Abstract
Direct observation of hydrogen and its effects is challenging due to its small size and high mobility. To understand the dominant embrittlement mechanisms leading to material failure, hydrogen and its effects must be studied at small scales and under operating conditions. We independently analyze hydrogen’s influence on the mechanical behavior for specific microstructural features (grains and grain boundaries, interfaces, precipitates, dislocations…), by nanoindentation, nanoscratching, and pillar compression during continuous hydrogen supply. We built a custom electrochemical cell for backside hydrogen charging, mounted on a commercial nanoindenter. This approach enables dynamic studies of hydrogen absorption and release and their effects on mechanics via time-resolved measurements. In addition, diffusible and trapped hydrogen effects are studied separately by tailored hydrogen charging protocols.
Hydrogen-metal interactions in iron alloys were studied with respect to composition, dislocation density, and grain size. Mechanical tests were complemented by scanning and transmission electron microscopy, Kelvin probe hydrogen permeation analysis, hydrogen quantification by thermal desorption spectroscopy, and hydrogen location by atom probe tomography. The studies were performed on Fe-X binary alloys, with X=Cr, Ni, Al, processed to achieve grain sizes >600 µm for single-crystal-like testing. Fe-Cr alloys were further cold rolled to increase the dislocation density, or annealed after high-pressure torsion to refine the grain size to ~200 nm. Time-dependent nanoindentation showed hydrogen-induced hardening, attributed to reduced shear stress for dislocation nucleation, increased lattice friction, stronger dislocation interactions, and dislocation peening. These effects are reversible within hours in coarse-grained alloys but persist for days in nanocrystalline materials due to hydrogen accumulation at grain boundaries.
The systematic data obtained can be used as a critical input for modeling hydrogen embrittlement mechanisms and support materials and process design for the sustainable use of hydrogen as an energy carrier.