Mechanisms of In Situ Growth of (FeAlCr)TiO Whisker Driven by Oxygen Partial Pressure and Reinforced Mechanical Properties in Corundum-Type Medium-Entropy Oxides.

Designing in situ whiskers to enhance the fracture toughness of ceramics presents challenges for structural applications. Herein, two corundum-type medium-entropy oxide ceramics (MEO-1 and MEO-2) are synthesized through solid-state reaction sintering under controlled oxygen partial pressures. MEO-1 is a single-phase corundum-type oxide (AlCrFeTi)O, while MEO-2 incorporates (FeAlCr)TiO whiskers to reinforce (AlCrFeTi)O. By regulating variable valence ions at different oxygen partial pressures, the whiskers achieved a maximum average length of 28.3 µm and a length-to-diameter ratio of 17.7 at 50.6 kPa, significantly enhancing the fracture toughness of the MEO-2, which possesses the optimal flexural strength, Vickers hardness, and fracture toughness of 321 ± 8 MPa, 22.4 ± 1.5 GPa, and 3.87 ± 0.12 MPa m, respectively. Bravais-Friedel-Donnay-Harker law (BFDH) simulation elucidates that oxygen partial pressures can effectively regulate ionic diffusion behavior and whisker growth. First-principles calculations demonstrate that the whisker growth direction [0 4 0] aligns with the high shear modulus direction, contributing to the strengthening mechanism. This work provides new insights for designing high-performance medium/high-entropy ceramics, highlighting the critical role of oxygen partial pressure in regulating whisker growth and improving mechanical properties.