Nanostructure and local polymorphism in “ideal-like” rare-earths-based high-entropy alloys

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Rare-earths-based hexagonal high-entropy alloys (HEAs) composed of the elements from the heavy half of the lanthanide series (from Gd to Lu, with the exception of Yb) and yttrium are much closer to an ideal solid solution than HEAs composed of other elements from the entire periodic system. Using the method of high-frequency levitation melting, three candidates for a physical realization of an ideal HEA were synthesized, an Y-Gd-Tb-Dy-Ho, a Gd-Tb-Dy-Ho-Lu and a Tb-Dy-Ho-Er-Tm, and a study of their structure and composition was performed to see how close to ideal HEA samples can be prepared. We found that all three HEAs exhibit a nanostructure of a hexagonal close-packed (hcp) matrix and rod-like cubic close-packed (ccp) precipitates of the lengths 200–600 nm and widths 50–100 nm. EDS analysis has revealed a general trend that the precipitates are slightly enriched in the elements with larger atomic radii relative to the matrix. The origin of the nanostructure that represents a local hcp ↔ ccp polymorphism at zero external pressure appear to be lattice distortions (equivalent to a chemical pressure), occurring due to the minute differences of the elements' atomic radii. The volume per atom is slightly larger in the ccp precipitates that are enriched in larger atoms, so that the lattice distortions can be better accommodated and minimized, which reduces the lattice strain energy that contributes to the mixing enthalpy ΔHmix ≠ 0. The employed synthesis route, which is standard for the preparation of alloys of high structural quality, did not lead to a physical realization of an ideal HEA in the most promising theoretical candidates.