Void growth based inter-granular ductile fracture in strain gradient polycrystalline plasticity

The precipitation hardened, high strength aerospace alloys (e.g. Al 7000 alloy series) suffer from loss of fracture toughness due to the heat treatment leading to intergranular ductile fracture. Depending on the quenching and aging processes, large precipitates at the grain boundaries with wide precipitate free zones might develop. Therefore the grain boundaries constitute a potential location for micro void formation and evolution under the effect of external loads. This is a common problem of such materials where there is considerable ductile intergranular fracture, which is normally attributed to the embrittlement effects of the environment in other type of alloys. In this context, for the modeling of such a degradation process, the current paper develops a physics based intergranular cracking model of polycrystalline materials where a strain gradient crystal plasticity model is combined with cohesive zone elements whose traction separation relation is based on the evolution of micro-voids at the grain boundaries. The framework successfully predicts the intergranular crack formation and propagation, taking into account different microstructural features, such as porosity, pore shape, grain orientation distribution, and grain boundary conditions.

Automated summary

High strength aerospace alloys, such as the Al 7000 series, are prone to loss of fracture toughness during heat treatment, leading to intergranular ductile fracture. This is often caused by the formation of large precipitates at grain boundaries and the development of precipitate free zones. Consequently, grain boundaries become potential locations for micro void formation and evolution under external loads, resulting in intergranular crack formation and propagation in the material.