Hydrogen-microstructure interactions in bcc Fe-Cr alloys at small scale

Hydrogen is a clean energy source as its combustion yields  only water and heat. However, as hydrogen prefers to accumulate in the concentrated stress region of metallic materials, a few ppm Hydrogen can already cause the unexpected sudden brittle failure, the so-called “hydrogen embrittlement”. The difficulties in directly tracking hydrogen limits the analysis to post-mortem probes ignoring hydrogen migration before and during testing, leading to debates about the governing mechanisms. Therefore, a more comprehensive understanding of hydrogen-metal interaction with microstructural features is necessary to prevent hydrogen-introduced damage and further contribute insights into developing hydrogen-resistant materials.

With the support of the project KSB Stiftung, the interaction of hydrogen with mechanical, chemical and electrochemical properties in ferritic Fe-Cr alloys is investigated utilizing an in-house built backside in situ nanoindentation method, which can characterize the time-dependent mechanical behavior of independent microstructural constituents within a material upon the simultaneous hydrogen charging [1]. The electrochemically produced hydrogen is generated beneath the sample and reaches the testing (upper) surface by bulk diffusion. Therefore, the sample surface remains intact after hydrogen charging, which allows the investigation of hydrogen effects by different characterization techniques, e.g., electron channelling contrast imaging, transmission electron microscopy, and atomic force microscopy. The device is assembled inside the nanoindentation chamber (Fig. 1a).

Single-phase ferritic alloys have high hydrogen diffusivity and low solubility, making them ideal for in situ studies during hydrogen charging and particularly targeting the effect of mobile hydrogen. We conducted in situ nanoindentation in Fe-15Cr, Fe-20Cr and Fe-2Al alloys with coarse grains in the millimeter range and low dislocation density achieved by annealing at 1300 °C for 4 hours. During the hydrogen charging, a hardening effect occurs while the Young's modulus remains unaffected. In addition, the hardening effect strengthens with increased applied current densities, corresponding to a larger amount of hydrogen, as displayed in Fig. 1b. This hardening effect is verified by an enhanced dislocation density in the cross-section underneath the nanoindentation imprints investigated via scanning transmission electron microscopy (Fig. 1c) and modelled accordingly [2]. The larger amount of dislocation is postulated to be originated from the hydrogen-facilitated homogeneous dislocation nucleation. The accumulated hydrogen is assumed to form the Cottrell clouds, which retards the dislocation motion and leads to the strengthening effect. In addition, a higher substitutional content results in a more pronounced hardening effect at the corresponding hydrogen saturation level, suggesting the hydrogen trapping ability of the analyzed elements. Besides, anisotropic hardness variation during hydrogen charging within different grain orientations of (100), (110), and (111) is noticed in Fe-21Cr alloy as well.