Microstructural Modeling of CSL Grain-Boundary Effects and Crack Growth in F.C.C. Polycrystals

Abstract

A new multiple-slip rate-dependent crystalline constitutive formulation that is coupled to the evolutionary equations of mobile and immobile dislocation densities is developed. Dislocation densities were modeled as internal state variables that provide a more detailed microstructural description of the material's inelastic deformation and interrelated physical mechanisms that control different failure modes. Specialized microstructurally-based finite-element schemes have been used to investigate the effects of crystallographic orientations of the grains and grain-boundaries (GBs), grain shape and size, (GB) misorientation and the dependency of GB strength and mechanical properties on specific CSL misorientations on the inelastic finite deformation and failure mode mechanisms in f.c.c. polycrystalline aggregates. A Voronoi algorithm was used to generate grains and GBs with random shapes and sizes. The combined effects of GB misorientation, structure and geometry, strain hardening, localized plastic shear slip, intensive regions of crystal lattice rotation and the evolution, interaction and accumulation of dislocation densities on the nucleation and growth of intergranular and transgranular failure modes in f.c.c. polycrystalline aggregates were studied. Results from this study are consistent with experimental observations that microstructures with desired material properties, such as resistivity to crack nucleation, can be achieved by the control of the crystallographic orientation distribution of the grain aggregate and CSL GB orientations. Results from this study show that transgranular failure modes are dominant in aggregates with a high frequency of Sigma-3 GBs, and intergranular fracture modes dominate the aggregate with a high frequency of Sigma-17b GBs.