Hydrogen hardening effect in heavily deformed single crystal α-Fe

► Hydrogen interaction with edge dislocations during deformation in α-Fe was studied. ► Hydrogen atoms have tendency to diffuse and stay around dislocation line. ► Yield strength is very sensitive to the presence of hydrogen in edge dislocations. ► After yielding hydrogen atoms are de-associated from the dislocations. ► This direct observation of hydrogen hardening confirmed the experimental findings.