Hydrogen embrittlement is a sudden failure, commonly observed in metallic materials, induced by even a few ppm of hydrogen. Here, we use a novel in-house in situ set-up for electrochemical backside hydrogen charging [1] to investigate the hydrogen-metal interaction mechanisms at a small scale. We performed time-resolved in situ nanoindentation tests on ferritic Fe-M (M=Cr, Al) model alloys with solute contents from 4 to 21 at.%. A hydrogen induced hardening effect, independent of the grain orientation, was observed, while the elastic modulus remained unchanged. The relative hardness variation follows an initial linear increase with increased hydrogen content until hydrogen absorption and desorption reach a steady state. This hardening effect is verified by an enhanced dislocation density in the cross-section underneath the nanoindentation imprints, quantified by scanning transmission electron microscopy (STEM) post-mortem analysis, and it was modeled accordingly [2]. A higher chromium content results in a more pronounced hardening effect at the corresponding hydrogen saturation level. The substitutional chromium atoms act as flat trapping sites for hydrogen, increasing its solubility as shown by thermal desorption analysis (TDA).

[1] M. J. Duarte, X. Fang, J. Rao, W. Krieger. G. Dehm. “Hydrogen-microstructure interactions by in-situ nanoindentation: a new backside electrochemical charging approach”, J Mater Sci. 56 (2021) 8732-8744.

[2] J. Rao, S. Lee, G. Dehm, M.J. Duarte. “ Diffusible hydrogen and its impact on nanohardness and dislocation structure in FeCr alloys by in situ nanoindentation”, (under revision, http://dx.doi.org/10.2139/ssrn.4037077).