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Page 8 of 21 Zhou et al. J Mater Inf 2022;2:18 https://dx.doi.org/10.20517/jmi.2022.27

Figure 7. (A) Heterogeneous microstructure of different layers in as-printed EHEA. (B,D) Cell-like microstructure with nanosized L1
[69]
2
precipitates uniformly distributed in the FCC matrix. (C) True stress-strain curves of EHEA fabricated by cast and SLM . SLM:
selective laser melting.
EHEAs, the cracking mechanisms of the as-printed samples transferred from the intergranular hot cracking
induced by coarse columnar FCC grains to the transgranular cold cracking resulted from the fracture of
[91]
brittle BCC grains under severe residual stress . He et al. fabricated a nearly fully dense and crack-free
[69]
AlCoCrFeNi EHEA by L-PBF, which showed a much superior strength to the as-cast sample . The
2.1
improvement in strength can be attributed to the heterogeneous eutectic microstructure, consisting of a
columnar, equiaxed and cell-like microstructure with nanosized L1 precipitates uniformly distributed in
2
[69]
the FCC matrix, as illustrated in Figure 7 . However, the L-PBF manufactured Ni Co Cr Fe Al W Mo
[68] 30 30 10 10 18 1 1
(at. %) EHEA showed serious softening above 650 °C, as illustrated by Yang et al. . Heat treatment may be
needed to further improve the high-temperature mechanical properties of as-printed EHEAs by optimizing
the microstructure and phase constitution.

AM of precipitation-strengthened HEAs
The tensile strength of FCC HEAs can be improved by introducing a hard ordered B2 phase. However, too
much B2 phase is detrimental to the ductility and even causes severe cracking during L-PBF. Unlike the
eutectic microstructure, another effective method to improve the tensile properties of as-printed FCC HEAs
is to introduce strengthening particles during AM or the subsequent heat treatment. According to the
literature, the strengthening phases in FCC HEAs can be generally classified into the coherent L1 phase and
2
the incoherent ceramic particles, like carbides, nitrides and oxides.

Carbides can be introduced into single-phase FCC HEAs by alloying carbon to the FCC HEA powder or
adding carbon to the molten pool during L-PBF, with the latter method being more flexible due to the easy
control of carbon content [75-77] . Kim et al. investigated the effects of carbon content on the microstructures,
[77]
tensile properties and deformation mechanisms of a CoCrFeMnNi HEA fabricated by L-PBF . As shown
in Figure 8, the carbide in situ formed during L-PBF was identified as Cr C and mainly distributed along
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