Phase decomposition can effectively enhance the mechanical properties of carbide ceramics and can overcome the difficulty of enhancing the mechanical properties of single-phase multicomponent carbide ceramics. In this work, a series of nonstoichiometric (TiZrVNb)C_x ceramics were prepared by spark plasma sintering (SPS) at different temperatures. The effects of the carbon content on the phase composition, microstructure evolution, and mechanical properties were investigated in detail. Phase decomposition occurred with decreasing carbon content. Two different solid solutions of (Ti,V)-rich and Zr-rich phases formed from the decomposition of equimolar single-phase solid solutions, namely, the Zr-poor phase and Zr-rich phase, respectively. The distribution of Nb element is relatively uniform. The semicoherent interfaces between the Zr-poor phase and the Zr-rich phase can harden and strengthen effectively under the synergistic effect of grain refinement. Ceramics with phase decomposition structures have apparent advantages compared to single-phase high-entropy carbides. This work provides an important train of thought for the microstructure tailoring and properties optimization of multi-component carbide ceramics.

To address the relatively mediocre mechanical properties of single-phase multi-component carbide ceramics, a phase transition from a single phase to multiple phases was proposed to achieve superior mechanical properties. A series of (TiZrV_xNb)C_0.8 ceramics with different V contents were fabricated by spark plasma sintering (SPS). The influence of the V content on the phase composition, microstructural evolution, and mechanical properties was investigated in detail. The transition behavior from a single phase to multiple phases is discovered and discussed. The formation of the Zr-rich phase and Zr-poor phase can be attributed to the increase in lattice distortion and mixed enthalpy caused by the addition of V. A nanometer lamellar structure with a semi-coherent interface obtained via in situ decomposition is reported for the first time in multi-component carbide ceramics. The semi-coherent interfaces with high dislocation density and strain concentration effectively improve the mechanical properties, grain refinement, and multi-phase formation. The optimal comprehensive mechanical properties of the Vickers hardness (26.3 GPa), flexural strength (369 MPa), and fracture toughness (3.1 MPa·m^1/2) were achieved for the sample with 20 mol% V.

The interest in refractory materials is increasing rapidly in recent decades due to the development of hypersonic vehicles. However, the substance that has the highest melting point (T_m) keeps a secret, since precise measurements in extreme conditions are overwhelmingly difficult. In the present work, an accurate deep potential (DP) model of a Hf–Ta–C–N system was first trained, and then applied to search for the highest melting point material by molecular dynamics (MD) simulation and Bayesian global optimization (BGO). The predicted melting points agree well with the experiments and confirm that carbon site vacancies can enhance the melting point of rock-salt-structure carbides. The solid solution with N is verified as another new and more effective melting point enhancing approach for HfC, while a conventional routing of the solid solution with Ta (e.g., HfTa₄C₅) is not suggested to result in a maximum melting point. The highest melting point (~4236 K) is achieved with the composition of HfC_0.638N_0.271, which is ~80 K higher than the highest value in a Hf–C binary system. Dominating mechanism of the N addition is believed to be unstable C–N and N–N bonds in liquid phase, which reduces liquid phase entropy and renders the liquid phase less stable. The improved melting point and less gas generation during oxidation by the addition of N provide a new routing to modify thermal protection materials for the hypersonic vehicles.

Transition metal carbides are promising candidates for thermal protection materials due to their high melting points and excellent mechanical properties. However, the relatively high thermal conductivity is still a major obstacle to its application in an ultra-high-temperature insulation system. In this work, the low thermal conductivity of dense (TiZrHfVNbTa)C_x (x = 0.6–1) high-entropy carbides has been realized by adjusting the carbon stoichiometry. The thermal conductivity gradually decreases from 10.6 W·m⁻¹·K⁻¹ at room temperature to 6.4 W·m⁻¹·K⁻¹ with carbon vacancies increasing. Due to enhanced scattering of phonons and electrons by the carbon vacancies, nearly full-dense (97.9%) (TiZrHfVNbTa)C_0.6 possesses low thermal conductivity of 6.4 W·m⁻¹·K⁻¹, thermal diffusivity of 2.3 mm²·s⁻¹, as well as electrical resistivity of 165.5 μΩ·cm. The thermal conductivity of (TiZrHfVNbTa)C_0.6 is lower than that of other quaternary and quinary high-entropy carbide ceramics, even if taking the difference of porosity into account in some cases, which is mainly attributed to compositional complexity and carbon vacancies. This provides a promising route to reduce the thermal conductivity of high-entropy carbides by increasing the number of metallic elements and carbon vacancies.