Orientation-related fatigue crack initiation behavior at twin boundary of Inconel 718 in vacuum environment of 650

•Fatigue crack initiation behavior related to twin boundary in IN718 was studied in vacuum at 650 ℃.•Twin boundary is the preferential cracking site rather than grain boundary or slip bands.•Dependency of twin boundary cracking on its angle with maximum tensile stress direction was revealed.•Crystallographic orientation related to twin boundary cracking in face centered cubic structure was elucidated. Complicated interaction of twin boundary with dislocation plays a pivotal role in mechanical properties. In this study, the effect of TB on the fatigue crack initiation of Inconel 718 superalloy was investigated in vacuum environment of 650 ℃. A three-point bending fatigue test with preprocessed plate specimen was applied to trigger multi-source crack initiation. Plenty of slip band patterns and crystallographic orientations were acquired by SEM and EBSD. Twin boundary cracking was observed as the main fatigue failure mode. Crack propagation was accomplished by connecting twin boundary cracks with transgranular cracking. Perfect deformation compatibility of twin boundary was achieved only when both symmetric slip systems operated the collinear slip directions on twin boundary. It was proved that TB cracking was not prone to occur when its angle with maximum tensile stress direction is less than 10°. Twin boundary is more vulnerable with larger angle, especially at about 56° or 85°. A new orientation case that slip systems parallel to twin boundary in one or both of matrix and twin are activated has been revealed to enrich orientation-related twin boundary cracking mechanism.