WEB Hydrogen effects on the mechanical behaviour of iron based alloys by in-situ nanoindentationThursday (24.09.2020) 15:55 - 16:10 S: Structural Materials 1 Part of:
The presence of hydrogen in a material can alter its behaviour and lead to early failure of a structure, either directly, by embrittlement mechanisms, or indirectly by accelerating other processes due to hydrogen diffusion. Hydrogen embrittlement refers to a brittle failure behavior of metallic alloys under the effect of hydrogen. It has been thoroughly studied for decades; however, there is still a lack of fundamental understanding and controversy about the governing mechanisms. Special attention is put on steels as they are capable of matching the needs of modern mobile and industrial infrastructure requirements regarding hydrogen energy-related applications.
Single-phase iron-based binary alloys were investigated by nanoindentation coupled to a novel in-situ electrochemical charging method developed in-house at the MPIE. One example are Fe-Cr alloys with different Cr content, with low dislocation density (below 1013 /m2), high hydrogen diffusivity, and low solubility, therefore ideal for in-situ studies. The mechanical properties of the alloys can be collected while supplying hydrogen at the backside. Hydrogen reaches the testing surface through solid-state diffusion. Therefore the sample surface remains intact after hydrogen charging, which allows the possibility to investigate the hydrogen influence in the microstructure by different characterization methods, e.g., electron channelling contrast imaging, electron backscatter diffraction, and atomic force microscopy. The cross-section underneath indentation imprints was examined by transmission electron microscopy to identify the microstructure variations before and after hydrogen charging.
The freshly polished Fe-Cr alloy surface shows a native oxide layer of ~3 nm containing Fe and Cr oxides and Fe hydroxides. During hydrogen charging, the hardness with an increased amount of hydrogen charged into the alloy, while the elastic modulus remains constant. A related decrease of the pop-in load indicates a reduction of the shear stress necessary for dislocation nucleation. Larger variations of both properties with higher Cr concentrations are also observed. The obtained results suggest that hydrogen acts as a solid-solution hardening solute in the low dislocation density ferritic Fe-Cr alloy. Substitutional chromium atoms that act as flat trapping sites for hydrogen retard its diffusion, resulting in a larger increase of hardness and a larger decrease with a higher concentration of Cr.