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Page 10 of 15 Ying et al. Microstructures 2023;3:2023018 https://dx.doi.org/10.20517/microstructures.2022.47

[47]
the HEA liquids may transform into network liquids through spinodal decomposition . The solidification
of the spinodal network liquids then results in the formation of a crystalline network structure. Moreover,
other mechanisms may also be factors of network structure. Literature [48-50] showed that the Rayleigh
instability may induce the fragmentation of dendrites during recalescence at large undercooling, resulting in
a network structure. Furthermore, another factor is the entropy effect. With increasing configurational
entropy, the growth morphology may transit from dendritic to faceted [51,52] , leading to a structure that is
different from the dendritic [32,53] , lamellar , or equiaxed grain structures [55,56] formed through conventional
[54]
casting processes. Using conventional methods, the microstructures can only be modified in the solid state
by thermal/mechanical treatments to introduce precipitates, structural defects, or refinement of the as-cast
grains [55,57] . The fluxing technique offers a unique route to directly develop the network structure in bulk-
sized samples through the solidification of undercooled liquids, highlighting the promising potential of the
fluxed N-HEAs in industrial applications.

Composition dependence of phase fractions
The present results showed that the network-like structure could be controlled by tuning the B content of
the alloys. The EDS results summarized in Table 1 show little difference between the Fe and Co contents in
the two phases, while the Ni and Cr contents were significantly different. The enthalpy of mixing (ΔH)
[58]
between elements is summarized in Table 3 , which shows that the absolute ΔH value between Cr and B
was the largest. Thus, it is reasonable that Cr and B prefer to segregate to form intermetallic phases in one
sub-network. This was also confirmed by the neutron diffraction results, which revealed the presence of
Cr B-type intermetallic phases. As long as the Cr element is still present in the FCC solid solution, the
2
volume fraction of Cr B may increase with increasing B content.
2

Structural origin of improved mechanical properties
As summarized in Figure 5D, the mechanical behavior is correlated with the volume fractions of the soft/
ductile FCC phase and hard/brittle Cr B-type intermetallic phase. The samples with a lower volume fraction
2
of hard/brittle Cr B intermetallic phase exhibit a lower hardness and yield strength but a higher compressive
2
strain. In situ synchrotron X-ray diffraction measurements of the tensile behavior of the B12 alloy revealed a
three-stage deformation process, with the ductile FCC phase yielding earlier, and the hard Cr B-type
2
intermetallic phase yielding later than the macroscopic yielding in the plastic region. The whole
deformation is inhomogeneous, indicating that the deformation is accommodated between the two phases,
maintaining the plastic compatibility. This heterogeneous deformation has also been observed in austenite-
ferrite dual-phase steels [59,60] . The Cr B-type intermetallic phase bears more stress after yielding, as evidenced
2
by the larger lattice strain. Similar to multiphase steel, where the hard phases bearing more stress ensure a
sufficient work-hardening capability , the hard Cr B-type phase in the B12 alloy may contribute to a
[37]
2
strain-hardening effect and an excellent combination of strength and ductility. Moreover, the FCC phase
also has high work hardening ability due to the multiple slip systems as well as other deformation
mechanisms such as stacking fault, twinning and phase transformation [6,39] . In this study, dislocations could
be observed, as revealed by the increasing trend of F-111 and F-222 intensity in the plastic region, and no
evidence of the involving stacking faults and phase transformation could be found from the synchrotron
experiments. Further investigation is needed to explain the work-hardening effect of the FCC phase at a
large strain.

In addition, although our in situ loaded sample only deformed to several percent, a tiny increase of lattice
strain in the FCC phase after 500 MPa could be observed, which means that stress was partitioned with the
FCC phase . A previous study of austenite-martensite dual-phase steel attributed the improved ductility to
[61]
stress transfer from the hard to the soft phase, forcing the two phases to deform together . The
[33]
cooperative deformation, as well as stress partitioning, could inhibit the strain localization and thus delay