Ultrafine dual-phase high-entropy carbide-boride ceramics with elemental distribution regulation: Microstructure and synergistic strengthening mechanism

Ultrafine dual-phase high-entropy (Ti,Zr,Hf,Nb,Ta)C–(Ti,Zr,Hf,Nb,Ta)B₂ ceramics were successfully synthesized via in-situ reactive hot pressing at 1800 °C, using nanoscale high-entropy carbide and zirconium boride powders as raw materials. Microstructural analysis revealed inhomogeneous elemental distribution between the two phases. Benefiting from the interdiffusion effect induced by in-situ reactions and the effective suppression of grain coarsening at low-temperature sintering, the material achieved full densification and a submicron-scale microstructure with average grain size of 0.2–0.8 μm, which are significantly refined compared to values reported. Meanwhile, the synergistic effects of the in-situ formed plate-like HEB phase, nanoscale (Zr,Hf)C precipitates with pinning effects, and solid solution strengthening collectively contributed to the excellent mechanical properties of the material. The ceramic with a raw composition of 40 mol% (Ti_0.2Zr_0.2Hf_0.2Nb_0.2Ta_0.2)C and 60 mol% ZrB₂ exhibited a flexural strength of 650 ± 37 MPa and a fracture toughness of 6.3 ± 0.4 MPa·m^1/2. This study provides a novel pathway for microstructural tailoring and performance enhancement in multi-component carbide-boride composite ceramics.