Sintering and High-Temperature Strength of (Ti,Hf,Ta)C Medium-Entropy Ceramics Consolidated by Biphasic Carbide Powders

This study reports for the first time on the preparation of medium-entropy (Ti,Hf,Ta)C-based powders with biphasic composition of Hf(Ti,Ta)C and Ti,Ta(Hf)C based on carbothermal reduction of TiO2, Ta2O5 and HfO2 with graphite. The synthesized (Ti,Hf,Ta)C-based powders had a fine particle size of 200 - 300 nm and low oxygen content of 0.42 wt%. After sintering at 2 100 & DEG;C for 1 h, single-phase (Ti,Hf,Ta)C ceramics were obtained. The effect of the Hf element con-tent on the densification and grain growth of (Ti,Hf,Ta)C medium-entropy ceramics was investigated and compared with monocarbide ceramics (TiC, HfC, TaC). The final sintered medium-entropy (Ti,Hf,Ta)C ceramics prepared by means of hot pressing at 2 100 & DEG;C had fine grains (0.92 & PLUSMN; 0.4 lm) and a relative density of 93.3 %. The Hf element signif-icantly inhibited the densification and grain growth of (Ti,Hf,Ta)C ceramics due to its lattice distortion and the slug-gish diffusion effects. The equimolar ratio (Ti,Hf,Ta)C corresponding to (Ti1/3Hf1/3Ta1/3)C had ultra-high strength at 1 600 & DEG;C (639 & PLUSMN; 38 MPa) and 1 800 & DEG;C (697 & PLUSMN; 26 MPa). The ultra-high strength of (Ti,Hf,Ta)C medium-entropy ceramics is the result of the collaborative optimization of the superfine microstructure (grain size of 0.92 & PLUSMN; 0.4 lm) and strong grain boundary strength.