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Page 6 of 17 Xiao et al. Microstructures 2023;3:2023006 https://dx.doi.org/10.20517/microstructures.2022.26

induced decrease in stacking fault energy promotes the formation of NTs for increased work hardening; and
(IV) the dense surface oxide film can keep the hydrogen from being absorbed and thus improves the HE
resistance .
[52]

It is noteworthy that similar mechanisms and/or strategies (i.e., tailoring of diffusion kinetics, regulation of
GB features, and so on) have been reported in other alloy systems like Al alloys and steels [57-59] . For instance,
Li et al. simultaneously achieved the improved mechanical strength and corrosion resistance of Mg-Li-Al
alloy by solid solution treatment because of the low diffusion kinetics of the solid solution phase .
[57]
Furthermore, the stress corrosion cracking resistance and the strength of the 7056 Al alloy with a high
content of Zn were also concurrently enhanced by tailoring GB architectures (i.e., the formation of GB
[58]
precipitates) .

In contrast to the above single-phase HEAs, Ronchi et al. demonstrated that hydrogen can produce a phase
transformation in the metastable 45Fe-35Mn-10Co-10Cr (at.%) HEA . Based on microstructural
[28]
characterization [Figure 4], it was found that hydrogen induces the γ- to ε-martensite phase transformation
in the metastable HEA, which preferentially occurs in the <101> and <111> oriented grains along Σ3
coincident site lattice boundaries. Additionally, high concentrations of hydrogen can promote the
formation of the extension twinning within the martensite , which may further contribute to enhancing
[28]
the HE resistance. Unfortunately, the mechanical response of the metastable HEA with hydrogen charging
was not demonstrated in the study.

Additionally, the effects of hydrogen on the mechanical response and embrittlement behavior of stable and
metastable HEAs (i.e., 20Fe-20Mn-20Ni-20Cr-20Co and 50Fe-30Mn-10Cr-10Co, at.%, respectively) pre-
charged with 100 MPa hydrogen gases have been experimentally investigated. Figure 5A and B show the
engineering stress-strain curves of the stable and metastable HEA with and without hydrogen charging,
respectively. Both HEAs with hydrogen charging exhibited severe degradation in tensile plasticity. It was
found that hydrogen-assisted cracking of the metastable HEA occurred via localized plasticity for both the
austenite and ε-martensite phases [Figure 5C-F] .
[60]

Moreover, regardless of electrochemical and gas hydrogen charging, the equiatomic FeCoCrNiMn HEA
system shows superior resistance to HE when compared with typical steels, i.e., 304, 316L and X80 [50,61,62] ,
which is expected to be an important candidate for HE-resistant HEAs. Thereafter, extensive efforts have
been devoted to further enhancing the HE resistance of the FeCoCrNiMn HEA by tailoring its
architectures [50,63-65] . For example, as shown in Figure 6A, through hydrogen-induced gradient NT structures,
the equiatomic FeCoCrNiMn HEA demonstrated an excellent property combination at 77 K .
[63]
Furthermore, the gradient-structured FeCoCrNiMn HEA shows both high yield stress (500-700 MPa) and
good ductility (15%-33%) under hydrogen environments [Figure 6B], where the gradient structures
containing surface NTs are introduced via the surface mechanical attrition treatment technique .
[66]
Additionally, it was reported that grain refinement can also contribute to improving the HE resistance of
equiatomic FeCoCrNiMn and FeCoCrNi HEAs [64,65] . Specifically, the hydrogen-charged FeCoCrNiMn HEA
with an average grain size of 1.9 μm exhibited a high tensile strength that was 1.5 times greater than that of
the hydrogen-charged HEA with a grain size of 22 μm and there was no significant decrease in tensile
elongation [Figure 6C and D] . Moreover, as shown in Figure 6E, incorporating the modified cellular
[64]
structures by selective laser melting and annealing treatment can also effectively improve the HE resistance
of the FeCoCrNiMn alloy . Such excellent resistance to HE is ascribed to the delayed crack initiation and
[67]
propagation by hydrogen-enhanced local plasticity with the formation of NTs and dislocation cells
[Figure 6F]. These above strategies should be further explored in many other HEA systems, like CoCrNi,
CoNiV, and so on.