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

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.