Figure 10 The SEM of (a) AlCoCrFeNiCu (b) AlTiCrFeCoNi HEA
The optical micrographs of the cross-sectional view of the HEAs specimen
is shown in Figure 11 and both alloys show dendritic structures. The
microstructure of AlCoCrFeNiCu HEA showed columnar dendritic structure
with homogenous white and dark contrast that was observed to be solid
solution dendritic and interdendritic. The speed of solidification
increases as the temperature gradient decreases during laser coating.
The increment in the velocity of solidification occurs from the bottom
to the top of the melt pool causing changes in the morphology of the
alloys [79, 80]. The bonding energy of Cu with other elements like
Co, Fe, Ni and Cr have been reported to be very small but, the mixing
enthalpy with these elements is very large [82]. During rapid
solidification, the possibility that Cu is displaced to the
interdendritic region is very high since the metal also has a relatively
low melting point; nucleation sites from the melt before eutectic
reactions occur in the interdendritic spaces could become an easy spot
for defects and increases the alloy’s possibility to fail at elevated
temperatures [83, 84]. The morphologies in Figure 11 (a) and (b)
showed crack free dendritic structures attributed to the pile-up of
solutes before solidification during deposition [85]. Fine equiaxed
grains observed in AlTiCrFeCoNi HEA are known to help limit
solidification cracking and improve mechanical properties. This is
attributed to the presence of the low melting point segregates that are
distributed over a large grain boundary area; and these grains allow
more transport of liquid between the grains [86, 87]. Nucleation
sites for the L12 (\(\gamma^{{}^{\prime}})\) phase were observed
in the scanning electron microscope for the AlCoCrFeNiCu HEA and is
attributed to copper clusters found in the FCC matrix and its
precipitation strengthening effect; thus, enhancing the hardness
property of the alloy at elevated temperatures [88, 89]. The
interdendritic spaces and morphology of the L12 phase
are both present in the alloy due to the nucleation from the melt before
the eutectic reaction. Lu et al. [90] also reported that high
entropy alloy AlCoCrFeNix recorded strengthening because
of the FCC/L12 phase observed.