An interface compatibility/equilibrium mechanism for delamination fracture in aluminum-lithium alloys

This work describes a mechanism for the initiation of delamination cracks in Al-Li alloys based on the soft/stiff character of adjacent grains. Small-scale-yielding, crystal plasticity simulations of divider grain configurations (L-T) reveal an elevated mean stress on grain boundaries. This mean stress increase drives a sharp localization of the Rice-Tracey parameter to the grain boundaries - elevation of the RT parameter indicates favorable conditions for void growth and triggering of delamination cracking, in agreement with the fractography of Ritchie and co-workers. Our simulation results and available experimental evidence indicate delamination initiates typically between soft/stiff grain pairs, often Bs (Bunge-convention Euler angles ϕ1=131°, Φ=83°,ϕ2=307°) or S (ϕ1=233°, Φ=151°,ϕ2=105°) orientations. The crystal plasticity results and a simple model of a soft/stiff material interface show that mean stress accumulation is a consequence of the mechanics of such an interface, and not necessarily tied to material inhomogeneities near the GBs (such as precipitate free zones).