  Phase selection in Ti-Zr-Hf-Al high-entropy alloys was investigated by
in-situ high-energy X$-$ray diffraction, single-crystal X$-$ray diffraction,
and density-functional theory based electronic-structure methods that address
disorder and vacancies, predicting formation enthalpy and chemical short-range
order (SRO). Samples with varying Al content were synthesized, characterized,
and computationally assessed to ascertain the composition-dependent phase
selection, as increased Al content often acts as a stabilizer of a
body-centered-cubic structure. Equiatomic TiZrHfAl was especially interesting
due to its observed bcc superstructure - a variant of $\gamma-$brass with 4
vacancies per cell (not 2 as in $\gamma-$brass). We highlight how vacancy
ordering mediates selection of this variant of $\gamma-$brass, which is driven
by vacancy-atom SRO that dramatically suppress all atomic SRO. As vacancies are
inherent in processing refractory systems, we expect that similar discoveries
await in other high entropy alloys or in revisiting older experimental data.
