Study of microstructural grain and geometric size effects on plastic heterogeneities at grain-level by using crystal plasticity modeling with high-fidelity representative microstructures

In-depth understanding of plastic heterogeneities at grain-level and local deformation behaviors is pivotal in exploring the mechanical response and fracture mechanism of thin metal sheets in micro-scale deformation. This work employs a state-of-the-art crystal plasticity spectral method in conjunction with comparable high-fidelity microstructures to study the coupling effect of micro-mechanical (intrinsic) heterogeneities introduced by grain size, morphology, orientation, and inter-granular interaction, as well as macro-mechanical (extrinsic) heterogeneities imposed by geometrical features (i.e., thickness and free surface). The study reveals that the extrinsic and intrinsic factors influence the plastic heterogeneity and fracture failure of metal foils via affecting: (i) the strain and stress fluctuations at grain-level, which are directly related to the fracture morphology observed experimentally; (ii) the morphology of local shear bands which accounts for the transformation of fracture modes with the decrease of the ratio (λ) of thickness to grain size; and (iii) statistical characteristics of plastic heterogeneities which interpret the sharp reduction of the fracture toughness with the decrease of λ. Moreover, the research further presents some findings about the influence of the examined factors on the stress and strain heterogeneities, lattice rotation, slip system activation, and the surface roughness of the material. This work therefore provides a well-rounded exploration of micro-scale plasticity of metal foils at grain-level, and serves as a physically motivated basis for the development and modeling of fracture and failure of metal polycrystals in micro-forming process.