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Page 10 of 17 Xiao et al. Microstructures 2023;3:2023006 https://dx.doi.org/10.20517/microstructures.2022.26

Figure 8. Oxygen-assisted failure mechanism in various HEA systems at intermediate-temperature regimes. Tensile curves of
(A) Ni-20Co-30Fe-6Al-4Ti-0.1B (at.%) HEA at 600 °C in air and vacuum conditions (Reproduced with permission [73] . Copyright 2022,
Elsevier) and (B) Ni-30Co-13Fe-15Cr-6Al-6Ti-0.1B (at.%) HEA at 775 °C with different surface treatments and testing environments
(Reproduced with permission [39] . Copyright 2022, Elsevier). HEA: High entropy alloy.

4Ti-0.1B (at.%) HEA, which not only improves the oxidation resistance of the HEA but also hinders the
inward oxygen diffusion and thus results in a substantial increase in tensile plasticity at 600 °C
[Figure 9A and B] . This suggests that enhancing oxidation resistance via tailoring the chemical
[73]
composition is a useful pathway for the innovation of ITE-resistant HEAs. Moreover, for the latter, our
previous work innovatively proposed a duplex-aging strategy in an L1 -strengthened 39.9Ni-30Co-13Fe-
[26]
2
15Cr-6Al-6Ti-0.1B (at.%) HEA. Such a duplex-aging strategy can controllably eliminate the intergranular
brittle phases, the so-called Heusler phase, therefore achieving a distinct brittle-to-ductile transition at
700 °C, as shown in Figure 9C. More strikingly, Cao et al. designed a heterogeneous columnar-grained
(HCG) structure in the 39.9Ni-30Co-13Fe-15Cr-6Al-6Ti-0.1B (at. %) HEA via regulating thermomechanical
[74]
treatments . This new type of HCG HEA exhibits superior resistance to intergranular fracture at 800 °C
compared with the equiaxed counterpart with severe brittleness along GBs, which could be originated from
the unique GB characteristics and distributions in the HCG HEA. Specifically, the HCG HEA shows an
unusually large tensile ductility of ~18.4% combined with a high yield strength of ~652 MPa at 800 °C, as
demonstrated in Figure 9D.

In addition, it is noteworthy that the serrated grain boundary (SEG) architectures can effectively solve this
intergranular premature cracking issue in an L1 -strengthened HEA (46.23Ni-23Co-10Cr-5Fe-8.5Al-4Ti-
2
2W-1Mo-0.15C-0.1B-0.02Zr, at.%) at 1000 °C. This kind of HEA with SEG structures shows a brittle-to-
ductile transition and achieves a superior strength as high as ~260 MPa while maintaining a uniform
elongation of ~6.5% [Figure 10] . This finding further demonstrates that SEGs can produce enhanced
[75]
resistance to intergranular crack nucleation and propagation at higher temperatures.

In addition to intergranular embrittlement in the intermediate-temperature regimes, some refractory HEAs
[76]
also suffer from obvious brittleness at room temperature , which is essentially attributed to the GB
segregation of the oxygen contaminant during fabrication, thereby weakening GB cohesion. Wang et al.
reported that GB engineering with the addition of either metalloid B or C in a NbMoTaW refractory HEA
can effectively alleviate the GB brittleness and changes the fracture morphology from intergranular to
transgranular fracture . The doped small-sized metalloids preferentially replace oxygen at GBs and
[76]