Vacancy-mediated Phase Selection in High-Entropy Alloys*

We highlight the importance of vacancy-mediated stability in refractory high-entropy
alloys (HEAs), where even small amounts of vacancies suppress chemical short-range
order (SRO) and drive vacancy ordering. As an example, phase selection in Ti-Zr-Hf-Al
alloys was investigated by in-situ high-energy X-ray diffraction, single-crystal X-ray
diffraction, and density-functional-theory electronic-structure methods that address
chemical disorder, including vacancies, predicting formation enthalpy, structure,
density, and chemical SRO. Equiatomic TiZrHfAl was especially interesting due to its observed bcc
superstructure, a variant of gamma-brass with 4 vacancies per cell. We highlight how
vacancies mediates selection of this variant, driven by vacancy-atom SRO that
dramatically suppress all atomic SRO. As vacancies are inherent in processing refractory
systems, similar discoveries may await in other high entropy alloys or in revisiting older
experimental data.