High-entropy materials have attracted considerable interest due to their unique, improved properties and large configurational entropy. Out of these, high-entropy ceramics (HECs) are of particular interest since the independent solubility of cations and anions results in increased configurational entropy. However, most HEC research considers only a single element occupying the anion sublattice, which limits the maximum attainable configurational entropy. Here, we expand our previous work on high-entropy borocarbides where both boron and carbon occupy the anion sublattice. By applying an ab initio based screening procedure, we identify six elements Li, Ti, V, Zr, Nb and Hf suitable for forming high-entropy borocarbides. With these elements, we propose six novel HEC compositions, and by computing their entropy forming ability, we identify that three are likely to form single-phase during synthesis. Material properties and lattice distortions for all proposed compositions are studied using density functional theory calculations with special quasirandom structures. The directional lattice distortions, a concept we introduce in this work, show that lattice distortions have an elemental and directional preference for certain HEC compositions. We also show that the novel inclusion of Li improves the mechanical properties of the proposed HECs, the details of which are studied using the electron localization function.

Transition metal borides have a unique combination of high melting point and high chemical stability and are suitable for high temperature applications (>2000 °C). A metastable dual-phase boride (Ti_0.25V_0.25Zr_0.25Hf_0.25)B₂ with distinct two hexagonal phases and with an intermediate entropy formation ability of 87.9 (eV/atom)⁻¹ as calculated via the density functional theory (DFT) was consolidated by pulsed current sintering. Thermal annealing of the sintered dual-phase boride at 1500 °C promoted the diffusion of metallic elements between the two boride phases leading to chemical homogenization and resulted in the stabilization of a single-phase high-entropy boride. Scanning electron microscopy, in situ high temperature x-ray diffraction, and simultaneous thermal analysis of the as-sintered and annealed high-entropy borides showed the homogenization of a dual-phase to a single-phase. The experimentally obtained single-phase structure was verified by DFT calculations using special quasirandom structures, which were further used for theoretical investigations of lattice distortions and mechanical properties. Experimentally measured mechanical properties of the single-phase boride showed improved mechanical properties with a hardness of 33.2 ± 2.1 GPa, an elastic modulus of 466.0 ± 5.9 GPa, and a fracture toughness of 4.1 ± 0.6 MPa m^1/2.

The processing of multicomponent (Ti_0.25V_0.25Zr_0.25Hf_0.25)B₂ ultra-high temperature hexagonal transition metal diboride in dual-phase and single-phase microstructures and investigation of oxidation behavior in the air at 1000 and 1500 °C are reported. The dual-phase diboride is a metastable phase composed of Hf-Zr-rich and Ti-V-rich phases that undergo phase transformation to a single-phase high-entropy diboride after thermal annealing. At 1000 °C, a B₂O₃ layer was formed on the material's surface, and the oxidation kinetics followed a para-linear behavior. At 1500 °C, a porous oxide layer was formed, facilitating oxygen diffusion and reaction with the diboride, resulting in linear oxidation kinetics. The prediction of the lifetime of the materials during high-temperature oxidation suggested that the high-entropy material outperforms the dual-phase diboride, making it most suitable for related applications. The superior performance of the high-entropy single-phase diboride was associated with the high-entropy and sluggish diffusion effects.

An entropy-stabilized multicomponent ultrahigh-temperature ceramic (UHTC) coating, (Ti0.25V0.25Zr0.25Hf0.25)B2, on a graphite substrate was in-situ sintered by spark plasma sintering (SPS) from constituent transition metal diboride powders. The (Ti0.25V0.25Zr0.25Hf0.25)B2 coating had a hardness of 31.2±2.1 GPa and resisted 36.9 GPa of stress before delamination, as observed at the interface. The temperature-dependent thermal properties of the multicomponent diboride (Ti0.25V0.25Zr0.25Hf0.25)B2 were obtained by molecular dynamics (MD) simulations driven by a machine learning force field (MLFF) trained on density functional theory (DFT) calculations. The thermal conductivity, density, heat capacity, and coefficient of thermal expansion obtained by the MD simulations were used in time-dependent thermal stress finite element model (FEM) simulations. The low thermal conductivity (< 6.52 W∙m−1∙K−1) of the multicomponent diboride coupled with its similar coefficient of thermal expansion to that of graphite indicated that stresses of less than 10 GPa were generated at the interface at high temperatures, and therefore, the coating was mechanically resistant to the thermal stress induced during ablation. Ablation experiments at 2200 °C showed that the multicomponent diboride coating was resistant to thermal stresses with no visible cracking or delamination. The ablation mechanisms were mechanical denudation and evaporation of B2O3 and light V–Ti oxides, which caused a decrease in the mass and thickness of the coating and resulted in mass and linear ablation rates of −0.51 mg·s−1 and −1.38 µm·s−1, respectively, after 60 s. These findings demonstrated the thermal and mechanical stability of multicomponent entropy-stabilized diborides as coatings for carbon materials in engineering components under extreme environments.

Entropy-stabilized Ultra High-Temperature Ceramics (UHTC) offer a groundbreaking solution to the challenges of extreme environments, showcasing enhanced mechanical properties, thermal stability, and resistance to oxidation at high temperatures. The consolidation of UHTC by ultra-fast high-temperature sintering (UHS) significantly reduces processing times and temperature and can produce dense high-performance ceramics with superior mechanical properties. This study reports the pressureless synthesis and consolidation of the entropy-stabilized (Hf0.25Zr0.25Ti0.25V0.25)B2-B4C composite through UHS within 1 minute, starting from transition metal diboride powders. B4C acts as an effective sintering aid, promoting the densification of the system and the formation of a nearly single-phase hexagonal diboride with a diboride-eutectic phase. Furthermore, a secondary minor hexagonal phase rich in V and Zr is formed close to the eutectic regions. Sintering currents of 40 A were necessary to reach densities higher than 90 % under pressureless conditions, achieving nano hardness higher than 27.3 GPa, comparable with high-entropy diborides produced by Spark Plasma Sintering. The study highlights the entropy-stabilized phase formation, diffusion, densification, and grain growth mechanisms involved during UHS. The work contributes to the understanding of entropy-stabilized ceramics produced by UHS as a faster and less energy-consuming process than conventional sintering methods.