Fatigue crack initiation mechanisms in Inconel 718 from MC carbide

Carbides are the primary inclusions in superalloys and crack initiations associated with carbides are crucial and unavoidable, which can affect the mechanical properties of the material. Carbide-related cracking characteristics at high temperatures have been extensively investigated experimentally, while the local deformation behaviors of carbides under fatigue loading have also been explored numerically. However, the multiscale interaction of carbides with dislocations remains not fully understood. In this study, the focus is on the various cracking mechanisms of carbides and the evolution of cracks in Inconel 718 superalloys studied by integrating quasi-in-situ three-point bending fatigue tests, transmission electron microscope observations, crystal plasticity and discrete dislocation dynamics simulations. This combination of multi-scale experiments and simulations enables a comprehensive understanding of the carbide-related crack nucleation mechanisms. The results reveal the presence of three distinct carbide-related cracking mechanisms, including interface debonding, inclusion cracks, and a newly observed mechanism: slip band cracks between uncracked carbides. Among these, the inclusion cracking dominates from the aspect of nucleation sequence and relative frequency. Through the comparison of various physical quantities, the normal stress valued 606 MPa and the maximum principal stress of 870 MPa can be used to determine the occurrence of interface debonding and inclusion crack. Meanwhile, elevated dislocation densities develop between adjacent carbides due to the coexistence of a high elastic strain gradient and limited plastic strain, indicating a potential site for crack initiation. These findings are crucial for the design of fatigue-resistant Inconel 718 components and for optimizing superalloy manufacturing processes.