Effects of mixing enthalpy and cooling rate on phase formation of Al_xCoCrCuFeNi high-entropy alloys

We investigated the influence of mixing enthalpy and cooling rate on phase formation and selected the melt-spun Al_xCoCrCuFeNi (x = 0, 0.1, 0.5, 0.8 and 1.0 in molar ratios) high-entropy alloys (HEAs) as a model system. The mean mixing enthalpy (ΔH_mix) of the alloy system is tuned from positive to negative by increasing Al molar ratios while the mixing entropy (ΔS_mix) only has an insignificant variation. Microstructure analyses revealed that the dominant phase in the Al_xCoCrCuFeNi HEAs changes from a face-centered cubic (FCC) to body-centered cubic (BCC) structure with the variation of the ΔH_mix. Accompanying with phase changing, the lattice constant of the FCC phase increases linearly with Al molar ratio regardless of cooling rate, indicating that the lattice expansion caused by the substitutional alloying of Al plays an important role in the phase evolution, in addition to the effect of the ΔH_mix. The increasingly negative enthalpy ΔH_mix with Al addition also leads to more pronounced phase separation with the formation of ordered intermetallic phases in the BCC-dominant HEAs than the FCC-dominant ones. Interestingly, when the magnitude of the mean ΔH_mix is small, both the coarsening of Cu-rich nanophase and decomposition of the solid-solution phase in the Al_0.5CoCrCuFeNi HEA are suppressed. This observation is in line with thermodynamic predictions that a weak ΔH_mix benefits the stabilization of the solid-solution phase.