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Xu et al. Microstructures 2023;3:2023015 https://dx.doi.org/10.20517/microstructures.2022.40 Page 3 of 9

were polished with a 2000-grit SiC paper, followed by electropolishing in an HClO :C H O = 1:9 solution
6
2
4
with a direct voltage of 20 V at room temperature. TEM specimens were mechanically ground to about
50 μm thickness, punched to Φ3 mm circle sheets, and then thinned by twin-jet electro-polished using a
mixture of 10% perchloric acid and 90% alcohol (vol.%) with a direct voltage of 30 V at a temperature of
-18 °C.

RESULTS AND DISCUSSION
The XRD patterns of the as-cast 18Al and 19Al alloys, as shown in Figure 1A. Both 18Al and 19Al alloys
have the FCC + BCC duplex phase microstructure. DSC curves of the 18Al and 19Al alloys are seen in
Figure 1B. Both alloys show only one exothermic peak, confirming that 18Al and 19Al EHEAs are at the
eutectic position, which can be attributed to their eutectic compositions. This phenomenon was also
detected in other EHEAs, such as in AlCoCrFeNi 2.1 [12] , Nb Fe Co Ni 1.00 [30] , CoCrFeNiTa 0.4 [31] , and
1.22
0.62
1.98
[32]
CoCrFeNiMo alloys .
0.8
Figure 2A and D show the scanning electron microscope (SEM) images of as-cast 18Al and 19Al alloys,
exhibiting that the as-cast 18Al and 19Al alloys both have a typical lamellar morphology of eutectic
microstructure. The electron backscatter diffraction (EBSD) inverse pole figures and phase maps of as-cast
18Al and 19Al EHEAs are seen in Figure 2B and C, Figure 2E and F, respectively. Different lamellar growth
directions were observed in the adjacent FCC grains of 18Al and 19Al EHEAs [Figure 2B and E], indicating
that these EHEAs possess a different lamellar arrangement in the grains with different orientations. The
EBSD phase maps in Figure 2C and F show that dark and light lamellae in Figure 2A and B are FCC and
BCC phases, respectively. The corresponding content of FCC and BCC phases in 18Al EHEA is about
59.2 vol.% and 40.8 vol.%. Compared with the 18Al EHEA, 19Al EHEA has a higher content of the BCC
phase, and the content of the BCC phase in 19Al EHEA is about 50.4 vol.%. Moreover, wider BCC lamellae
in 19Al EHEA were observed in Figure 2F. In 18Al EHEA, L1 and B2 phases with nanoindentation
2
hardness of 5.0 ± 0.6 GPa and 6.2 ± 0.7 GPa, respectively, while in 19Al EHEA, L1 and B2 phases with
2
nanoindentation hardness of 4.6 ± 0.5 GPa and 5.7 ± 0.9 GPa, respectively. These results are consistent with
[19]
the previous research that the B2 phase is harder than the L1 phase .
2
To better understand the microstructural features of as-cast 18Al and 19 Al EHEAs, we performed
transmission electron microscopy (TEM) observation equipped with energy dispersive spectroscopy (EDS).
Figure 3A and D exhibit the alternating lamellae microstructure of as-cast 18Al and 19Al EHEAs,
respectively. According to the selected area electron diffraction patterns (SADPs) in Figure 3B and E, the
dark and light lamellar in 18Al and 19Al EHEAs are L1 and B2 phases, respectively. L1 and B2 phases can
2
2
be seen as ordered modes of FCC and BCC phases. EDS maps [Figure 3C and F] and SADPs reveal that the
Ll lamellae in 18Al and 19Al EHEAs enriched in Co and Cr while B2 phases in 18Al and 19Al EHEAs
2
enriched in Ni and Al but depleted in Cr and Co. We noted that the B2 phases in 18Al are not well enriched
in Ni and Al, mainly due to the B2 phases being eroded away during the TEM sample preparation, as seen
in Figure 3A. The average widths of the BCC lamellae in 18Al and 19Al EHEAs are ~0.3 μm and ~0.5 μm,
respectively. It is widely reported in Fe, Cr contained EHEAs that it is easy to precipitate in the form of
spherical particles in the B2 phases owing to the Cr element showing a limited solid solubility [14,33-35] . While
we failed to observe particles in the B2 lamellar of our 18Al and 19Al EHEAs, as exhibited by the STEM
images of Figure 3A and D. This phenomenon may be ascribed to the removed Fe element in our EHEAs,
which is similar to other EHEA . In the recently-reported Ni Co Cr Fe Al W and
[27]
10
30
30
18
10
2
Al Co Fe Ni EHEAs, the orientation relationship between the L1 and B2 phases is determined to
18.86
18.36
43.53
19.25
2
be [011]L1 // [ 11]B2 and (11 )L1 // (01 )B2, meeting the classical K-S relationship [33,36] . This semi-
2
2
coherent interface is usually accompanied by a great number of lattice misfit dislocations [33,36] . During the