work, a phase field framework is proposed to model this unique and peculiar behaviour of NC metals by coupling elastic and inelastic deformations occurring within the grains with grain boundary mechanisms (grain boundary migration or grain rotation). Extra misorientation dependent energy contribution is considered to model suppressions of phase transformations in functional materials. Coevolution of grain boundaries and intra-granular bulk dislocation plasticity is studied with respect to elastic and plastic anisotropies by employing grain-scale Hill yield plasticity. Furthermore, a full 3D phase field model of deformation twinning in FCC metals is developed to explicitly investigate the grain/ twin boundary interactions.