Extreme Stress Gradients in Cyclically Loaded Polycrystalline Alloys

During cyclic loading, dislocations accumulate at microstructural features such as grain boundaries and form complex defect aggregates within the interior of individual grains; such structures lead to intragranular variations of elastic strain and orientation. With increasing cycles, these variations lead to potentially large stress/strain gradients and inevitably will affect the fatigue performance of the material. Grain averaged elastic strains were tracked via high energy X-ray diffraction microscopy during high cycle fatigue loading of two polycrystalline alloys, a Ni-based superalloy (LSHR) and a binary alloy (Al-Li). Following cyclic loading, individual grain(s) within the bulk of the specimen were further probed by dark field X-ray microscopy, a high-resolution X-ray characterization technique, to characterize the 3D elastic strain and orientation fields around internal dislocation structures. For the LSHR sample, The elastic strains from DFXM exhibit steep gradients in the elastic strain were measured in a region in direct proximity to the twin boundary, on the order of 400 MPa over a 30 micron span. For the Al-Li sample, discrete slip bands are identified with the associated orientation and elastic strain values surrounding these regions. This study provides new insights of fatigue induced dislocation structures, such as persistent slip bands, in the bulk of polycrystalline materials, which are unable to be captured with standard 2D surface or destructive techniques.