The inferior fracture toughness of high-entropy diboride ceramics (HEBs) severely limits their application in extreme environments. To address this challenge, nanoscale Al2O3 was introduced as the reinforcement to improve the fracture toughness of (Hf0.2Zr0.2Ti0.2Ta0.2Cr0.2)B2 and (Hf0.2Zr0.2Ti0.2Ta0.2Mo0.2)B2. Excellent mechanical properties were achieved for 15 vol% Al2O3 doped (Hf0.2Zr0.2Ti0.2Ta0.2Cr0.2)B2 and (Hf0.2Zr0.2Ti0.2Ta0.2Mo0.2)B2 with a fracture toughness of 6.52 MPa·mm1/2, 6.78 MPa·mm1/2 and a flexural strength of 518 MPa and 532 MPa, respectively. The strengthening/ toughening mechanisms can be ascribed to the pinning effect exerted by Al2O3 particles that are uniformly distributed within the ceramic matrix. Furthermore, the incorporation of Al2O3 substantially decreases the porosity of the ceramic material. This alteration facilitates a more thorough densification of the internal microstructure, which in turn markedly improves the mechanical responses of the HEBs. Besides, the inherent high-elastic modulus of Al2O3 contributes to the flexural strength of the ceramic material, thereby further augmenting its mechanical properties.
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