Micro-damage initiation in ferrite-martensite DP microstructures: A statistical characterization of crystallographic and chemical parameters

Damage mechanisms occurring in a DP-steel microstructure at low strains, under monotonic and cyclic deformation modes have been studied by a comprehensive statistical analysis of scanning electron microscopy observations. The study aims to define local microstructural configurations of high damage susceptibility. In particular, the role of grain and interphase boundaries at triggering micro-damage features was investigated. SEM-based electron backscatter diffraction (EBSD) and electron channeling contrast imaging (ECCI) were utilized. Focused ion beam (FIB) milling was additionally applied for a 3-dimensional investigation of the crack morphology and corresponding arrangement of phases. Microstructural configurations of high damage susceptibility were deduced and confirmed through a quasi in-situ deformation experiment. Damage initiation is shown to be highly dependent on the distribution and morphology of the martensitic islands. Voids are most likely to nucleate at interphase boundaries under monotonic loading, particularly at triple junctions between ferritic grain boundaries and martensite. On the other hand, decohesion cracks are frequently observed inside of martensite where they are remarkably restricted to prior austenite grain boundaries (PAGbs). The low fracture strength of martensitic PAGbs is discussed in the light of local chemical analysis of different types of interfaces using atom probe tomography (APT).