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Page 2 of 17 Xiao et al. Microstructures 2023;3:2023006 https://dx.doi.org/10.20517/microstructures.2022.26

Keywords: High-entropy alloys (HEAs), environmental embrittlement (EE), hydrogen embrittlement, intermediate-
temperature embrittlement, EE-resistant HEAs

INTRODUCTION
High-performance structural materials with promising strength and ductility combinations are highly
[1-4]
desirable for a wide range of engineering applications . Distinct from traditional single-principal-element-
guided materials, like steels , aluminum alloys [9,10] , titanium alloys [11,12] and Co/Ni-based superalloys [13-17] ,
[5-8]
the multiple-principal-element paradigm has significantly motivated the rapid development of a new class
of metallic structural materials, namely, high-entropy alloys (HEAs), which are also known as multi-
principal-component and chemically complex alloys [18-22] .

Given their extraordinary physical, thermal and mechanical properties [23-25] , emerging HEAs have been
expected to benefit many potential engineering applications in the aerospace, automotive, nuclear power,
petrochemical and electronic manufacturing fields, as well as many other industries [26-28] . Like most
conventional metallic alloys [29,30] , HEAs unavoidably face enormous challenges regarding environmental
embrittlement (EE) failure, which is critical to the safety and reliability of engineering structures.
Intermediate-temperature embrittlement (ITE) and hydrogen embrittlement (HE) are considered to be two
crucial EE issues in various advanced metallic structural materials [31-38] . Specifically, due to the synergetic
effect of the local stress concentration in the vicinity of grain boundaries (GBs) and environmental GB
attacks, most polycrystalline high-temperature structural alloys often undergo serious intergranular
embrittlement in intermediate-temperature regimes (i.e., 600-800 °C) [39,40] . Similarly, when exposed to
hydrogen environments, the deformation capability of some once ductile metallic alloys dramatically
degraded, resulting in poor fracture resistance and intergranular failures [41-46] . This is because the hydrogen
generally traps at the GBs and then reduces their cohesive strength. The nucleation and propagation of
cracks preferentially occur at GBs during tensile deformation, consequently leading to intergranular
fracture [47,48] .

Notably, such undesired EE behavior, including HE and ITE, has been frequently observed in different HEA
systems [39,49] . To date, many research groups have devoted their efforts to addressing EE problems in new
types of HEAs and have made significant achievements. Therefore, in this study, we provide an overview of
the recent important discoveries in the EE of representative HEAs. Two major types of typical EE (ITE and
HE) behavior and mechanisms (or microstructural factors) are discussed separately, as schematically
illustrated in Figure 1. The key strategies for improving the resistance to EE in the HEA systems are
highlighted. In this review, different HEA systems are introduced and discussed, as presented in Table 1.
Finally, the challenges and future research trends for the development of EE-resistant HEAs are briefly
summarized.

HE MECHANISMS AND MITIGATION STRATEGIES IN HEA SYSTEMS
In this section, we summarize the significant progress made in the development of HE-resistant HEAs and
the associated governing mechanisms and novel strategies.

In the past few years, numerous studies on HE resistance have been mainly focused on single-phase HEAs,
such as the CoCrNi, CoNiV and FeCoCrNiMn systems [49-52] . Generally, hydrogen diffuses into alloys and
can be introduced via electrochemical and gas hydrogen charging. Here, we summarize the HE behavior of
HEAs in different hydrogen charging approaches. For the former case, when hydrogen is incorporated via
electrochemical hydrogen charging, Soundararajan et al. demonstrated a relatively high HE resistance of an