Hydrogen decelerates fatigue induced grain boundary migration in nanostructured iron

The difficulty to reveal trapping sites of hydrogen in metals, how hydrogen interacts with lattice defects and potentially changes their behavior, still prevents a generalized understanding of hydrogen (H)-embrittlement. This is specifically the case for nanostructured materials, where direct characterization techniques would require an exceptional lateral and time resolution, given the small grain size and high diffusivity of H. The tendency of nanostructures for grain coarsening under mechanical or thermal loads, adds further complexity to this issue. Cyclic high pressure torsion uses this peculiarity and allows to conclude whether H is located at grain boundaries or changes the deformation behavior. If hydrogen is trapped at grain boundaries, the kinetics of fatigue induced grain coarsening should clearly differ compared to the uncharged reference samples, while a change of the deformation behavior would manifest in a different texture evolution compared to the reference. The experiments clearly reveal that H prevents grain growth up to accumulated strains of ε_acc = 100, while it still decelerates boundary migration at even larger accumulated strains of ε_acc = 500. The results give thereby indirect proof of preferential H-defect-interaction. The occurrence of grain boundary deceleration rather than its acceleration strongly suggests that grain boundary pinning dominates over an amplifying effect on dislocation and disconnection mobility. Thus, the results indicate the importance of H-grain boundary interaction but also question the role of the hydrogen enhanced localized plasticity (HELP) theory in nanostructured iron.