Micro-fracture at prior austenite grain boundaries (PAGbs) in martensite: a coupled crystallographic-chemical investigationWednesday (28.09.2016) 16:00 - 16:15 Part of:
Martensite is a primary constituent in most advanced high strength steels (AHSS), for which martensite cracking (DM) is a typical micro-damage mechanism. Using high resolution electron backscatter diffraction, EBSD, multiple DM features were analyzed for a ferritic-martensitic dual-phase steel after deformation is applied in different modes. Statistical analysis of the micro-damage features indicates a high influence of the interphase boundary morphology on damage initiation. Using ARPGE software, developed by C. Cayron in Grenoble, the prior austenite grain structure is reconstructed for the fractured martensitic islands using the EBSD data. Our results show enhanced embrittlement of prior austenite grain boundaries (PAGbs) inside martensite, where martensitic cracks are mostly restricted to PAGbs under both, monotonic and cyclic deformation, indicating highly stored transformation-induced residual stresses and/or favored segregation of solute elements at these boundaries. Special influence is played by the crystallographic character of PAGbs as fracture toughness increases at regions where early nucleation of martensite takes place, following the coordinated nucleation (CN) model. Using atom probe tomography (APT), high carbon segregation is detected at the matensitic PAGbs. Intrinsic residual stresses stored at the vicinity of PAGbs is currently under analysis through focused ion beam (FIB) milling, followed by relaxation strain assessment by digital image correlation (DIC).