Effect of microstructure on fatigue crack deviation in AA2050-T84

Precipitation hardened AA2050 exhibits an unusual crack deviation phenomenon during fatigue testing: when the intended crack propagation direction is not parallel to macroscopic rolling direction, after initial propagation perpendicular to the loading direction, the crack deviates and further abnormally propagates in other direction which can be parallel to the direction of applied loading. Such a behaviour renders difficult the prediction of cracking in AA2050 and, thus, of AA2050 fatigue lifetime. In the present work using multi-scale microstructure characterization (SEM/EBSD/EDS, transmission electron microscopy (TEM) and atom probe tomography (APT)) of AA2050-T84 after fatigue tests, the effect of different microstructure parameters on crack deviation is carefully investigated keeping constant the macroscopic mechanical conditions (sample geometry, loading direction and amplitude, frequency). The respective influence of grain structure (including that of grain boundaries), intermetallics and precipitate-free zones is evaluated. The grain boundaries with misorientation angle ranging from 40 to 60 degrees are observed to be the origin of the deviation phenomenon: the principal crack deviates at grain boundaries and further propagates along grain boundary. Numerous Fe-and Mn-rich intermetallics found in the volume of AA2050-T84 do not represent the major cause for the crack deviation, although, fostering it (only 25% of cracks were observed to deviate in the regions containing the intermetallics). The cause of failure at grain boundaries was deduced from APT results and TEM observation of precipitate free zones comparing T84 and T34 states (the latter do not reveal the crack deviation). In addition, it was observed that because of unrecrystallized state of AA2050-T84, the grains feature "layered"strained structure favourable to deviation inside the grains when a critical internal misorientation is reached along the crack path.