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M o 3 5 N i 1 5 R h 1 5 R u 3 5 , F e 1 4 M o 3 5 N i 1 5 R h1 5 R u 2 1 , M o2 5N i2 5R h2 5R u2 5, and F e 2 0 M o 2 0 N i 2 0 R h 2 0 R u 2 0 ( a t. % ) alloys were designed b y referring t o hexagonal close-packed ( h c p ) N b - M o - R u - R h - P d high-entropy alloys ( H E A s ) reported b y Liu e t a l., with the help o f Pearson ' s Crystal Data. X-ray diffraction profiles o f the F e 2 0 M o 2 0 N i 2 0Rh2 0 R u 2 0 and F e 1 4 M o 3 5 N i 1 5 R h 1 5 R u 2 1 alloys prepared via the conventional arc-melting and subsequent annealing a t 1700 K for 1 h show a n h c p structure. Further scanning electron microscopy observations combined with elemental mapping via energy-dispersive X-ray spectroscopy confirmed the single h c p structure. The F e 2 0 M o 2 0 N i 2 0 R h 2 0 R u 2 0 H E A annealed a t 1700 K for 1 h exhibited a mixing entropy (Sm i x) normalized b y the gas constant (R ) o f 1.846, 14% higher than the configuration entropy (Sc o n f i g) normalized b y R (Sc o n f i g/R = l n 5 ) . This study reveals two new ultrahigh-mixing-entropy alloys ( U H M i x E A s ) that satisfy Sm i x > Sc o n f i g, the F e 2 0 M o 2 0 N i 2 0 R h 2 0 R u 2 0 and F e 1 4 M o 3 5 N i 1 5 R h 1 5 R u 2 1 alloys. The evaluation o f Sm i x/Sc o n f i g for the present U H M i x E A s and referential Co-containing H E A s from early studies revealed that Sm i x/Sc o n f i g of the former are constant whereas those o f the latter increase a t the magnetic transition (Curie) temperature o r below.

Os-free alloys with compositions of Fe12Ir20Re20Rh20Ru28 and Ir25Re25Rh25Ru25 (at%) from an Ir-Re-Rh-Ru system with and without Fe are prepared via conventional arc-melting and subsequent annealing to examine their formation into a single hexagonal close-packed (hcp) structure as high-entropy alloys (HEAs). These alloys are derived by referring to Yunseko et al., who reported the formation of HEAs with a single hcp structure in a near-equiatomic composition (similar to Ir20Re20Rh20Ru20) achieved via a chemical reaction. The aim of the present study is to exclude Os from the prototypical HEA (the quinary exact equiatomic Ir20Re20Rh20Ru20 alloy) to prevent hazardous osmium tetroxide (OsO4) from volatilizing in a bulk sample. Fe12Ir20Re20Rh20Ru28 alloy is set as the target alloy by replacing Os with Fe0.6Ru0.4 in the prototypical HEA. The X-ray diffraction (XRD) profile of the Fe12Ir20Re20Rh20Ru28 alloy annealed at 2273 K for 1 h shows an hcp structure, and further scanning electron microscopy (SEM) observations combined with elemental mapping via energy dispersive X-ray spectroscopy (EDX) confirmed a single hcp structure. The XRD profiles of the other samples (the Fe12Ir20Re20Rh20Ru28 alloy in both as-prepared and annealed at 2000 K for 1 h states, and the Ir25Re25Rh25Ru25 alloy in both as-prepared and annealed at 2273 K for 1 h states) exhibit a combination of hcp structures, based on XRD, SEM, and EDX observations. The Fe12Ir20Re20Rh20Ru28 HEA yielded by the conventional solidification method reflects a significant development, as it is an Os-free alloy and the first single-phase hcp-HEA that includes an element from the 3d late transition metal.