Hydrogen effects on metallic materials studied via small-scale in-situ techniques
It has been known for more than a century that hydrogen has a detrimental effect on the mechanical properties of various metals and alloys, i.e. the so-called hydrogen embrittlement. However, the effects and mechanisms are still not well known. To properly understand the sole effect of hydrogen on materials, critical experiments should be designed according to different microstructures, environmental conditions, loading conditions, etc. For example, ferritic steels have a high diffusivity of hydrogen at room temperature (~10-9 to 10-7 m2 /s) such that pre-charged hydrogen can easily diffuse out in ex-situ tests, and therefore in-situ setup should be considered. By using hydrogen plasma and a small-scale loading module, the in-situ testing of ferritic steels can be realized in an environmental scanning electron microscope (ESEM). Results show that, in a coarse-grained Fe-3wt.%Si steel upon monotonic loading, besides a ductility loss, the hydrogen plasma can cause a transition from ductile dimples to brittle-like features on the fracture surface. In-situ cyclic loading test in the same material with oligo-crystalline structure confirms the hydrogen-assisted fatigue crack growth in the ferritic structure results from a restricted plastic deformation and has a dependency on the loading rate (frequency). This setup has been proven valid in the hydrogen embrittlement study for other materials such as austenitic stainless steels and medium-Mn steels with austenite and ferrite.
Nanomechanical testing is useful in revealing materials local behavior at fine-scale such that some global complexities can be easily avoided. Concerning hydrogen-material interaction, an in-situ electrochemical charging cell has been developed and utilized in a nanoindentation instrument. By using a glycerol-based electrolyte, the specimen can be cathodically charged for up to several days without significant change in the surface integrity even at a microscale. Testing on two high-entropy alloys shows that hydrogen-charging solely can induce irreversible slip lines accumulated as surface steps (CoCrFeMnNi, Cantor alloy) or cause phase transformation from γ-austenite to ε-martensite (Fe-30Mn-10Co-10Cr-0.5C, at.%). Based on further characterizations, the hydrogen-induced internal stress is concluded responsible for these changes on the sample surface.