H is an element that plays an increasingly important role in the production and efficient usage of energy. Besides its direct use as an energy source, it influences the way we produce and consume energy decisively in an indirect way; In high-strength metallic materials (Rm > 600 MPa), especially steels, the usability and service life is regularly limited by H embrittlement, leading to failures which are notoriously hard to predict. Despite the close to 150-year long history of continuous research and enormous economic significance of H related failures, little is known experimentally about the underlying chemical mechanisms, which are inherently playing out on the atomic scale. It is known structurally that H in shallow trapping sites, combined with diffusional paths are enabling dislocation movement and with it localised plasticity and resulting in crack nucleation and propagation. Which trapping sites and diffusional paths is to date mostly speculative. This is to a large extent because it is highly challenging to image light elements on the atomic scale and find out which trapping sites and diffusion paths were active.
Using atom probe tomography (APT), the quantitative detection of H in materials is in principle possible, but two main factors were limiting the application of APT for the analysis of H in materials so far: Firstly, spurious H commonly present in ultra-high vacuum (UHV) systems obscures the H originally present in the material. Secondly, trapped H tends to escape during sample transfer from ambient conditions into UHV at room temperatures. In the present work, we have used deuterium oxide (D2O) with its very low natural abundance as the H source to avoid interference, combined with a quick-transfer device enabling sample transfer in several minutes rather than hours. A similar approach was recently employed by Chen et al. to verify the presence of D at VC in s steel [1,2].
This setup allowed us to detect and quantify the amount and distribution of D at grain boundaries in alpha iron, in dependence of the crystallographic nature of the grain boundary. It will also allow us to extend the analysis to dislocation structures, where the H is known to influence the flow stress. These folly quantitative results will greatly aid theory and simulation of hydrogen embrittlement.
 D. Haley et al., Int. J. Hydrogen Energy 39 (2014) 12221-12229.
 Y.S. Chen et al., Science (80-. ). 355 (2017) 1196.