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

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## Abstract

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.

## Keywords

High-energy X-ray diffraction X-ray diffraction Misorientation Statistics Hydrogen embrittlement High-pressure torsion## Notes

### Acknowledgements

We would like to acknowledge the contributions of Dr. Ian Robertson (Department of Materials Science and Engineering, University of Wisconsin-Madison) and Dr. Shuai Wang (Department of Mechanical and Energy Engineering, Southern University of Science and Technology) for preparing the samples and performing the high-pressure torsion experiment. Dr. Akihide Nagao (JFE Steel Corporation) performed the high-pressure hydrogen charging and thermal desorption spectroscopy. We also thank the staff of the Cornell High Energy Synchrotron Source, especially Dr. Peter Ko (Cornell High Energy Synchrotron Source, Cornell University). We are also grateful for the assistance of Christopher Budrow (Sibley School of Mechanical and Aerospace Engineering, Cornell University) in performing the diffraction experiment, the help of Dr. Darren Pagan (Cornell High Energy Synchrotron Source, Cornell University) and Dr. Mark Obstalecki (Air Force Research Laboratory) in developing the indexing method, and the help of Dr. Kelly Nygren (Cornell High Energy Synchrotron Source, Cornell University) on hydrogen embrittlement.

### Compliance with Ethical Standards

### Conflict of interest

On behalf of all authors, the corresponding author states that there is no conflict of interest.

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