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Zhou et al. J Mater Inf 2022;2:18 https://dx.doi.org/10.20517/jmi.2022.27 Page 9 of 21

Figure 8. (A) High-angle annular dark field STEM and corresponding EELS (TEM-electron energy loss spectroscopy) mapping results to
[77]
confirm nanosized carbides. (B) Representative tensile stress-strain curves for three SLM-built C-HEAs . C-HEA: C-containing HEA.
[75,76]
the grain boundaries. Similar phenomena were also observed in other as-printed C-containing HEAs .
With increasing carbon content, the tensile strength and ductility were simultaneously improved. The
improvement in strength can be attributed to the Cr C precipitation, high back stress and easy formation
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of deformation twins. The high-density deformation twins and reduced pore defects are the main reasons
for the ductility improvement observed.

Nitrides or N-rich structures can also be used to strengthen the FCC HEA matrix by doping TiN particles to
the HEA powder through ball-milling or in situ formation of ordered nitrogen complexes under a nitrogen
atmosphere during L-PBF. For instance, Li et al. fabricated a CoCrFeNiMn HEA matrix composite with
nano-TiN particle reinforcements via L-PBF [81,82] . Although the strength of the as-printed HEA composite
was significantly improved due to the refined microstructures and pinning effects of TiN, the ductility of the
alloy was greatly sacrificed. In addition, Zhao et al. found that the strength and ductility of the as-printed
[92]
CoCrFeNiMn HEA were simultaneously enhanced by using a reactive N + Ar atmosphere during L-PBF ,
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as shown in Figure 9A. Figure 9B illustrates the schematic diagram of this L-PBF process. During
fabrication, N atoms were dissolved into the molten pool to form an ordered nitrogen complex (inset of
Figure 9B), which facilitated the dislocation multiplication, leading to a higher dislocation density with
smaller dislocation cells. Thus, the improvement in yield strength is mainly attributed to the dislocation
strengthening. In contrast, the introduction of nitrogen atoms resulted in a more heterogeneous
microstructure of the matrix, leading to a higher work hardening rate and stabilizing the plastic
deformation.

The oxide-dispersion-strengthening mechanism has been widely applied to traditional alloys fabricated by
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AM, such as steel, superalloys and Ti alloys . Similarly, this method can also be used to strengthen HEAs
[83,96-98]
by in situ forming oxides during L-PBF or blending oxides with pre-alloyed powders . Chen et al.
proposed a new method to develop an oxide-dispersion-strengthened (ODS) HEA using a L-PBF
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Mn-doped FeCoCrNi HEA powder . The oxide particles in the as-printed ODS HEA were proved to be
MnO and Mn O . As illustrated in Figure 10, MnO was formed by the in situ oxidation reaction between
2 3
Mn and oxygen during the L-PBF process, while Mn O came from the surface oxide of the Mn powder.
3
2
These oxide particles hindered the dislocation movement during the tensile test, leading to a significant
Orowan strengthening effect. At a high plastic strain, voids around the oxide particles were formed,
reducing the tensile ductility to a certain degree. Moreover, it is interesting to find that this in situ formation
of Mn-containing oxides can help to improve the high-cycle fatigue resistance and compressive creep