Comparative Study of Fatigue Crack Growth in FCC and ODS BCC Materials with Emphasis on Intergranular Propagation

While transgranular cracking mechanisms have been extensively studied, intergranular propagation is less understood and is often attributed to environmental effects, carbide and oxide cracking. Recently, a novel mechanism for intergranular crack growth was identified in FCC alloys under cyclic loading, highlighting the specific conditions and features that drive this process. The performed research in this work extends the investigation to BCC alloys to evaluate whether the observed mechanisms are broadly applicable. The intergranular fatigue crack initiation and growth were investigated in polycrystalline Sanicro 25 (FCC alloy) and Fe-10Al-4Cr-4Y₂O₃ (BCC ODS nanocomposite). Fractured surfaces were examined using scanning electron microscopy. The underlying dislocation structure of persistent slip bands (PSB) was scrutinized by transmission electron microscopy, and HR-EBSD correlations were performed. Persistent slip markings (PSM), which arise from cyclic plastic strain localization, were identified as critical in intergranular crack initiation by influencing grain boundary cohesion and displacing adjacent grains. A qualitative comparison clarifies distinctions between PSMs and striations, addressing common misinterpretations in fractography. The specific morphological and crystallographic features of PSMs and PSBs in both investigated materials are systematically analyzed. The observed differences are discussed in the context of their respective deformation mechanisms and microstructural constraints. These findings contribute to a more comprehensive understanding of intergranular fatigue crack initiation and propagation, particularly in relation to crystallographic slip activity and grain boundary interactions in materials with differing crystal structures.