Through-thickness inhomogeneity of environmentally assisted cracking (EAC) in AA5083-H128 alloy

The through-thickness inhomogeneity of environmentally assisted cracking (EAC) for AA5083-H128 thick plate is investigated based on sensitization treatments, microstructure characterization, slow strain rate tests (SSRT) and J-based fracture tests. More continuous Mg-rich precipitates with a higher Mg concentration are identified along the grain boundaries of sensitized specimens excised from near the top of the plate than those of specimens excised from the middle of the plate. The larger amount and grain boundary coverage of Mg-rich precipitates present after sensitization for samples excised from the top of the plate was consistent with both mechanical tests (i.e. SSRT, J-based fracture) that showed greater EAC susceptibility than samples excised from the middle of the plate. In the J-based fracture tests, sensitized specimens tested in the S-T orientation exhibited greater EAC susceptibility than L-T specimens. In the L-T tests, more grain boundary delamination, likely caused by hydrogen embrittlement from the hydrogen evolution associated with the corrosion of the Mg-rich phases, was exhibited on the fracture surfaces for sensitized specimens removed from the top of the plate compared to the middle. Despite this embrittlement, this grain boundary delamination contributed to the greater crack growth resistance exhibited by the sensitized L-T samples due to delamination toughening.