Hydrogen (H) is the most abundant element in the Universe. It has a great potential to replace the C-based fuels as a more sustainable energy solution. In order to support and implement this transition towards a more sustainable H economy, understaning the interaction of H with its surrounding infrastructure requires immediate attention. H embrittlement (HE) is a phenomenon that causes abrupt loss in the load bearing capacity of large engineering structures in the presence of H. Its limited understanding poses a hurdle for the transition to a H based economy. High strength steels are particularly prone to HE where even less than 1 part per million by weight (ppmw) H is sufficient to dramatically degrade the mechanical properties

Medium Mn steels consisting of a dual-phase, austenite-ferrite microstructure have been employed for its enhanced transformation induced plasticity (TRIP) effect to achieve an improved strength-ductility combination. However, its multiphase microstructure with difference in H solubilities and diffusivities makes HE studies on this material system challenging. A recent study elucidated to the presence of strong H trapping sites with an activation energy of up to 50 kJ/mol in such steels, hinting towards H trapping at the austenite-ferrite phase boundaries (PBs).

Here, a medium Mn steel (0.2C-10Mn-3Al-1Si) is heat treated to produce a microstructure with a high density of PBs which will enable us to bypass the need for site specific specimen preparation for APT. Additionally, the microstructure revealed an approximately equal fraction of semi-coherent and incoherent PBs. We investigate and visualize interaction of H by systematically probing vacancy clusters, dislocations, and austenite-ferrite PBs via atom probe tomography (APT).