Hydrogen Embrittlement: New Mechanistic Insight from Experiments and Simulations

Hydrogen-induced embrittlement of metals is a major concern for industries ranging from nuclear fission and oil & gas to emergent technologies, such as fusion power and hydrogen fuel systems. First reported over 150 years ago by W. H. Johnson, it has been extensively studied since. Yet many open questions remain, and understanding of hydrogen uptake and embrittlement is far from complete.

A key question is how combined driving forces, such as stress and temperature drive hydrogen redistribution. We have developed a non-equilibrium thermodynamics framework to capture these effects, including a fully mechanistic model for the heat of transport. This can be used to determine when each effect dominates, enabling the development of guidelines for design.

Using crystal plasticity coupled with hydrogen diffusion simulations we also study how microstructural heterogeneity alters local hydrogen distribution. The results show that local hydrogen concentrations may substantially differ from the mean, and a statistical framework is developed to capture this effect in component-scale simulations.

In hydrogen embrittlement, the interaction of hydrogen with dislocations plays a central role, with previous studies reporting that it can facilitate or hinder dislocation glide. Using Bragg Coherent X-ray Diffraction Imaging (BCDI), we have been able to make the first direct measurements of hydrogen-induced strain field shielding in 316L during insitu hydrogen charging. Our observations also show hydrogen-enabled dislocation glide and rapid dislocation climb, suggesting that hydrogen enhances point defect generation.

At the macroscopic scale we have developed a new experimental rig that allows the simultaneous application of hydrogen charging and mechanical loading during synchrotron radiography and X-ray powder diffraction. Our in situ tests on 316L show a hitherto unreported rapid hydrogen embrittlement mechanism which only appears during mechanical loading in hydrogen atmosphere. Using X-ray imaging, XRD and electron microscopy the underlying mechanisms can be clarified. A next challenge will be to use these mechanistic insights to enable more reliable, quantitative predictions of hydrogen embrittlement.

About the presenter

Felix Hofmann is Professor of Engineering Science at the University of Oxford. His research focusses on understanding the evolution of materials in extreme environments, such as irradiation, high or low temperature and hydrogen, with a particular focus on the behaviour of crystal defects. His group extensively uses and develops synchrotron X-ray techniques, micro-scale thermal transport measurements, advanced electron microscopy tools and insitu experiments. His research has focussed on tungsten and steels for fission and fusion environments and more recently on materials for hydrogen fuel systems. He has served on beamtime allocation panels at DLS and at the ESRF, and currently co-chairs the later. Together with colleagues from UKAEA and MIT he hosts the “International Nuclear Engineering Colloquium”, a monthly fully-online seminar series, which regularly draws a worldwide audience of 200+ expert participants. In Oxford he is the incoming director of graduate studies and previously served as deputy director of graduate studies and finals examiner. He is a college lecturer at Trinity College, Oxford.