Multiscale simulation of the competition between transgranular and intergranular ductile fracture

The competition between intergranular and transgranular fracture in aluminium alloys with precipitate free zones (PFZs) along grain boundaries is investigated using a multiscale approach. Ductile fracture occurs both in the grains and in the soft PFZ through the nucleation of voids at second phase inclusions, their growth and subsequent coalescence. A grain level unit cell model has been developed in order to capture the link between the microstructure, the flow properties and the ductility, which is very much dictated by the inter-vs. trans-granular mode of cracking. The constitutive description used in both regions is an extension of the Gurson model, accounting for void shape effects, void rotation and hardening. When the yield stress in the interior of the grains is high (resp. low), the fracture is purely intergranular (resp. transgranular). For intermediate yield stress, part of the fracture is intergranular and part of the fracture is transgranular. It is also shown that the elongation of the grain in the principal loading direction (resp. in the transverse direction) favours transgranular (resp. intergranular) mode of failure.