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Zhou et al. J Mater Inf 2022;2:18 https://dx.doi.org/10.20517/jmi.2022.27 Page 5 of 21

[20,59-65]
Figure 3. Comparison of tensile yield strength and uniform elongation of various HEAs fabricated by AM: FCC HEAs ; eutectic
[68-74] [75-83] [84-89]
HEAs ; HEAs strengthened by ceramics ; HEAs strengthened by L1 . FCC: face-centered-cubic; HEAs: high-entropy alloys.
2
precipitation-strengthened HEAs with desirable properties is a significant challenge due to the chemical
complexity. Another key issue is the difficulty in obtaining suitable AM processing parameters for these
precipitation-strengthened HEAs. Machine learning provides an effective method to quickly screen out
[58]
alloys with desirable properties and suitable AM processing parameters without tedious experiments .
Thus, this review focuses on some new trends regarding AM precipitation-strengthened HEAs and the
applications of machine learning in designing new-type alloys and also obtaining suitable AM processing
parameters.

AM OF HEAS
AM processes, including PBF and DED, have shown many significant advantages in fabricating HEA
components compared to traditional processing methods, such as casting, wrought and welding. Until now,
many kinds of HEAs have been manufactured by AM methods, including single-phase HEAs (FCC or
BCC) [20,59-67] , EHEAs (FCC + BCC) [68-74] and precipitation-strengthened HEAs [75-89] . Figure 3 summarizes the
tensile yield strength against the uniform elongation of as-printed HEAs at room temperature. Due to their
poor workability and the high cost of raw materials, only a few studies have been conducted on the AM of
[66,67]
BCC HEAs . There is still no report on the tensile properties of BCC HEAs fabricated by AM methods.
[20,59-65]
As-printed FCC HEAs show large ductility but low strength . In addition, for EHEAs manufactured by
L-PBF, their tensile properties vary over a wide range, which may be caused by the relative proportions
between the FCC and BCC phases and different microstructural features [68-74] . Another approach to
strengthening the soft FCC matrix is to introduce second hardening phases, such as incoherent ceramic
[83]
particles (carbides [75-80] , nitrides [81,82] , oxides , and so on), and coherent L1 2 [84-89] . Among these second
hardening phases, it was found that the coherent L1 phase can significantly improve the tensile strength of
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the as-printed FCC HEAs without seriously sacrificing ductility.

AM of single-phase HEAs
FCC HEAs can be well manufactured by AM methods due to their excellent workability. The defects,