Dual role of TiC in solid-solution evolution and high-temperature oxidation resistance of (NbTaMoW)C

TiC addition promoted single-phase formation and simultaneously improved oxidation resistance of (NbTaMoW)C. • TiC makes Ti-Mo-containing subsystems more favorable than Mo-containing ones and lowered mixing Gibbs free energy of HECs. • Enhanced oxidation resistance results from TiC-altered defect energetics and the formation of a denser oxide layer. Understanding component effects is important for the rational design of high-entropy carbides (HECs). This work investigates the effects of TiC addition on solid solution formation and high-temperature oxidation resistance of (NbTaMoW)C through experiments and theoretical calculations. The solid-solution onset and single-phase formation temperatures of the Ti-containing HEC were 200 °C and 50 °C lower than those of the (NbTaMoW)C, respectively. Theoretical calculations show that TiC addition makes Ti-Mo-containing subsystems more energetically favorable than the Mo-containing ones in the Ti-free system, and it lowers the mixing Gibbs free energy of the final HEC, thermodynamically favoring solid-solution formation. Oxidation at 800–1000 °C showed that (TiNbTaMoW)C exhibited improved oxidation resistance, with lower mass gain, smaller oxidation rate constants, and a denser oxide layer. Defect-related calculations further show that TiC increases carbon-vacancy formation energy, stabilizes oxygen at vacancy sites, and raises the energetic penalty for interstitial oxygen incorporation, thereby providing guidance for rational compositional design of HECs.