Crack surface morphology and grain misorientation in fatigued aluminium alloy AA7050 samples after interrupted ageing and retrogression-reageing treatments

We present the investigation of fatigue crack surface morphology and crystallographic grain orientation in samples of aluminium alloy AA7050 that had been subjected to interrupted ageing (T6I4-65) and retrogression-reageing (RRA) heat treatments. Both T6I4-65 and RRA ageing treatments generate bimodal (particle size) microstructural features. A single tensile overload was applied and its influence on the crystallographic grain orientation was evaluated using electron backscatter diffraction (EBSD). The results reveal that T6I4-65 and RRA heat treatments contribute to the increased occurrence of planar slip features in the form of small and large flat facets, respectively. T6I4-65 condition leads to a greater degree of lattice misorientation in the vicinity of fatigue crack tip than the RRA condition which is likely to be the evidence of increased plastic deformation due to the overload.