  The structure and thermodynamic properties of Ti$_x$(HfNbTaZr)$_{(1-x)/4}$
from Refractory High Entropy multicomponent Alloys to pure titanium are
investigated employing comprehensive MCSQS realizations of the disordered
atomic structure and DFT calculations. We showed that to model the random
structure in a limited supercell, it is necessary to probe a large space of
random configurations with respect to the nearest neighbor's shells. Mimicking
the randomness with the many-body terms does not lead to significant
improvements in the mixing energy, but modeling the random structure with the
few nearest neighbor pairs leads to improvements in the mixing energy.
Furthermore, we demonstrated the existence of weak to medium SRO for the two
equimolar compositions. Chemical ordering is investigated by associating a
large number of MCSQS realizations to DFT energy calculations, and SRO results
are rationalized in terms of the crystallographic structure of the pairs of
elements and binary phase diagrams. When Ti is added to
Ti$_x$(HfNbTaZr)$_{(1-x)/4}$ alloys, the mixing energy remains slightly
positive for all $x$. For $x$ > 0.4, a phase transition in favor of an hcp
structure is observed in agreement with the predictions of the Bo-Md diagram.
At $x$ = 0.5, a dual phase is predicted. Ti content in this class of alloys
could be a practical way to select phase structure and tailor the elastic
properties to specific applications.
