A study of microstructure-driven strain localizations in two-phase polycrystalline HCP/BCC composites using a multi-scale model

In this work, we present a 3D microstructure-based, full-field crystal plasticity finite element (CPFE) model using a thermally activated dislocation-density based constitutive description and apply it to study the deformation of a two-phase hexagonal close packed (HCP)-body center cubic (BCC) Zr/Nb composite. The microstructure models were created using a synthetic grain structure builder (DREAM.3D) and a meshing toolset for the 3D network of grains, grain boundaries, and bimetal interfaces. The crystal orientations, grain shapes, and grain sizes for each phase were initialized based on the measured data. With this novel technique, we aspire to couple the evolution of microstructural heterogeneities with the evolution of spatially resolved mechanical fields during the deformation of complex composites. Here, we apply it to understand the role that microstructure plays in the development of the local concentrations in strain and strain rate that can trigger plastic instabilities, such as shear banding. Our chief findings are that 1) local areas of relatively high (and relatively very low) strain concentration occur at triple junctions or quadruple points and then connect via straining to create a banded configuration that extends across the polycrystalline layer, 2) this event starts in the Zr phase and not in the Nb phase, and 3) the triggering hot spots in strain occur at junctions that join grains with very dissimilar reorientation propensities and vice versa for cold spots. In order to determine how such influential localizations can be prevented during processing via application of intermediate annealing treatments, we used the model to also explore the effects of annealing-induced changes in accumulated dislocation density, crystallographic texture and grain shape on the development of strain localizations during subsequent deformation. We found that while it is difficult to avoid strain localizations at grain junctions, when provided a microstructure containing a few large grains spanning the thickness, elongated grain shapes, and reduced dislocation density, the linkage of hot spots in the form of a band can be postponed. At the end we show that when an additional softening mechanism is introduced, these localized strain concentration areas can lead to shear bands. (C) 2015 Elsevier Ltd. All rights reserved.