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

Table 1. Summary of different categories of HEAs (at.%)
EE issues HE ITE
HEA systems Equiatomic CoCrNi 39.9Ni-20Co-30Fe-6Al-4Ti-0.1B
Equiatomic CoNiV 29.9Ni-30Co-13Fe-15Cr-6Al-6Ti-0.1B
Equiatomic FeCoCrNiMn 46.23Ni-23Co-10Cr-5Fe-8.5Al-4Ti-2W-1Mo-0.15C-0.1B-0.02Zr
97(CoCrNi)-3Mo 100-x(NbMoTaW)-xB
Equiatomic FeCoCrNi
45Fe-35Mn-10Co-10Cr
50Fe-30Mn-10Cr-10Co

HEAs: High entropy alloys; EE: environmental embrittlement; HE: hydrogen embrittlement; ITE: intermediate-temperature embrittlement.

Figure 1. Schematic showing EE phenomena and associated microstructural factors. EE: Environmental embrittlement.

equiatomic CoCrNi medium entropy alloy (MEA) with ultimate tensile strength as high as ~1 GPa, as
shown in Figure 2A . The authors argued that the superior HE resistance is primarily attributed to the
[51]
enhanced dynamic strain hardening caused by hydrogen-promoted mechanical nanotwinning [Figure 2B].
However, an inconsistent observation of this equiatomic CoCrNi MEA can be found under gas hydrogen
charging, where the CoCrNi MEA displayed a pronounced ductility reduction (~70.9%) compared to the
uncharged sample. Interestingly, it was further shown that boron doping (400 at. ppm) can obviously
decrease the hydrogen-induced ductility loss (~45.8%) in this MEA, which is basically ascribed to the GB
decoration of boron [Figure 2C-E]. The boron segregation not only significantly increases the cohesive
strength of GBs but also reduces the hydrogen diffusivity along GBs, resulting in improved HE resistance .
[49]
Therefore, it should be noted that different hydrogen charging approaches can create distinctive mechanical
responses in the same HEA. This is because hydrogen atoms are largely concentrated on the surface region
of the samples via electrochemical charging, whereas hydrogen is more homogeneously distributed in the
gas hydrogen-charged samples . In addition to the effect of boron on the resistance to HE, it was also
[53]
reported that a small addition of boron (0.2-1.6 at.%) can improve the malleability of brittle eutectic HEAs,
which is primarily attributed to the transition in eutectic morphology from lamellar eutectic to dendrite
eutectic. Such a transition is believed to result from the increased constitutional undercooling caused by
boron additions .
[54]
Additionally, the HE resistance of the equiatomic CoCrNi MEA can be further enhanced via Mo doping
(~3 at.%). As shown in Figure 3, Mo can promote the formation of nanotwins (NTs) in the hydrogen-
[55]
charged specimen during the deformation process . It was claimed that the existence of NTs can impede
the local accumulation of hydrogen and disturb the continuous motion of dislocation interacting GBs,
thereby inhibiting the intergranular decohesion and improving the HE resistance. Nevertheless, it is