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Xiao et al. Microstructures 2023;3:2023006 https://dx.doi.org/10.20517/microstructures.2022.26 Page 9 of 17

Figure 7. Mechanism and prediction of HE in various alloy systems. (A) Schematic of embrittlement process at crack tip. (B) Schematic
of crack tip hydrogen concentrations C cleave and C emit that control embrittlement as a function of KI app . (C) Predicted embrittlement due
app
to nanodiffusion and blocking of dislocation emission: KI reaches KIC prior to reaching K . (D) Predicted embrittled and unembrittled
Ie
domains of hydrogen concentration for six alloys and Ni. The transition region corresponding to the upper and lower limits of K is
Ie
indicated by thick black lines (Reproduced with permission [68] . Copyright 2021, American Physical Society). HE: Hydrogen
embrittlement.
recognized that coherent L1 -type precipitate-strengthened HEAs are some of the most promising
2
candidates for high-temperature structural applications due to their exceptional thermal and mechanical
properties at a wide range of temperatures [20,70-72] . Unfortunately, like many high-strength metallic structural
materials, such L1 -strengthened HEAs also usually exhibit temperature-dependent premature tensile
2
failure. As shown in Figure 8, it has been claimed that environmentally (i.e., oxygen)-assisted GB damage
plays a vital role in premature intergranular failure at intermediate-temperature regimes [39,73] . Additionally,
the potentially formed second phases at GBs also act as sites for crack initiation and propagation, leading to
brittle intergranular fracture during tensile deformation .
[39]
Therefore, extensive efforts have been made to overcome this ITE issue in these L1 -strengthened HEAs.
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Several advances have demonstrated that the ITE resistance can be effectively improved in some typical
HEA systems through careful compositional optimization and structural regulation. For the former, it was
found that Cr doping can introduce compact protective oxide layers in a 39.9Ni-20Co-(30-x)Fe-xCr-6Al-