Yutato Arai1,Manami Saito1,Yasuo Kogo1
Tokyo University of Science1
Yutato Arai1,Manami Saito1,Yasuo Kogo1
Tokyo University of Science1
Refractory high entropy alloys and ceramics are considered as candidates for advanced aerospace heat resistant materials such as hypersonic vehicle cruised at Mach 5 or higher and heated at above 2000<sup>o</sup>C or higher during operation. Recently, the materials design for refractory high entropy alloys and high entropy ceramics have been proposed. These are combinations of refractory metals (Ti, Zr, Hf, Cr, Nb, Ta, V, Mo, W, etc.) or transition metal diborides, carbides and nitrides, which is well known as ultra-high temperature ceramics (UHTCs). However, designs for these materials are independent and design guidelines for boundary area of high entropy alloys and high entropy ceramics have not been developed.<br/>In the present study, we focused on TiZrHfNbTaB<sub>x</sub>, which is considered as a refractory high entropy material owing to their high melting temperature (>2000<sup>o</sup>C) and becomes high entropy alloy and high entropy ceramics depending on the content of B (x).<br/>The objective of this study is the design of microstructure of TiZrHfNbTaB<sub>x</sub> using calculation thermodynamics and the evaluation of the relationship between the degradation behavior of TiZrHfNbTaBx in air, microstructures, and mixing entropy.<br/>As raw materials, chunks for B, Ti, Zr, Hf, Nb and Ta (purity: 99at%) was used. TiZrHfNbTaBx (x = 0-1 and 10) was fabricated by arc-melting method. Thermodynamic calculation software (FactSage 8.1, CRCT: Centre for Research in Computational Thermochemistry, Canada) was used to estimate equilibrium phase of TiZrHfNbTaBx. The weight gain by oxidation was evaluated by using thermo gravimetric analysis (TGA) up to 1200<sup>o</sup>C in air.<br/>Calculation phase diagram (CALPHAD) reveals that TiZrHfNbTaB<sub>x</sub> (x = 0-1) is composed of metal phase with BCC structure and MB phases (M = Ti, Zr, Hf, Nb, Ta) with orthorhombic structures. For x = 10 ((Ti<sub>0.2</sub>Zr<sub>0.2</sub>Hf<sub>0.2</sub>Nb<sub>0.2</sub>Ta<sub>0.2</sub>)B<sub>2</sub>), MB<sub>2</sub> structure, which is a general structure for UHTCs, is estimated as the equibilium phase.<br/>TiZrHfNbTaB<sub><sup>x</sup></sub> (x = 0-1) fabricated by arc-melting reveals that TiZrHfNbTa phase with BCC structures and TiZrHfNbTaB (MB phase) are formed and the amount of TiZrHfNbTaB phase increases with the increase of x. For x = 10, the uniform (Ti<sub>0.2</sub>Zr<sub>0.2</sub>Hf<sub>0.2</sub>Nb<sub>0.2</sub>Ta<sub>0.2</sub>)B<sub>2</sub> with MB<sub>2</sub> structure is formed. These results are good agreement with the estimation by CALPHAD. Contrary, the observation by SEM with EDX analysis shows that the content of Nb and Ta is higher than that of Ti, Zr and Hf in MB phase, and vice versa in metal phase.<br/>The weight gain begins at ~700<sup>o</sup>C for TiZrHfNbTaB and ~850<sup>o</sup>C for TiZrHfNbTa and TiZrHfNbTaB<sub>0.1</sub>. It after exposed up to 1200<sup>o</sup>C for TiZrHfNbTa, TiZrHfNbTaB<sub>0.1</sub>, TiZrHfNbTaB and (Ti<sub>0.2</sub>Zr<sub>0.2</sub>Hf<sub>0.2</sub>Nb<sub>0.2</sub>Ta<sub>0.2</sub>)B<sub>2</sub> is 12, 10, 13% and 3%, respectively. The lowest weight gain of TiZrHfNbTaB<sub>x</sub> is observed for (Ti<sub>0.2</sub>Zr<sub>0.2</sub>Hf<sub>0.2</sub>Nb<sub>0.2</sub>Ta<sub>0.2</sub>)B<sub>2</sub> despite it has the lowest mixing entropy among TiZrHfNbTaB<sub>x</sub> fabricated in the present study. These results indicate that the crystal structure and microstructure strongly affect the oxidation behavior of TiZrHfNbTaB<sub>x</sub>. The relationship between mixing entropy, crystal structure, microstructure and the degradation by oxidation will be discussed in the presentation.