Effect of Ti addition on the structural, thermodynamic, and elastic properties of Ti$_{x}$(HfNbTaZr)$_{(1-x)/4}$ alloys

Abstract

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.