Preparation of High-Entropy (Zr0.25Hf0.25Ta0.25Ti0.25)C-Ni-Co Composite by Spark Plasma Sintering

In the present work, the thermodynamic formation possibility and stability of high-entropy (Zr0.25Hf0.25Ta0.25Ti0.25)C ceramic single phase were theoretically calculated by first principles, and a novel (Zr0.25Hf0.25Ta0.25Ti0.25)C high-entropy ceramic-Ni-Co composite was successfully prepared by spark plasma sintering (SPS). The first principles calculation indicated the atomic size difference delta for (Zr0.25Hf0.25Ta0.25Ti0.25)C was 3.23 pct, which was < 6.6 pct, and solid solution would possibly happen. The calculated DFT formation enthalpy of the (Zr0.25Hf0.25Ta0.25Ti0.25)C system was - 1.78 eV. The negative formation enthalpy indicated that (Zr0.25Hf0.25Ta0.25Ti0.25)C was thermodynamically stable. The experimental results showed a new fcc (Fm-3m) single rock salt crystal structure ceramic phase was formed after SPS sintering. Two phases (ceramic phase and metal phase) were presented in the material. Besides, for the (Zr0.25Hf0.25Ta0.25Ti0.25)C phase, the Hf, Ta, Zr and Ti distributed uniformly, and no secondary phase was detected. The metal phase was homogeneous along the ceramic phase. No pores and gaps existed between the ceramic phase and metal phase. Good bonding was obtained, caused by some Hf, Ta, Zr or Ti atoms diffusing in the metal phase. The fracture toughness and hardness of the high-entropy (Zr0.25Hf0.25Ta0.25Ti0.25)C ceramic-metal composite were 14.2 MPa m(1/2)and 1620 HV(30). The excellent comprehensive performance was caused by good bonding between the ceramic phase and metal phase and mass disorder in the (Zr0.25Hf0.25Ta0.25Ti0.25)C phase.