Effect of grain orientations on fracture behavior of polycrystalline metals

Abstract

Currently no systematic approach exists to explicitly quantify the combined effects of anisotropic grain orientations and grain boundary behavior on the fracture of polycrystalline materials. A multiscale cohesive finite element (CFEM) based computational framework is developed to predict the fracture properties such as KIC and JIC of polycrystalline metals as functions of microstructural attributes. The focus is on characterizing the effects of crystallographic texture on fracture toughness. The framework uses computationally generated statistically equivalent microstructure sample sets with varying proportions of textured grains, allowing statistical variations and distributions of the fracture behavior due to microstructural variabilities and the influences of intergranular and transgranular fracture mechanisms to be quantified and analyzed. A crystal plasticity formulation is used to model the anisotropic deformation in the grains. A misorientation-dependent interfacial relation is used to model texture-sensitive crack growth through grains and grain boundaries. The framework allows exploration of the effects of microstructure on the macroscopic fracture measures via the manifestation of the different fracture mechanisms. Calculations carried out for Mo under 2D and a more general 2.5D plane strain conditions capture and delineate the competing effects between (a) intergranular and transgranular fracture and (b) plasticity and crack growth on the overall fracture toughness of the material. Both the 2D and 2.5D models use the same sets of microstructures; the 2.5D model uses columnar grains. The results indicate that, as the fraction of grains with preferred orientation increases, transgranular fracture dominates relative to intergranular fracture. Consequently, the relative contribution of plastic dissipation associated with transgranular fracture is enhanced, resulting in higher overall fracture toughness for the material setting analyzed. The effect is characterized in terms of the effective grain size distribution, fraction of grains with favourably oriented slip systems, and misorientation-dependent GB characteristics. Finally, an analytical correlation is established between the overall fracture resistance and the microstructure attributes.