Abstract
The microstructural origins and early behavior of crack formation in AA6451 samples under three-point bending has been investigated using a multiscale electron microscopy-based approach. Two different heat treatments were investigated: solution treated and naturally aged (T4) and artificially aged (T6). Electron backscatter diffraction and cross-sectional scanning electron microscopy imaging showed that, prior to crack formation, grain boundary ledges formed. These ledges formed adjacent to grain boundaries whose surface traces were oriented parallel to the bend axis and their planes forming angles below 70° with the sample surface. The grain boundary misorientation angle did not show any correlation with grain boundary ledge or crack formation. The sub-surface deformation fields associated with the grain boundary ledges were strongly affected with the precipitate state, with grain refinement and ultrafine grain formation occurring in artificially aged samples in the precipitate free zone and more diffuse deformation structures forming in the non-aged samples. In both cases, constituent particles were not found to have a significant influence on damage nucleation processes.
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