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].