Fatigue crack initiation in an austenitic stainless steel: Role of grain boundaries and triple points

A novel material proposed for use in sodium cooled fast nuclear reactors as a steam generator containment material is Stainless Steel 709 - an austenitic stainless steel. The proposed lifetime for such fast reactors is several decades and over those years fatigue failure at room and elevated temperature (~650°C) is a major issue for this material. The goal of this study is to investigate fatigue damage accumulation and crack nucleation in SS709 at room and elevated temperature. The role of grain boundaries as generators of strain heterogeneity, and ultimately initiators of fatigue cracks, has been emphasized in the past. To satisfy kinematic compatibility at grain boundaries, “compatibility stresses” are generated in the boundary vicinity and have been verified by observation of secondary slip near grain boundaries, i.e., slip not directly attributed to the bulk grain orientation Schmid factor effect. In this work, focus is placed on measuring plastic strain fields in the vicinity of grain boundaries and triple points to study the grain boundary influence on the activation of secondary slip systems. Schmid factors are calculated from the electron back scattered diffraction (EBSD) data and shear strains of each of the twelve slip systems are inferred from the digital image correlation (DIC) measurements. The results show a discrepancy between the magnitude of Schmid factors and the activation of slip systems. To address the discrepancy, modified Schmid factors accounting for compatibility stresses are then calculated and are compared to the bulk Schmid factors.