Hydrogen-enhanced microbanding in an austenitic FeMnAlC low-density steel: Effect on hydrogen embrittlement resistance

We have investigated the influence of 101 mass ppm hydrogen content on the room temperature deformation structure and mechanical behavior of an austenitic Fe30Mn6.5Al0.3C (wt.%) low-density steel by several electron microscopy techniques, such as electron channeling contrast imaging (ECCI), electron backscatter diffraction (EBSD), and scanning transmission electron microscopy (STEM). The steel exhibits a high hydrogen embrittlement resistance associated with a moderated increase in strength (yield stress increase of 10%) and ductility (increase in the elongation to fracture of ∼ 8%). Analysis of the deformation structure reveals that hydrogen influences the deformation behavior by promoting deformation mechanisms associated with inhomogeneous plasticity (hydrogen-enhanced deformation banding (HEDB)) and strain localization (hydrogen-enhanced microbanding (HEMB)). These deformation mechanisms are ascribed to hydrogen-induced effects on dislocation plasticity, resulting in macroscopic kink bands, sub-micron localized strain gradients, and localized shear at cell blocks. We find that HEMB plays a relevant role in the deformation behavior of sub-micron localized strain gradients by promoting plastic relaxation and the enhanced storage of geometrically necessary dislocations within them. These effects mitigate the activation of damage mechanisms and enhance the strain-hardening capacity, contributing to the high HE resistance of the steel, comparable to that of high HE-resistant fcc alloys and steels.