Hardening effect of diffusible hydrogen on BCC Fe-based model alloys by in situ backside hydrogen charging

•The effect of diffusive vs. deeply trapped hydrogen is deduced by time-dependent evolution of mechanical properties for ferritic alloys using novel in-situ nanoindentation during backside continuous hydrogen supply.•A linear relationship between hardness increase and hydrogen ingress is observed in the initial stage, with a more pronounced hardening effect for higher content of the studied substitutional elements.•An increase in dislocation density due to hydrogen is quantified by transmission electron microscopy.•Our data support the interpretation that hydrogen facilitates dislocation nucleation in the initial stage following the defactant theory; and we link the hardness increase to the dislocation density and friction stresses arising from diffusible hydrogen.