Micro-damage initiation in DP-steel: a multi-scale mechanical characterization of grain and interphase boundariesWednesday (28.09.2016) 12:15 - 12:30 Part of:
Typical microstructures of advanced high strength steels possess multi-phase compositions of an inherent mechanical contrast, where different damage mechanisms are mutually activated with applied deformation. In ferritic-martensitic dual-phase (DP) steels, typical damage mechanisms include martensite cracking, martensite-ferrite interphase damage, and ferritic void formation. The relative contribution of each mechanism depends on varying DP-steel microstructural parameters. In this study, two DP-steel microstructures, with the same chemical composition, are produced by varying heat treatment parameters. The difference in phase properties between both alloys is assessed by nano-indentation method. A comprehensive statistical analysis is applied for both materials to characterize different damage mechanisms at low strains, in order to understand the role of grain and interphase boundaries at triggering micro-damage in DP-steels, under monotonic and cyclic deformation modes. SEM-based electron backscatter diffraction (EBSD) and electron channeling contrast imaging (ECCI) techniques are utilized. Focused ion beam (FIB) milling is additionally applied for a 3-dimensional investigation of the damage features. The results of this statistical analysis are confirmed through a quasi in situ deformation experiment, through which the microstructural development is tracked under monotonic tensile loading up to micro-damage initiation. It is shown that damage initiation is highly dependent on the distribution and morphology of the martensitic islands, where void nucleation is highly susceptible at interphase boundaries under monotonic loading, particularly at triple junctions between ferritic grain boundaries and martensite. On the other hand, cracks inside martensite are remarkably restricted to prior austenite grain boundaries (PAGbs). Fracture strength of grain and interphase boundaries is assessed via a local micro-scaled bending experiment. Prospects of grain boundary engineering of DP-steels are discussed in the light of comparing the micro-mechanical behavior of both DP-steels.