First-Principle Study on the Stability of Lightly Doped (Nb1-x Ti x )C Complex Carbides and Their Verification in 1045 Steel.

The mixing Gibbs free energy and formation enthalpy difference of different Ti-doped (Nb1-x Ti x )C complex carbides were calculated using the Cambridge Serials Total Energy Package (CASTEP) module of Materials Studio 2019 software. The calculation results predict that (Nb1-x Ti x )C complex carbides have higher stability than pure NbC and TiC. Therefore, three lightly Ti-doped (Nb1-x Ti x )C complex carbides with theoretical densities close to that of the 1045 steel were designed for calculations. The calculation results show that the formation energy of (Nb1-x Ti x )C complex carbides decreases with an increase in the Ti content. These designed (Nb1-x Ti x )C complex carbides have mechanical stability, and their bulk modulus, shear modulus, Young's modulus, and hardness are all lower than those of pure NbC. The electronic performance results show that these three structures show good conductivity, and the 3d orbitals of Ti atoms and the 4d orbitals of Nb atoms are strongly hybridized with the 2p orbitals of C atoms. The Nb-C and Ti-C bonds exhibit strong covalent bonds. To verify the stability of the (Nb1-x Ti x )C complex carbides, the prepared (Nb0.8Ti0.2)C complex carbide was added to the 1045 steel as a refiner. After observing under a transmission electron microscope (TEM), we found that the (Nb0.8Ti0.2)C complex carbide could exist stably as a face-centered cubic structure, which provided a method for the design and synthesis of complex carbides used for refiners.