Entropic stabilization and descriptors of structural transformation in high entropy alloys

With first-principles theoretical analysis of the local structure using Bond Orientational Order parameters and Voronoi partitioning, we establish (a) HCP$\rightarrow$BCC structural transformation in high-entropy alloys (HEAs) Nb$_x$(HfZrTi)$_y$ at 16% Nb-concentration, and (b) that the internal lattice distortions (ILDs) peak at the transition. We demonstrate that the relative stability of HCP and BCC structures is driven by energetics, while the overall stability is achieved with contribution from the vibrational entropy that exceeds the configurational entropy of mixing. We show that along with atomic size mismatch, low average number ($< $5) of valence electrons and disparity in the crystal structures of constituent elements are responsible for larger ILDs in Nb$_x$(HfZrTi)$_y$ than in HEAs like Nb$_a$Mo$_b$W$_c$Ta$_d$.