Probing the Mechanism of Hydrogen Embrittlement in Body-Centered Cubic Titanium Alloys Using Multiscale Characterization Techniques
Kaila Bertsch | 21-LW-037
The presence of hydrogen in structural metals causes severe loss of ductility and sudden, unpredictable failure. This embrittlement severely limits material selection and lifetimes in a wide range of hydrogen-rich environments such as in marine, corrosion, renewable energy/hydrogen gas, and aerospace applications. Despite decades of research on this phenomenon, the underlying physical mechanism of hydrogen environment embrittlement (HEE) remains incompletely understood due to apparently contradictory effects of hydrogen on material response. At the macroscale, hydrogen appears to create brittle failure, while at the microscale plasticity appears to be enhanced. Thus, a central challenge is that understanding HEE requires probing the effects of hydrogen on the mechanical response across length scales.
To bridge this knowledge gap, we compared the tensile response of body-centered cubic (BCC) alloys, including Ti alloys and commercial purity Fe, with and without internal hydrogen across scales. The microstructural evolution under tensile loading was analyzed from micro- to meso- to macroscale using scanning electron microscopy (SEM), electron backscatter diffraction (EBSD), focused ion beam (FIB) machining, and transmission electron microscopy (TEM) to illuminate the full effects of hydrogen on mechanical response and failure. Ti alloys, both conventional and produced by additive manufacturing (AM) with tailored microstructures, were found to fail before the onset of plasticity for the given conditions. It was found that the initial high-temperature conditions used during hydrogen-charging and annealing created additional brittleness in these alloys, confounding the influence of hydrogen alone. In iron, it was found that hydrogen enhances plasticity and dislocation organization at the microscale in a way that adds an extra constraint to grain boundaries at the mesoscale. This makes grain boundaries an easier pathway for failure, creating failure pathways that appear brittle at the macroscale, despite significant plasticity. The results from BCC alloys in this study supported observations made in face-centered cubic (FCC) crystal structure alloys reported in the literature, confirming for the first time that grain-boundary constraints are a driving mechanism for HEE in structural metals.
This research has yielded a better understanding of what causes HEE, which is critical to understanding how to reduce its impact in applications such as corrosion, transportation infrastructure, and hydrogen/renewable energy, which are relevant particularly to Climate and Energy Resilience, Materials Science, and Accelerated Materials and Manufacturing initiatives and core competencies at LLNL, developing science and technology tools and capabilities to meet future national security challenges, and to enhancing the fundamental understanding of aging nuclear weapons systems. This project helped to spur new research into hydrogen–metals interactions that are of critical importance to other DoE and NNSA interests, setting the stage for PI Kaila Bertsch to successfully fund an external proposal related to hydrogen isotope diffusion in metals funded by the Tritium Sciences Program within DoE/Pacific Northwest National Laboratory (FY22-FY23), and helping to address DOE's energy and environmental security missions. This project supported development of early career scientists Kaila Bertsch and postdoc Jibril Shittu; the experience gained by Dr. Bertsch as PI supported her successful proposal for a different Exploratory Research LDRD project beginning FY23 (23-ERD-034) and her conversion to staff scientist in FY22. This project supported new LLNL infrastructure, including acquisition of an in situ corrosion-tensile testing machine, and new capabilities, implementing the transmission Kikuchi diffraction technique for the first time at LLNL. The infrastructure, capabilities, and research from this project are also synergistic with other parts of the recent LDRD portfolio, including 20-SI-004 "Predicting and Controlling Corrosion" and 20-LW-015 focused on simulations of hydrogen–metals interactions, supporting an overall development in expertise in hydrogen–metals interactions, hydriding, and failure in metals.