MULTI-SCALE DIFFRACTION STUDY OF REVERSIBLE/IRREVERSIBLE DEFORMATION MECHANISMS IN THE NI-BASED SUPERALLOYS DURING FATIGUE

In this study, a nickel-based polycrystalline is subjected to cyclic loading. The subsequent fatigue damage has been investigated with in-situ neutron-diffraction, thermal characterization for a single-phase, transmission-electron microscopy (TEM) and polychromatic X-ray microdiffraction (PXM). Different stages of fatigue damage are observed including bulk hardening, softening, and eventual saturation evident in the diffraction patterns and the thermal-evolution features. An increase in dislocation density is responsible for hardening within the early cycles. The transition to saturation cycles is characterized by the anisotropy of the lattice-strain evolution. Inhomogeneity of the thermal response and irreversible compression of the lattice planes and statistical dislocation structures are observed in the final saturation fatigue cycles. Analysis of the PXM-Laue patterns reveals cyclically-deformed microstructure near the grain boundaries, which are composed of the lattice rotations and grain subdivisions. The PXM results are in good agreement with the TEM results. Combined simulation/experimental analysis allows determination of slip-system dependent dislocation density in individual grains.