A model for predicting fracture mode and toughness in 7000 series aluminium alloys

A model is proposed, which predicts the toughness of 7000 series aluminium alloys in a variety of situations, including two alloy compositions, different quench rates from the solution treatment temperature and ageing states from underaged to overaged. The model is derived in three steps. The energy dissipated by transgranular fracture is first calculated, using a simplified cohesive zone approach. The energy dissipated by intergranular fracture is then calculated using a critical strain criterion, and the total dissipated energy is then estimated using an averaging by the respective area fractions of the two modes, which are themselves dependent on the respective energies of the two main fracture mechanisms. The model input parameters are the material's mechanical properties such as yield stress and strain-hardening rate, geometrical features and related properties of the precipitate-free zones, and area fraction of the grain boundaries covered with precipitates. The model predicts all the main features of the evolution of the toughness/yield strength compromise with changing quench rate or ageing treatment. It allows to predict the evolution of toughness when dramatic changes in the occurrence of fracture modes are observed. Finally, using the model it is possible to predict the effect of changes in individual parameters on the overall fracture behaviour.