Mechanisms of In Situ Growth of (Fe₄Al₃Cr)_0.25TiO₅ 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.