  High-manganese twinning-induced plasticity (TWIP) steels exhibit high strain
hardening, high tensile strength, and high ductility, which make them
attractive for structural applications. At low tensile strain rates, TWIP
steels are prone to hydrogen embrittlement (HE). Here though, we study the
hardening and strengthening resulting from electrochemical hydrogen-charging of
a surface layer of a Fe-26.9Mn-0.28C (wt.%) TWIP steel. We observed a 20%
increase in yield strength following the electrochemical hydrogen-charging,
accompanied by a reduction in ductility from 75% to 10% at a tensile strain
rate of 10-3s-1. The microstructural evolution during tensile deformation was
examined at strain levels of 3%, 5% and 7% by electron backscatter diffraction
(EBSD) and electron channeling contrast imaging (ECCI) to study the dislocation
structure of the hardened region. As expected, the microstructure of the
hydrogen-hardened and the uncharged regions of the material evolve differently.
The uncharged areas show entangled dislocation structures, indicating slip from
a limited number of potentially coplanar slip systems. In contrast, hydrogen
segregated to the grain boundaries, revealed by the deuterium-labelled atom
probe tomography, delays the dislocation nucleation by blocking dislocation
sources at the grain boundaries. The charged areas hence first show the
formation of cells, indicating dislocation entanglement from more non-coplanar
slip systems. With increasing strain, these cells dissolve, and stacking faults
and strain-induced {\epsilon}-martensite are formed, promoted by the presence
of hydrogen. The influence of hydrogen on dislocation structures and the
overall deformation mechanism is discussed in details.
