Akbar Bagri1 2 John Hanson5 Jonathan Lind6 Peter Kenesei3 Robert Suter7 Silvija Gradecak2 Michael Demkowicz4

1, Johns Hopkins University, Baltimore, Maryland, United States
2, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States
5, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States
6, Lawrence Livermore National Laboratory, Livermore, California, United States
3, Argonne National Laboratory, Argonne, Illinois, United States
7, Carnegie Mellon University, Pittsburgh, Pennsylvania, United States
4, Texas A&M University, College Station, Texas, United States

We use two non-destructive, synchrotron-based techniques—X-ray absorption tomography (XRAT) and near-field high-energy X-ray diffraction microscopy (HEDM)—to assess the character of grain boundaries resistant to hydrogen embrittlement in Ni-base alloy 725. Our study yields the grain size distribution and the full, five-parameter grain boundary character distribution. These analyses show that the microstructure of alloy 725 comprises elevated densities of Σ9 and coherent Σ3 grain boundaries. We also use the registered HEDM and XRAT reconstructions to connect the grain boundary character to intergranular fracture inside a hydrogen-embrittled Ni-base alloy 725 sample. This investigation provides unprecedented insight into the connection between the crystallographic character and susceptibility to hydrogen-assisted fracture of individual grain boundaries. We find that boundaries formed along planes with low Miller-index facets are especially resistant to hydrogen-assisted crack propagation and their presence results in toughening of the microstructure. Our investigation opens new paths toward predicting and improving the performance of Ni-base alloys exposed to hydrogen.