Hydrogen-prompted heterogeneous development of dislocation structure in Ni

First documented in 1875, the deterioration of mechanical properties of hydrogen-containing metals is a longstanding yet unsolved problem in materials science. In this work, the evolution of dislocation structures in differently orientated grains (i.e., near [100], [110], and [111]) of the uncharged and hydrogen-charged (400 and 1200 ppm) polycrystalline Ni were systematically investigated by combining electron backscatter diffraction, focused ion beam and scanning transmission electron microscopy. By using site-specific characterization methods, for the first time, we discover that hydrogen-enhanced localized plasticity (HELP) is orientation-dependent, with the following sequence: [100] > [111] > [110]. Massive incompatibility between differently orientated grains, induced by the orientation dependence of HELP, contributes to the premature intergranular fracture of Ni, especially for the 400 ppm H-charged Ni. Our results suggest that optimizing orientation distribution is a potential approach for enhancing metals' resistance to hydrogen damage. The relative contribution of HELP and hydrogen-enhanced decohesion (HEDE) mechanisms in hydrogen embrittlement of Ni is also analyzed quantitatively for 400 and 1200 ppm H-charged samples. In the 400 ppm H-charged Ni, a strong synergistic interaction exists between HELP and HEDE mechanisms, and the HELP mechanism plays a critical role in premature fracture. By contrast, in the 1200 ppm H-charged Ni, the HELP effect on final failure is much less significant and HEDE is the dominant embrittlement mechanism.