Computational Study of Microstructurally Short Crack Propagation in 7xxx Series Aluminium Alloys

Abstract

Intergranular Environmentally-Assisted Cracking (EAC) is known to be an issue of concern in new-generation 7xxx series Al alloys with high zinc content, Zn > 8 wt%, when exposed to humid air. The cracking process occurs in a brittle manner almost exclusively along grain boundaries (GBs) and has been attributed to hydrogen embrittlement. How the highly heterogeneous grain structures found in these partially recrystallised materials impact the growth behaviour of microstructurally short cracks is currently 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. A new model to study EAC in 7xxx series alloys is also presented, which is implemented in the D¨usseldorf Advanced Material Simulation Kit (DAMASK). It includes the coupling of crack tip H generation and transport, crystal plasticity, and trapping of H at dislocations, the GBs, and crack tip stress field, to predict crack propagation at the microstructural length scale. When applied to study EAC in 7xxx series Al alloys, the novel implementation of different local H source functions at the crack tip has provided insight into the dominant rate- controlling processes associated with intergranular EAC. Additionally, while simulating realistic grain structures, the model could reproduce predictions for the crack velocities present in experimental 4-point-bend (4PB) performance tests. Furthermore, the model provided insight into whether the total amount of H, or the rate at which it is generated, is ultimately more important in determining EAC growth rates.

Date of Award	26 Mar 2025
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Timothy Burnett (Main Supervisor) & Philip Prangnell (Co Supervisor)