Finite temperature thermal expansion and elastic properties of (Hf _1-x Ta _x )C ultrahigh temperature ceramics

HfC, TaC, and their solid solution ceramics have been identified as the best materials for ultrahigh-temperature ceramics. However, their temperature-dependent properties, such as their thermal expansion and elastic properties, are mostly unknown. Here, we investigated the thermal expansion and temperature-dependent elastic properties of (Hf _1-x Ta _x )C solid solutions over the entire composition range by first principles calculations. Ta-rich solid solutions showed higher elastic properties and metallicity, and these two opposing properties yielded unique thermal expansion behavior with increasing Ta content. The dynamic instability of TaC above its critical volume (23.84 Å ³ /mol) made it difficult to determine its temperature-dependent elastic properties at ultrahigh temperatures (>1800 K) using the harmonic phonon calculation under the quasi-harmonic approximation. In terms of elastic properties, TaC and Ta-rich solid solutions are the best choice for high-temperature applications; however, the optimal compositions should be determined considering the poor oxidation resistance of the Ta-rich phases. Our results present the important intrinsic properties of (Hf _1-x Ta _x )C solid solutions as well as those of HfC and TaC.