Abstract: A series of five (5RE_0.2)Ta₃O₉ high-entropy ceramics (RE = La, Gd, Nd, Dy, Sm, Eu, Ce) is successfully synthesized via a solid-state method. Their temperature-dependent fracture toughness is systematically investigated across the 25-1200 °C range, using in situ three-point bending tests coupled with the digital image correlation method. The fracture toughness exhibits a non-monotonic variation: it peaks at 2.3 MPa m^1/2 at room temperature, then sharply declines to a minimum of 1.5 MPa m^1/2 at 400 °C. Subsequently, an anomalous rebound to approximately 2.1 MPa m^1/2 is observed at 800 °C, followed by a further decrease, stabilizing between 1.7 and 1.9 MPa m^1/2 at 1200 °C. High-temperature X-ray diffraction and high-resolution transmission electron microscope results confirm an irreversible orthorhombic-to-tetragonal phase transformation occurring between 600 and 800 °C. Aberration-corrected STEM provides direct atomic-scale evidence of a high-density dislocation network interacting with crack tips, revealing dislocation pinning, bridging, and crack deflection. The combined action of phase-transformation-induced lattice strain and dislocation-mediated toughening effectively suppresses linear crack advance and enhances energy dissipation, thereby accounting for the partial recovery of fracture toughness at 800 °C. These direct observations establish a distinctive synergistic toughening mechanism in high-entropy ceramics, in which irreversible phase transformation and thermally activated dislocation networks cooperate to improve high-temperature fracture resistance.

Key words: (5RE_0.2)Ta₃O₉ high-entropy ceramics, Three-point bending tests, Digital image correlation analysis, Phase transformation, Dislocation, Toughening mechanism