Unveiling the Corrosion Mechanisms of High‐Entropy RETaO ₄ Through In Situ Observation

Abstract The ambiguous understanding of calcium‐magnesium aluminosilicate (CMAS) corrosion mechanisms in RETaO ₄ has hindered performance optimization through rare‐earth compositional engineering. This study systematically investigates the corrosion behavior of 3–10 component RETaO ₄ systems. In situ X‐ray diffraction/Transmission electron microscope coupled with Electron backscatter diffraction analysis unveils dynamic reaction pathways during the pre‐corrosion heating stage, identifying the crystallization and growth patterns of dominant corrosion product pyrochlore‐structured (Ca _2‐a‐b RE _a Al _b )(Ta _2‐c‐d Mg _c Si _d )O ₇ . A reaction‐diffusion mechanism of CMAS corrosion for RETaO ₄ is proposed, highlighting the different behaviors of various rare earth elements in the corrosion process. Among eight types of RETaO ₄ , La _1/6 Nd _1/6 Sm _1/6 Eu _1/6 Gd _1/6 Dy _1/6 TaO ₄ exhibits the best corrosion resistance, with a relatively thin corrosion layer and the ability to avoid element segregation and localized infiltration. These findings establish composition‐property relationships for designing next‐generation corrosion‐resistant thermal barrier coatings.