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Page 2 of 9 Xu et al. Microstructures 2023;3:2023015 https://dx.doi.org/10.20517/microstructures.2022.40

their ordered types (such as L1 , B2 and D0 ) [5-10] . Generally, considering the as-cast mechanical properties
2
19
of HEAs, FCC-structured HEAs exhibit good ductility but relatively low strength, while BCC or HCP-
structured HEAs show high strength but limited ductility [11-13] . These single mechanical properties, poor
castability and composition segregation can seriously deteriorate the further engineering applications of the
as-cast HEAs [14,15] .

Lu et al. first reported the concept of eutectic high entropy alloys (EHEAs) and developed a cast
AlCoCrFeNi EHEA with alternating FCC(L1 )/BCC(B2) lamellar morphologies and an outstanding
2.1
2
[16]
combination of high strength and large ductility . Recently, the mechanical properties of AlCoCrFeNi
2.1
EHEA were enhanced further by thermo-mechanical processing, that is, cold rolling in multi-steps to a
reduction in thickness of ~90%, followed by annealing [15,17,18] . More recently, Shi et al. designed a
directionally solidified Al Fe Co Ni EHEA with ~50% uniform tensile elongation, three times that of
41
20
19
20
conventional cast EHEAs and comparable strength, which provides novel guidance in developing new
structured materials with large elongation and high fracture toughness . Up to now, A wide range of cast
[19]
EHEAs with outstanding mechanical properties and different compositions have been reported [20-26] .
In the past years, Co-Cr-Ni-Al alloys have shown latent capacities to form conventional eutectic
microstructures, which can be verified by Calculation of Phase Diagrams (CALPHAD) methods [27-29] .
However, the microstructures for good mechanical properties and underlying deformation mechanisms of
this type of EHEA still need more investigation. We initially designed Co 20-x/3 Cr 20-x/3 Ni 50-x/3 Al to investigate
10+x
the Al content on the microstructures and mechanical properties of our CoCrNiAl alloys. Surprisingly, we
found that a nominal composition of Co 20-x/3 Cr 20-x/3 Ni 50-x/3 Al (x = 8 and 9) (hereinafter named 18Al and
10+x
19Al, respectively.) can form eutectic microstructures with regular L1 /B2 lamellar morphologies, which is
2
similar with the lamellar structures in most studied as-cast EHEAs [12,14,27-29] . Electron probe microanalyzers,
scanning and transmission electron microscopes were adopted to character the eutectic microstructures,
chemical compositions and deformation mechanisms to reveal the origin of the good properties.

MATERIALS AND METHODS
CoCrNiAl alloys were prepared using arc-melting constituent elements with a purity of > 99.9 (wt.%). These
two alloys were re-melted five times to improve the chemical homogeneity under a Ti-gettered argon
atmosphere. The molten alloys were then drop-cast into a water-cooled copper mold with dimensions of
10 mm × 10 mm × 60 mm. Dog-bone-shaped tensile specimens with a cross-section area of 3.0 × 0.9 mm
2
and a gauge length of 10 mm were cut from the cast ingots by electrical discharge machining. Room-
temperature tensile tests were conducted in a CMT4305 universal electronic tensile testing machine with a
strain rate of 1 × 10 s . At least three tensile experiments were repeated to improve the reproducibility.
-3 -1
Crystal structures of these as-cast specimens were examined by X-ray diffraction (XRD) with Cu Ka
radiation (Rigaku SmartLab). The 2θ scanning was performed in the range of 20°-100° at a scanning speed
of 5°/min. Thermal behaviors of the as-cast HEAs were investigated by a differential scanning calorimeter
(DSC) operated in an argon atmosphere at a heating/cooling rate of 10 °C /min. Nanoindentation tests were
performed at least five tests for each phase using a G200 Nano Indenter system. During each indentation
test, the depth was increased from 0 nm to 100 nm over a period of 20 s, and kept constant for 10 s. The tip
contact did not extend beyond each phase. The microstructures were characterized by a field emission
scanning electron microscope (Carl Zeiss Supra55) equipped with an electron backscatter diffraction
(EBSD) detector (Oxford Instrument), electron probe microanalyzer (EPMA, SHIMADZU, 8050G) and
transmission electron microscope (TEM) (FEI Tecnai G2 F20 operated at the voltage of 200 kV). The
chemical characterizations of different phases were conducted using an energy dispersive spectroscopy
(EDS) system attached to a transmission electron microscope (TEM). The EBSD and EPMA specimens