High entropy ceramics are the research hotspot in recent years. Transition metal boride medium entropy and high entropy ceramics have become important candidate materials for extreme environment resistance, due to their excellent mechanical properties, chemical reaction inertia and high melting point. In this work, the effects of B₄C excess content on the densification, microstructure and high temperature flexural strength of medium entropy boride ceramics were studied for the first time, and the preparation process of (Ti,Zr,Hf)B₂ powder with low oxygen content and high sintering activity was synthesized. The entropy ceramic density of (Ti,Zr,Hf)B₂ prepared by hot pressed sintering method at 1800℃ is over 99%, and the grain size of (Ti,Zr,Hf)B₂ ceramics with excess content of 15 wt% B₄C is 5.0±2.1μm. As the excess content of B₄C increases to 25 wt%, the grain size is significantly refined to 2.4±0.7μm. The part of excess B₄C reacts with Si₃N₄ introduced by ball milling, to produce BN phase by in situ reaction, while the other part of B₄C exists in the matrix as a second phase. The introduction of BN and B₄C phases can effectively suppress grain growth during the sintering process of medium entropy ceramics, while also improving the high-temperature flexural strength of the ceramics. When B₄C exceeds 25 wt%, the (Ti,Zr,Hf)B₂ medium entropy ceramics have the highest high-temperature flexural strength at 1600℃ of 631±62 MPa.

Excellent irradiation resistance is the basic property of nuclear materials to keep nuclear safety. The high-entropy design has great potential to improve the irradiation resistance of the nuclear materials, which has been proven in alloys. However, whether or not high entropy can also improve the irradiation resistance of ceramics, especially the mechanism therein still needs to be uncovered. In this work, the irradiation and helium (He) behaviors of zirconium carbide (ZrC)-based high-entropy ceramics (HECs), i.e., (Zr_0.2Ti_0.2Nb_0.2Ta_0.2W_0.2)C, were investigated and compared with those of ZrC under 540 keV He ion irradiation with a dose of 1×10¹⁷ cm⁻² at room temperature and subsequent annealing. Both ZrC and (Zr_0.2Ti_0.2Nb_0.2Ta_0.2W_0.2)C maintain lattice integrity after irradiation, while the irradiation-induced lattice expansion is smaller in (Zr_0.2Ti_0.2Nb_0.2Ta_0.2W_0.2)C (0.78%) with highly thermodynamic stability than that in ZrC (0.91%). After annealing at 800 ℃, ZrC exhibits the residual 0.20% lattice expansion, while (Zr_0.2Ti_0.2Nb_0.2Ta_0.2W_0.2)C shows only 0.10%. Full recovery of the lattice parameter (a) is achieved for both ceramics after annealing at 1500 ℃. In addition, the high entropy in the meantime brings about the favorable structural evolution phenomena including smaller He bubbles that are evenly distributed without abnormal coarsening or aggregation, segregation, and shorter and sparser dislocation. The excellent irradiation resistance is related to the high-entropy-induced phase stability, sluggish diffusion of defects, and stress dispersion along with the production of vacancies by valence compensation. The present study indicates a high potential of high-entropy carbides in irradiation resistance applications.

High-temperature mechanical properties of medium-entropy carbide ceramics have attracted significant attention. Tailoring the microstructure is an effective way to improve these high-temperature mechanical properties, which can be affected by the evolution of the enthalpy and entropy, as well as by lattice distortion and sluggish diffusion. In this study, the effects of equiatomic Zr/(Ti,Nb) substitution (Zr content of 10–40 at%) on the microstructure and high-temperature strength of (Ti,Zr,Nb)C medium-entropy ceramics were investigated. The grain size of the (Ti,Zr,Nb)C medium-entropy ceramics was refined from 9.4±3.7 to 1.1±0.4 μm with an increase in the Zr content from 10.0 to 33.3 at%. A further increase in the Zr content to 40 at% resulted in a slight increase in the grain size. At 1900 ℃, the (Ti,Zr,Nb)C medium-entropy ceramics with the Zr contents of 33.3 and 40 at% exhibited ultra-high flexural strengths of 875±43 and 843±71 MPa, respectively, which were higher than those of the transition metal carbides previously reported under similar conditions. Furthermore, relatively smooth grain boundaries, which were detected at a test temperature of 1000 ℃, transformed into curved and serrated boundaries as the temperature increased to 1900 ℃, which may be considered the primary reason for the improved high-temperature flexural strength. The associated mechanism was analyzed and discussed in detail.