  Refractory high-entropy alloys are under consideration for applications where
materials are subjected to high temperatures and levels of radiation, such as
in the fusion power sector. However, at present, their scope is limited because
they are highly brittle at room temperature. One suggested route to mitigate
this issue is by alloying with Ti. In this theoretical study, using a
computationally efficient linear-response theory based on density functional
theory calculations of the electronic structure of the disordered alloys, we
study the nature of atomic short-range order in these multi-component
materials, as well as assessing their overall phase stability. Our analysis
enables direct inference of phase transitions in addition to the extraction of
an atomistic, pairwise model of the internal energy of an alloy suitable for
study via, e.g. Monte Carlo simulations. Once Ti is added into either the
NbMoTaW or VNbMoTaW system, we find that there is competition between chemical
phase ordering and segregation. These results shed light on observed chemical
inhomogeneity in experimental samples, as well as providing fundamental insight
into the physics of these complex systems.
