Figure 6 D.O.E Plot Showing the Effect of Laser Parameters on the Microstructure of AlCoCrFeNiCu and AlTiCrFeCoNi HEAs after stage 1 and 2 of Experiments
Effect of Laser Parameters on the Phase and Microstructural Analysis3.3.1 The Phase Analysis
The analysis of the X-ray diffraction patterns of AlCoCrFeNiCu and AlTiCrFeCoNi HEAs are shown in Figures 7 and 8 at laser powers of 800 W and 1200 W and at varying scanning speeds. The analysis shows a solid solution of BCC and FCC structures due to the elemental compositions of both alloys. Typical diffraction peaks associated with Al-FCC, Co-HCP, Cr-BCC, Fe-BCC, Ni-FCC and Cu- FCC for the AlCoCrFeNiCu HEA, and Al-FCC, Co-HCP, Cr-BCC, Fe-BCC, Ni-FCC and Ti- HCP for AlTiCrFeCoNi HEA were identified in the results [57, 58]. It was observed, with the origin Pro 8 software, that there were significant effects on the laser parameters on the phase structures of the alloys as the peaks positions of each sample were relatively close when compared on the same scale despite the variations in parameter [59, 60]. The changes in compositions of both alloys, that is; Cu in the first alloy and Ti in the second alloy showed different amount of phases in each alloy respectively. Although more BCC and FCC peaks are seen in both alloys attributed to the elements having lower melting points dissolving in the matrix of the elements having a higher melting point also reported by Moravcik, Cizek [61], [62, 63]. The elemental effect of Cobalt and Nickel forming the FCC phase and Copper segregating to the inter dendritic region to form a Cu-rich FCC phase makes the AlTiCrFeCoNi HEA more of a BCC solid solution structure [64, 65]. Moreso, the AlTiCrFeCoNi HEA compared to the AlCoCrFeNiCu HEA had more peaks suggesting either an excessive grain refinement induced by the rapid solidification process, or the lattice distortion effect [61, 66].
For the AlCoCrFeNiCu HEA, the highest peak at 45° showed the BCC phases and most peaks observed were mostly BCC, however, at a high speed of 12 mm/s, the FCC peak is detected. This significant peak occurrence is attributed to the reduction of grain size from the rapid solidification occurring at a high scan speed as higher scanning speed has been reported to lead to grain refinement [37, 67, 68]. This FCC peak also confirms the segregation of Cu to the inter dendrites as the peak consists mostly of Cu [69, 70]. The AlCoCrFeNiCu HEA in Figure 7
has its most dominant diffraction peak at 2 θ= 44.5 and AlTiCrFeCoNi HEA was at 35.174 .These phases consist of Iron-Nickel and Aluminium Nickel respectively with solid solubility at 912 C according to TCFE6 thermodynamic database and Thermo Calc [71]. The intermetallic AlNi3 phase (L12, FCC) is said to be responsible for an increase in the mechanical properties of the AlCoCrFeNiCu alloy [72]. The large Al content in AlCoCrFeNiCu and Al/Ti content in AlTiCrFeCoNi favoured the formation of more BCC phases over FCC phase and is attributed to the large atomic size of the elements [25[73]]. The BCC structure is known to have lower atomic packing density than the FCC resulting in the accommodation of larger solute atoms [74, 75]. The gradual change in lattice parameters means fewer weak phases exist within the layers. The XRD spectrum of the AlTiCrFeCoNi HEA in Figure 8 shows the highest peak consisting of BCC (Cr-Fe) (Fe-Ti) phases with no significant changes with change in parameters. Titanium is observed to have an impact on the composition of the phases due to its large atomic radius difference in contrast to other elements while favouring the formation of more BCC phases over FCC phase [76]. The high entropy effect is responsible for the decrease in Gibbs free energy stabilizing the solid solution in the alloy system [77, 78].