Mechanisms of In Situ Growth of (Fe<sub>4</sub>Al<sub>3</sub>Cr)<sub>0.25</sub>TiO<sub>5</sub> Whisker Driven by Oxygen Partial Pressure and Reinforced Mechanical Properties in Corundum-Type Medium-Entropy Oxides.

Abstract

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 (Al_0.41Cr_0.26Fe_0.31Ti_0.02)₂O₃, while MEO-2 incorporates (Fe₄Al₃Cr)_0.25TiO₅ whiskers to reinforce (Al_0.40Cr_0.25Fe_0.30Ti_0.05)₂O₃. 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^1/2, 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.