Fe₂₀Mo₂₀Ni₂₀Rh₂₀Ru₂₀ and Fe₁₄Mo₃₅Ni₁₅Rh₁₅Ru₂₁ Ultrahigh-Mixing-Entropy Alloys with Single Hexagonal Close-Packed Structure

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Mo₃₅Ni₁₅Rh₁₅Ru₃₅, Fe₁₄Mo₃₅Ni₁₅Rh₁₅Ru₂₁, Mo₂₅Ni₂₅Rh₂₅Ru₂₅, and Fe₂₀Mo₂₀Ni₂₀Rh₂₀Ru₂₀ (at%) alloys were designed by referring to hexagonal close-packed (hcp) Nb–Mo–Ru–Rh–Pd high-entropy alloys (HEAs) reported by Liu et al., with the help of Pearson’s Crystal Data. X-ray diffraction profiles of the Fe₂₀Mo₂₀Ni₂₀Rh₂₀Ru₂₀ and Fe₁₄Mo₃₅Ni₁₅Rh₁₅Ru₂₁ alloys prepared via the conventional arc-melting and subsequent annealing at 1700 K for 1 h show an hcp structure. Further scanning electron microscopy observations combined with elemental mapping via energy-dispersive X-ray spectroscopy confirmed the single hcp structure. The Fe₂₀Mo₂₀Ni₂₀Rh₂₀Ru₂₀ HEA annealed at 1700 K for 1 h exhibited a mixing entropy (S_mix) normalized by the gas constant (R) of 1.846, 14% higher than the configuration entropy (S_config) normalized by R (S_config/R = ln 5). This study reveals two new ultrahigh-mixing-entropy alloys (UHMixEAs) that satisfy S_mix > S_config, the Fe₂₀Mo₂₀Ni₂₀Rh₂₀Ru₂₀ and Fe₁₄Mo₃₅Ni₁₅Rh₁₅Ru₂₁ alloys. The evaluation of S_mix/S_config for the present UHMixEAs and referential Co-containing HEAs from early studies revealed that S_mix/S_config of the former are constant whereas those of the latter increase at the magnetic transition (Curie) temperature or below.

Mixing entropy (S_mix) normalized by the configuration entropy (S_config) plotted against the absolute temperature (T) for the Fe₂₀Mo₂₀Ni₂₀Rh₂₀Ru₂₀ ultrahigh-mixing-entropy alloy (UHMixEA) and other referential UHMixEAs and high-entropy alloys. The thick and thin curves represent stable and metastable states, respectively.