Unlocking Strength-Toughness Dilemma in High-Entropy Borides by Intragranular Microstructural Reconstruction

High-entropy boride ceramics hold great promise as ultra-high-temperature structural materials but are hindered by the well-known strength-toughness trade-off. Conventional extrinsic toughening approaches, such as composite reinforcement and microstructural refinement, offer limited improvements as they fail to modify the inherent intragranular tendency for brittle fracture. Here, we report an approach to overcome this limitation by constructing intragranular energy dissipation units through an extreme non-equilibrium process. By employing heavy direct current sintering with TiSi₂ addition, high densification (> 93% relative density) was achieved at a substantially reduced sintering temperature of 1000°C and an ultrahigh heating rate exceeding 5300°C/min. This process promotes selective diffusion of cations, leading to compositional redistribution within grains and forming compositional gradients and dislocation networks. These microscopic features collectively hinder crack propagation. The resulting ceramic demonstrates attractive mechanical properties with a flexural strength of 887 MPa and a fracture toughness of 7.1 MPa·m^1/2. These findings demonstrate a viable pathway for the intrinsic toughening of high-entropy ceramics through intragranular microstructural engineering.