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

Abstract

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 aluminium 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 aluminium 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.