Environmentally assisted cracking (EAC) requires a combination of stress, suitable environment and a sensitized microstructure. The EAC of AA5083 can take place in ambient conditions where there is a supply of humid air and the microstructure is susceptible to sensitisation in service with exposure to slightly elevated temperatures over long time periods.
EAC, proceeds via initiation, propagation and final failure. However initiation can be sub-divided to include the initial state of the sample, the evolution of the sample surface to create the precursor conditions to promote cracking followed by the first increment of crack growth. Propagation proceeds from short cracks to sustained crack growth and needs to incorporate phenomenon of crack coalescence and non-propagating cracks.
Using time-lapse X-ray computed tomography (CT) we have been able to track the growth of a crack population from initiation through propagation to final failure at high resolution ((1.6 µm)3 voxel size). In a post-mortem follow-up we have correlated the cracks that caused final failure to the electron microscopy of the fracture surface and also investigated in detail the initiation sites with 3D serial sectioning using a Xe+ plasma focused ion beam (PFIB) combined with electron backscatter diffraction (EBSD) and energy dispersive X-ray spectroscopy (EDX).
The cracks were grown in a round bar tensile test specimen subjected to slow strain rate testing in humid air following a sensitization treatment and pre-exposure in a saltwater solution. We observed 44 cracks initiate and grow and positively identified that 42 of them initiated at surface breaking second phase particle clusters. These clusters were predominantly AlFeMnSi but also could contain or were Mg2Si. No positive identification of beta Al3Mg2 was made in these regions but it is possible it had fully dissolved by the time it was observed. Cracking proceeded with a wide range of velocities which at the microscale could be been seen to stop and start at different intervals. There was a large distribution of crack sizes at the time of failure with a significant population of non-propagating cracks existing at a size <300 µm. Whilst crack growth proceeded via intergranular route prior to final microvoid coalescence failure the crack proceeded in a different mode which we have termed Type 2 cracking which appears to be an environmentally assisted mode of tearing.