In-situ study of planar slip in a commercial aluminum-lithium alloy using high energy X-ray diffraction microscopy

Aluminum-lithium alloys offer high specific-strength and -stiffness which makes them attractive for aerospace applications. However, several material related challenges have limited the use of these alloys including a tendency for grain boundary cracking due to localized damage known as delamination. Prior studies of these alloys have found evidence of a relationship between delamination and the occurrence of intense planar slip due to the shearing of the δ' (Al₃Li) precipitate phase. In this study, far-field high-energy X-ray diffraction microscopy is used to quantify the effects of slip activity and subsequent stress response of the Al-Cu-Li alloy (AA2099) in both individual grains and various grain populations. Results show that grains with high Schmid factors, interpreted as an indicator of propensity for single slip at the elastic-plastic transition, are most likely to undergo grain softening consistent with precipitate-shear-driven planar slip. Conversely, grains with low Schmid factors are less prone to this deformation mode. A corresponding rise in triaxiality is found in grains which soften. The implications of pairings of grains exhibiting dissimilar micromechanical behaviors for delamination are discussed.