Entropy stabilized single-phase (Hf,Nb,Ta,Ti,Zr)B2 solid solution powders obtained via carbo/boro-thermal reduction

Abstract This report deals with the synthesis of entropy stabilized (ES) single-phase (Hf,Nb,Ta,Ti,Zr)B2 powders, with specific surface area of about 1.6 m2/g, in a AlB2-type structure using individual transition metal (TM) oxides and, for the first time, elemental boron and carbon as precursors to feed the carbo/boro-thermal (CBT) reduction. Elemental B and C were intimately mixed into a mixture of five TM oxide powders, TM = Hf, Nb, Ta, Ti and Zr through an high energy planetary milling. ES single-phase TM diboride solid solution powders were obtained by a synthesis process consisting of a CBT reduction followed by solid solution formation. A B:C molar ratio = 1.27 (per 1 M mass of metals) was adjusted leading the CBT reduction to completion (i.e., full conversion of TM oxides), applying the synthesis temperature of 2123 K under vacuum. The micro-strain intended as deviation of some inter-planar distances of the entropy stabilized AlB2-type solid solution lattice, was analyzed by x-ray diffraction: a strong anisotropic micro-strain was found, and was attributed to the compositional disorder due to the coexistence of differing TM with different atomic radius.