Room-temperature low-cycle fatigue and fracture behaviour of asymmetrically rolled high-strength 7050 aluminium alloy plates

The asymmetrical rolling (ASR) process with shear deformation is considered as a promising technology to adjust/improve through-thickness microstructure homogeneity and integrated properties of high-strength alu-minium alloy plates. But the advantages may come with caveats that are the subject of our research. In this paper, the room-temperature low cycle fatigue properties and fracture behaviour of the ASR-ed AA7050 alu-minium alloy plates are compared with the symmetrical rolling (SR) one. It is shown that after either type of rolling the plates exhibit similar low-cycle fatigue lives but the SR-ed one displays a better cyclic deformation ability and slightly higher fatigue lives at high strain amplitudes. It is demonstrated that the severe surface localized deformation contributes to the formation of slip relief on the surface and subsequently initiates micro cracks that are propagated via transgranular and/or intergranular fracture modes along with obvious fatigue striations. Recrystallised grains with coarse grain boundary precipitates and wide precipitate-free zones near the upper/bottom layers as well as numerous and larger secondary particles in the ASR-ed plates may cause early crack initiation for a short crack initiation life. However, the SR-ed plate with more frequent subgrains near surface layers, numerous fine subgrains and less indissoluble particles could possess better crack initiation/propagation resistance and cyclic loading behaviour. Fine subgrains with higher microhardness/strength can facilitate passing of dislocations or slip bands into adjacent grains so as to delay crack propagation such as via energy-intensified transgranular fracture for extending fatigue life. Properly balancing the through-thickness strain/deformation distribution and the formation of recrystallization/indissoluble particles via implementing a feasible ASR process becomes a critical issue to achieve fracture-resistant microstructures for high-strength aluminium alloy plates. The underlying causes/mechanisms regarding the differences of microstructures and mechanical behaviour are revealed and discussed based on modelling through-thickness temperature/strain distribution and detailed microstructure characterization.