Fe<sub>20</sub>Mo<sub>20</sub>Ni<sub>20</sub>Rh<sub>20</sub>Ru<sub>20</sub> and Fe<sub>14</sub>Mo<sub>35</sub>Ni<sub>15</sub>Rh<sub>15</sub>Ru<sub>21</sub> Ultrahigh-Mixing-Entropy Alloys with Single Hexagonal Close-Packed Structure

Abstract

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 (Smix) normalized by the gas constant (R) of 1.846, 14% higher than the configuration entropy (Sconfig) normalized by R (Sconfig/R = ln5). This study reveals two new ultrahigh-mixing-entropy alloys (UHMixEAs) that satisfy Smix > Sconfig, the Fe20Mo20Ni20Rh20Ru20 and Fe14Mo35Ni15Rh15Ru21 alloys. The evaluation of Smix/Sconfig for the present UHMixEAs and referential Co-containing HEAs from early studies revealed that Smix/Sconfig of the former are constant whereas those of the latter increase at the magnetic transition (Curie) temperature or below.