Hydrogen-induced compatibility constraints across grain boundaries drive intergranular failure of Ni

A multi-scale experimental approach was used to determine the fundamental mechanisms responsible for the hydrogen-induced transition in failure mode from ductile transgranular to intergranular in polycrystalline Ni during uniaxial loading. Hydrogen accelerated the evolution of the deformation microstructure, producing smaller dislocation cells and microbands, and causing significantly different orientation deviations to develop in neighboring grains, while inducing less evolution of texture, less grain rotations, less elongation of the grains parallel to the tensile axis, and greater out-of-surface distortion of the grains. These observations are explained in terms of the hydrogen-enhanced plasticity mechanism, which results in a redistribution of hydrogen that stabilizes the deformed microstructure and increases the hydrogen coverage on the grain boundaries. The stabilization of the microstructure manifests as a reduced ability of grains to cooperatively accommodate evolving deformation structures, which introduces an additional compatibility constraint across grain boundaries. The combination of this compatibility constraint across grain boundaries, the locking of the microstructure in a specific configuration by hydrogen, and the hydrogen-weakening of the grain boundaries drives the hydrogen-induced intergranular failure.