Os-Free Fe₁₂Ir₂₀Re₂₀Rh₂₀Ru₂₈ High-Entropy Alloy with Single hcp Structure Including Fe from Late Transition Metals

Abstract

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 (∼Ir₂₀Os₂₀Re₂₀Rh₂₀Ru₂₀) achieved via a chemical reaction. The aim of the present study is to exclude Os from the prototypical HEA (the quinary exact equiatomic Ir₂₀Os₂₀Re₂₀Rh₂₀Ru₂₀ 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 Fe_0.6Ru_0.4 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.

SEM and EDX images and XRD profiles of samples annealed at 2273 K for 1 h. (top) Fe₁₂Ir₂₀Re₂₀Rh₂₀Ru₂₈ and (bottom) Ir₂₅Re₂₅Rh₂₅Ru₂₅ alloys that formed single and comboned hcp structures, respectively.