Assessing the effect of composition on the structural, mechanical and thermal stability of high entropy ceramic...

Refractory metal based High Entropy Ceramic Carbides (HEC) are promising materials for extreme environmental applications such as energy, tooling, aerospace and defense owing to their superior thermal and mechanical stability. Present work uses density functional theory calculations to identify optimum compositions of a new class of HECs comprised of carbides of refractory metals such as Ti, Nb, Hf, Ta, and W exhibiting superior structural, mechanical and thermal stability. Total 13 compositions were evaluated by varying the atomic fractions of Ti-Hf, Ti-Nb, Ti-Ta, and Ti-W while keeping the other three metals at a fixed atomic fraction of 0.2. Compared to the pure metal carbides, the HEC compositions displayed excellent elastic modulus and hardness, but relatively low melting points. Due to the dominance of entropy of mixing over enthalpy of mixing, along with favorable thermodynamic and structural stability parameters, all studied HEC compositions exhibited a tendency to form stable two/multi-phase solid solutions rather than intermetallic phases or phase separation. The mechanical and thermal stability of HECs showed a strong correlation with the nature and strength of metal-carbon covalent bond. For instance, the Nb rich Ti_0.1Nb_0.3W_0.2Hf_0.2Ta_0.2C showed highest hardness of 35.59 GPa attributing to the presence of strong and localized covalent bonding. Further, the Ta rich Ti_0.05Ta_0.35Nb_0.2Hf_0.2W_0.2C showed highest melting point (3495 K) and the Hf rich Ti_0.1Hf_0.3Nb_0.2W_0.2Ta_0.2C showed highest fracture toughness of 4.03 MPa.m^1/2. Overall, this study highlights the importance of evaluating composition-property relationships to identify optimum HEC compositions for a target application.