EUROPEAN CONGRESS AND EXHIBITION ON ADVANCED MATERIALS AND PROCESSES - EUROMAT 2021

Dual-Phase (DP) steels are commonly used in the automotive industry and are known for their favorable strength and ductility. Under deformation, damage initiates in the microstructure, which is composed of relatively soft ferrite and hard martensite, in critical locations, and can propagate and coalesce until fracture occurs. Although the various types of damage and their usual initiation sites are rather well known, the exact deformation mechanisms that control the damage initiation are still under debate. While high-resolution SEM-DIC could potentially resolve the strain fields at sub-micron scales [Hoefnagels et al., Strain 2019], the damage initiation sites are not known in advance, leading to unacceptably large areas that need to be tracked. In this work we therefore aim to predict damage initiation sites that can be observed upon subsequent in-situ SEM-DIC deformation testing.
The damage prediction is based on the principle wherein it is presumed that damage initiates at typical microstructural configurations, "hotspots", such as a thin martensite “notch”. First, ex-situ tensile tests are performed, with microstructure characterization before and after deformation to construct the hotspot, which essentially is an average of microstructures at many damage initiation sites. The hotspot is then used to find locations in the new undeformed microstructure by cross-correlation of the hotspot with respect to a large area of the microstructure.
Several predicted damage locations are then characterized in detail using Electron Backscatter Diffraction and are tracked under deformation using SEM-DIC, yielding strain fields at high spatial resolutions (<100 nm). These measurements result in unexpected observations of martensite plasticity before martensite “cracking” occurs, as martensite is usually assumed to fail in a brittle manner. Moreover, other martensite notches show much more plastic deformation without damage, leading to new insights into the mechanisms that dominate damage in DP steels.