Page 72 - Read Online

Xiao et al. Microstructures 2023;3:2023006 https://dx.doi.org/10.20517/microstructures.2022.26 Page 13 of 17

systems exhibit complex structures and substantial compositional constitutions, which could significantly
influence hydrogen diffusion and the associated mechanical responses. Therefore, the effects of structures
(i.e., phase structures, vacancies and dislocations) and constitutions (substitutional and interstitial atoms) in
HEAs on hydrogen-induced mechanical behavior and the underlying mechanisms should be carefully
studied. Notably, a quantitative model has been proposed by Nag et al. and Kamachali et al., which
demonstrated that the solute-solute interactions and internal stresses can significantly affect the
thermodynamic properties and strengthening mechanisms of HEA systems [78,79] . If the H atoms are
incorporated into the HEAs with highly-diverse chemistries, the effects of complex chemistry on the
mechanical responses and deformation mechanisms in the HEAs with H atoms should be experimentally
and theoretically carried out. The role of complex chemistry in the formation of “surfaces” in crack
formation and propagation also needs to be explored.

(2) As earlier demonstrated, hydrogen atoms are preferentially partitioned into GBs and then reduce the
cohesive strength of GBs. Therefore, the structural features of GBs play a key role in determining the HE
resistance of alloys. Based on this, it could be an effective routine to achieve enhanced HE resistance in
HEAs via elaborately designing the GB architectures (e.g., introducing precipitates at GBs and creating GB
segregation). Therefore, further research should be focused on the innovative design of novel GB structures
in the HEAs.

(3) For HE-resistant materials, hydrogen local concentration is a critical parameter in governing the
resistance to HE. When exposed to hydrogen environments for sufficient durations, a low hydrogen
concentration stems basically from low hydrogen solubility and sluggish hydrogen diffusion in most
metallic alloy systems. Therefore, the effect of a specific element on hydrogen solubility and hydrogen
diffusion in different HEA systems should be systematically investigated. We believe that studies along this
direction could help to significantly accelerate the design of HEAs that are intrinsically resistant to HE.

(4) It is well known that atomic-scale microstructural traps can substantially limit hydrogen diffusion. For
instance, Chen et al. showed that hydrogen can be trapped in the core of finely dispersed V-Mo-Nb carbides
[30]
in ferritic steels . Furthermore, it was also reported that hydrogen concentration is observed at carbon-rich
[80]
dislocations and incoherent interfaces between niobium carbides and the surrounding steel . Therefore, it
could be an effective strategy to enhance the resistance to HE in HEAs via engineering hydrogen traps, i.e.,
[81]
nanoscale carbides and low-energy dislocation nanostructures . In addition, state-of-the-art
microstructural characterization techniques like cryogenic atom probe tomography should be employed to
directly observe the hydrogen distribution of HEA systems.

(5) Similar to Ni-based superalloys, L1 -strengthened HEAs have shown significant promise in high-
2
temperature applications. However, it has been recognized that oxygen-assisted GB damage plays an
important role in the ITE issue. Therefore, improving oxidation resistance may be an efficient avenue that
can hinder the oxygen-accelerated GB damage. Furthermore, as earlier reported, serrated GBs and
columnar-like grains can effectively address the ITE issues. It should be noted that such unique GB
structures can greatly reduce the diffusion kinetics of the oxygen and then delay intergranular cracking. As a
result, careful tailoring of GB features should be carried out to further improve the resistance to ITE in
HEAs. In contrast, it has been recognized that creating hydrogen traps and reducing diffusion kinetics can
effectively improve the resistance to HE. We expect that L1 -strengthened HEAs with unique GB features
2
are also highly HE resistant, owing to the suppressed diffusion kinetics (serrated GBs and columnar-like
grains) and high-density hydrogen traps (L1 /matrix interfaces).
2