Statistical Characterization of Intragrain Misorientations at Large Strains Using High-Energy X-Ray Diffraction: Application to Hydrogen Embrittlement

High-energy X-ray diffraction (HEXD) has become a powerful technique for studying deformation and failure in structural metals over the last two decades. In this work, we used multi-grain HEXD to investigate hydrogen embrittlement in polycrystalline nickel by quantifying the effects of solute hydrogen on the grain-scale plastic response. Five polycrystalline samples were probed: one undeformed control sample with no added hydrogen, one with no added hydrogen compressed in a high-pressure torsion (HPT) anvil, one with no hydrogen compressed and torqued by HPT, one charged with hydrogen compressed by HPT, and one with hydrogen compressed and torqued by HPT. Most current HEXD data reduction algorithms can only be used at strains less than 5–10%, while the largest strains in our samples exceed 21%. To process the data, we developed a new forward projection-based processing method to calculate discrete single grain orientation distributions (DSGODs) from the data. We used three statistics-based state variables to characterize the measured DSGODs. We found the grains in the two materials (hydrogen-charged and uncharged) had the same amount of intragrain misorientation, but how that misorientation was distributed in orientation space was different. We identified two types of grains in the hydrogen-charged samples, but only a single type in the uncharged samples. Our results suggest solute hydrogen does not affect all grains within a deforming aggregate equally. These mesoscale results and new state variables can help bridge the gap between micro- and macroscale observations of the effects of solute hydrogen on plasticity.