Early stage strain localization at grain boundaries during fatigue studied using in situ electron channeling contrast imaging and high-resolution electron backscattered diffraction

In situ high-resolution electron backscattered diffraction is combined with electron channeling contrast imaging to study the interactions of dislocations and associated stress fields near grain boundaries in austenitic stainless steel 316L samples at the early stages of fatigue. The influence of fatigue stress amplitude on strain irreversibilities at grain boundaries was also investigated. It was found that dislocation pile-ups formed at 150 or 220 MPa are fully or partially released by reversing the loading direction, and the stress fields associated with those pile-ups are also fully or partially reversible between maximum tension and compression. In contrast, the dislocation networks formed at 300 MPa remain stable and irreversible. The cause of irreversibility at 150 MPa and 220 MPa was found to be dislocation debris generated by reversible dislocation glide, while at 300 MPa the irreversibility is mainly a result of tangles formed by multiple slip systems intersecting each other.