Fatigue-induced microstructural deformation and multimode defect-assisted cracking of laser powder bed fused superalloy at 650 °C

Multi-scale characterization was conducted to investigate the interior failure behavior of a laser powder bed fused (LPBF) nickel-based superalloy under fatigue cyclic loading at operating temperature of 650 °C. The results reveal a shift in crack nucleation sites with increasing fatigue life–from additive manufacturing defects to crystallographic facets. Six distinct interior fatigue failure modes were identified, each involving defect-assisted crack nucleation surrounded by faceted features. Microcracks preferentially propagated in a transgranular fracture mode under localized shear stress, leading to grain fracture and facet formation. Variations in grain orientation influenced local fracture behavior, resulting in the formation of highly uneven facets. Competition between surface, subsurface and interior crack nucleation modes was observed, particularly at lower stress levels, indicating a transition in dominant fatigue mechanisms. At elevated temperatures, facet cracking was driven by a synergistic mechanism involving anti-phase boundary shearing, precipitate bypassing, and stacking fault shearing. These findings advance the understanding of defect-microstructure interactions and provide a basis for improving fatigue life prediction and design strategies for high-temperature LPBF components.