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Page 12 of 21 Zhou et al. J Mater Inf 2022;2:18 https://dx.doi.org/10.20517/jmi.2022.27

where is the yield strength, is the friction stress, refers to grain refinement strengthening (k is the
Hall-Petch slope and d is the grain size) and refers to the dislocation strengthening (α is a constant, M is the
Taylor factor of 3.06, G is the shear modulus, b is the Burgers vector of full dislocations and ρ stands for the
dislocation density). This revealed that the enhanced yield strength of the as-printed FCC HEA was mainly
derived from the dislocation strengthening compared with the grain refinement strengthening.

Solid solution strengthening arises from the interaction of solutes with dislocations. The different atomic
sizes lead to severe lattice distortion of HEAs, which impedes the dislocation movement and leads to
[100]
pronounced solid solution strengthening . Ishimoto et al. developed a TiNbTaZrMo bio-HEA with super
[101]
yield strength by L-PBF . The ultrafast cooling rate during L-PBF promoted the formation of fine grains,
suppressed the elemental segregation and realized an oversaturated solid solution. The combined effects of
grain refinement and solid solution strengthening were regarded as the main reasons for the excellent
strength of the TiNbTaZrMo bio-HEA fabricated by L-PBF. Zhang et al. systematically evaluated the factors
that affect solid solution strengthening by comparing the microstructures of CoCrFeNiX (X = Al, Nb or
0.4
Ta) fabricated by powder plasma arc AM in terms of mixed entropy, mixed enthalpy, atomic radii
[102]
difference, electronegativity, valence electron concentration and melting point . Compared with Al, Ta
and Nb with high melting temperatures had a more significant influence on the formation of the
topologically close-packed solid solution phase, which was the main reason for the enhanced strength.
Furthermore, elements with significant differences in electronegativity tended to be enriched in the second
solid solution phase. Therefore, it is crucial to control multiple variables when improving the mechanical
properties of HEAs by introducing a solid solution phase.

TRIP is an effective method to break the strength-ductility trade-off by providing materials with extensive
work hardening ability. The TRIP effect can be triggered by reducing the phase stability. Thus, metastable
HEAs were developed by tuning the stacking fault energy of the matrix phase through the selection of
particular alloy chemistry. In addition to the alloy composition optimization, Agrawal et al. also found that
[103]
the TRIP effect can be tailored through L-PBF methods . As shown in Figure 12, the as-printed metastable
Fe Mn Co Cr Si (at. %) HEA showed a much higher work hardening ability than the as-cast sample. Due
40 20 20 15 5
to the thermal cycling of L-PBF, heat accumulation in the previously solidified layer occurred, which
promoted the formation of a high-temperature γ (FCC) phase in that layer under an ultrafast cooling rate.
Therefore, the as-printed metastable Fe Mn Co Cr Si HEA showed a dual phase (γ + ε) microstructure,
40
15
5
20
20
while the as-cast sample showed an ε phase (HCP)-dominant microstructure [Figure 12B and C]. Upon
deformation, most of the metastable γ phase was transformed into the ε phase [Figure 12D] under tensile
stress, which contributed significantly to the work hardening ability of the as-printed sample. In addition to
the TRIP effect, the TWIP effect also occurred in ε grains, thereby inducing the non-basal plasticity.
The strengthening mechanisms of the precipitation-strengthening HEAs fabricated by L-PBF vary with
precipitate types and sample states. Due to the ultrafast cooling rate of L-PBF, the coherent precipitate, such
as the L1 phase, is difficult to separate from the matrix. Thus, dislocation strengthening is the main reason
2
for the excellent strength and ductility of the as-printed HEAs containing the L1 -forming elements. The
2
high-density L1 phase can be introduced by subsequent aging treatment, leading to the significant
2
precipitation strengthening effects. During deformation, both L1 precipitation and high-density dislocation
2
networks can work as effective obstacles to the dislocation motion. Thus, the high strength of the as-printed
sample after aging treatment can be attributed to the synergistic strengthening mechanism of the
precipitates and dislocations. The plastic deformation mode is mainly dominated by SFs, which are
activated in most grains and exhibit a nano-spaced SF network, multiple SFs and Lomer-Cottrell locks.
[89]
These unique microstructures provide sufficient work hardening ability to the as-printed FCC HEA . In