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.