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Page 6 of 9 Xu et al. Microstructures 2023;3:2023015 https://dx.doi.org/10.20517/microstructures.2022.40

Figure 4. Mechanical responses of as-cast 18Al and 19Al EHEAs tested at room temperature. (A) The engineering stress-strain curves of
18Al and 19Al EHEAs were tested at room temperature. The inset shows the corresponding true stress-strain curve. (B) The strain
hardening rate vs true strain plots of 18Al and 19Al EHEAs.

(GNDs) were prevented and piled up at FCC/BCC boundaries of 19Al in this deformation stage, resulting in
larger long-range back stress in L1 phases [37-39] . The GNDs piled up at the FCC/BCC boundaries also
2
generated forward stress in the B2 phases , promoting the plastic deformation of the B2 phases. When
[37]
both the L1 and B2 phases were plastically deformed, the softer L1 phases would undergo larger plastic
2
2
strain, leading to a heterogeneous deformation [37,38] . To accommodate the heterogeneous deformation,
enough strain gradients must be present near the heterogeneous FCC/BCC surfaces, thereby producing a
more remarkable HDI hardening in 19Al. Finally, although the as-cast 18Al and 19Al EHEAs show an
[37]
abnormal inability to sustain high strain-hardening rates over a narrow region III, they both have sufficient
uniform tensile strains (> 9%).

Figure 5A and B exhibit bright-field (BF) TEM images of alternating lamellae in the ~9% strained 18Al and
19Al EHEAs. Two kinds of {111} plane traces were detected in the L1 lamellae of 18Al EHEA, as shown in
2
Figure 5A. Figure 5B is a bright-field (BF) micrograph showing stacking faults (SFs) in the L1 lamellae of
2
19Al EHEA at [110] zone axis. In addition, strain-induced stacking faults were also observed in
AlCoCrFeNi EHEA and additively manufactured EHEAs . This scenario suggested that the stacking
[14]
[40]
2.1
fault was another significant deformation type of our EHEAs besides the planar dislocation slip mentioned
above in the L1 lamellae. Figure 5A and B show a high density of dislocations occurring in the L1 lamellae,
2
2
while no obvious dislocations are detected in the B2 phases. In the recently-reported AlCoCrFeNi EHEA,
2.1
dislocations in the B2 lamellae can be detected in ( 10) and (110) slip bands, and these dislocations could be
hindered by the spherical precipitates enriched in the Cr element .
[14]
We investigated the fracture surfaces of 18Al and 19Al EHEAs to further reveal the damage and fracture
mechanisms of the developed as-cast EHEAs at room temperature. These images of two EHEA samples in
Figure 5C and D unveiled a similar fracture morphological character, namely, trench-type microstructures
and several blocky phases with the cleavage character on the fracture surface [14,16,33] . More specifically, these
two EHEAs both featured two types of fracture modes, i.e., brittle-type fracture in the BCC phase
accompanied by a ductile fracture in the FCC phase. To better understand the blocky phases on the fracture
surface of two EHEAs, we performed the EPMA analysis for the flat fracture surface of the 18Al EHEA. The
EPMA images in Figure 5E reveal that the blocky phase is enriched in Ni and Al, but depleted in Co and Cr.
The EPMA results indicate that blocky NiAl-rich phases are BCC precipitates. These BCC precipitates may