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Chen et al. J Mater Inf 2023;3:10 https://dx.doi.org/10.20517/jmi.2023.06 Page 11 of 19

Figure 6. The superior mechanical properties of eutectic high entropy alloys overcome the strength-ductility trade-off in conventional
[47,65,164-171]
high entropy alloys .
[177] [177,178]
image correlation (DIC) and the loading-unloading-reloading (LUR) tests , as illustrated in Figure 7.

In addition to the HDI strengthening, twinning-induced plasticity (TWIP) is another plausible
strengthening mechanism in EHEAs, although, in the first place, it applies to materials of low stacking fault
[179,180]
energy (SFE) with enhanced strain hardening capability and delayed plastic instability . Through the so-
called dynamic Hall-Petch effect, twin boundaries can act as obstacles to dislocation motion while
[181] [182]
permitting some partial dislocations to glide . Diao et al. reviewed the deformation twinning (DT)
mechanism in HEAs and proposed the preferential conditions for the activation of the DT mechanism,
including (a) large deformation strain; (b) low deformation temperature; (c) high strain rate; and (d) large
grain size. With the first three conditions being met, a sufficient high dislocation density will be generated,
leading to a high local stress for twin nucleation; while the last condition is to ensure there is sufficient space
[183]
[180]
for high twinning activities (i.e. twin thickening) . For instance, Shi et al. uncovered a sequentially
activated DT mechanism in the ultrafine-grained Al Fe Co Ni EHEA which resulted in an outstanding
19 20 20 21
combination of yield strength (~1.2 GPa) and tensile ductility (~24.0%).

Besides DT, transformation-induced plasticity (TRIP) is another strengthening mechanism that can be
[184]
activated in an alloy with an even lower SFE . For conventional HEAs, the deformation induced phase
transformation can be activated by heuristically adjusting the alloy chemical composition to lower
[184,185]
SFE . However, it is difficult to activate or control the TRIP effect in EHEAs through compositional
[186]
tuning because of the narrow compositional range for a eutectic alloy. Liu et al. reported the phase
transformation from the BCC/B2 phase to the FCC phase in the near-eutectic AlCoFe Ni alloy and
2 2 [187]
attributed this phenomenon to enhanced atom diffusion under high-temperature torsion. Wu et al.
found a phase transformation from B2 to BCT structure in the Al Co Cr Fe Ni W EHEA during tensile
18 30 10 10 30 2
deformation, which was thought to be responsible for the deformability of the B2 phase. These results are
interesting and warrant further research. In particular, it still remains open whether EHEAs with the TRIP
and/or TWIP effect can be designed through the aforementioned data-driven approach.

SUMMARY
To sum up, we provide a critical review of the recent development of EHEAs in this article by focusing on
the various approach for compositional design, from the empirical to data-driven methods. Through the
heuristic empirical methods based on binary eutectics (i.e., combination versus substitution), people have
successfully developed a number of EHEAs; however, this trial-and-error approach is ineffective in