In Situ Investigation of the Role of Slip in Crack Initiation in Hydrogen Embrittled Inconel 725

Hydrogen embrittlement (HE) of metals continues to cause unexpected failures because it remains imperfectly understood. Prevailing theories emphasize either hydrogen (H)-induced loss of cohesion or H-enhanced plasticity as the key factors responsible for HE. We use in situ tensile testing to clarify the role of slip in the initiation of intergranular cracks in hydrogen embrittled UNS N07725 (Inconel alloy 725). I designed an experiment that allows electrochemical H charging of alloy 725 specimens, tracks surface plastic deformation using digital image correlation (DIC) during in situ tensile testing in a scanning electron microscope (SEM), and correlates the plastic strain distribution to the underlying microstructure, determined with electron backscatter diffraction (EBSD). My work reveals that intense, localized slip in alloy 725 occurs predominantly along grain boundaries, rather than in grain interiors. As H content increases, slip along grain boundaries decreases. At low H concentration, cracks initiate predominantly in the vicinity of slipping boundaries, albeit not at the locations of greatest slip, but rather at locations where slip is arrested, especially at intersections between slipping boundaries and pre-existing thin twin lamellae. This observation suggests that the role of slip in crack initiation is to generate local stress concentrations that promote decohesion. At higher H concentration, a greater fraction of cracks is seen to initiate at locations where no localized slip occurs, suggesting that slip is not essential for crack initiation in H embrittled alloy 725. These aforementioned findings are inconsistent with the hydrogen enhanced localized plasticity theory of HE. They support the hydrogen enhanced decohesion theory of HE. Moreover, they emphasize the importance of plastic strain heterogeneities in crack initiation, especially at lower H concentrations, and reveal a potentially important role of strain gradient hardening in crack initiation events involving such heterogeneities