Computational study of the geometrical influence of grain topography on short crack propagation in AA7XXX series alloys

Abstract

Intergranular Environmentally-Assisted Cracking (EAC) has recently been reported to be an issue of concern in new-generation 7000 series aluminium alloys, such as AA7085, when exposed to humid air. The cracking process occurs in a highly brittle manner almost exclusively along grain boundaries (GB's) and has been attributed to hydrogen embrittlement, probably by GB decohesion within the stress field at the crack tip. Currently, how the highly heterogeneous grain structures found in these partially recrystallized materials impact the growth behaviour of microstructurally short cracks is poorly understood. In particular, there is expected to be a high sensitivity to the grain structure in the transition from initiation to sustained propagation, where the local mechanical driving force is very sensitive to the crack path. Volume Elements, VE's, with synthetic grain structures have been generated from real microstructure and texture data, so that the effects of important grain structure variables can be explored in crystal-plasticity simulations, to understand the extent to which typical grain-structural features affect the driving force for short-crack growth. Specifically, by considering the effect of different uncrystallised grain aspect ratios and embedding recrystallised grains in the model, the strain energy release rate has been calculated as a function of crack path. This has revealed large reductions and fluctuations in the driving force for short cracks in relation to the local grain structure encountered by the crack tip, which have been estimated by the model.