Material design for TiZrHfNbTaBx: A boundary material of refractory high-entropy alloys and ceramics

The refractory high-entropy ceramics, TiZrHfNbTaBx (x= 0, 0.1, 1, 10), which vary from high-entropy alloys to high-entropy ceramics depending on the B-content, were fabricated using the arc-melting method. TiZrHfNbTa (x = 0), TiZrHfNbTaBx (x = 0-1), and (TiZrHfNbTa)B-2 (x =10) showed BCC, BCC with MB (M = Ti, Zr, Hf, Nb, Ta), and MB2 structures, respectively. The Vickers hardness and Young's modulus of these materials increased with an increasing B-content because these ceramic properties are superior to those of metals. The weight gain during oxidation, measured using thermogravimetric analysis, revealed that the highest and lowest weight gains were observed for TiZrHfNbTaB and (TiZrHfNbTa)B-2, respectively. Their weight gain depends on their crystal structures rather than their B-contents.