Fatigue crack initiation in additively manufactured Inconel 718: effect of microstructure and defects.

In contrast to other popular AM alloys such as 17-4 PH stainless steel or Ti-6Al-4V where fatigue crack initiation exclusively occurs from volumetric defects, fatigue cracks in additively manufactured Inconel 718 (IN-718), that is in a machined surface condition, often initiate from persistent slip bands (PSBs); therefore, there is competition between PSB- and defect-mediated crack initiation. A crystal plasticity (CP) simulation of cyclic loading of a polycrystalline aggregate is used to investigate the factors governing such competition, which is then validated by experiments. A physics-based, free slip distance (FSD)-dependent hardening law is proposed, which enables the CP model to capture the heterogeneous strain distribution in IN-718. Taking into consideration the contrast in slip activity among the adjacent layers of material, the locations and lives for crack initiation can also be calculated. The distribution of a localization parameter defined as a function of FSD and Schmid factor within a grain is found to correlate well with the locations of PSBs. The maximum values of the localization parameters within a microstructure are shown to correlate well with the experimentally obtained crack initiation lives. The presence of volumetric defects in IN-718 generally do not impact strain localization behavior unless their size is large compared to the grain size.