Tuning structure and CMAS corrosion behavior in high-entropy RE disilicate via average RE3+ ionic radius

Abstract

This study employed the average RE³⁺ ionic radius as a key design parameter to simultaneously tailor the microstructure and CMAS corrosion behavior of high-entropy RE disilicates (Gd_x1Y_x2Er_x3Yb_x4Lu_x5)₂Si₂O₇ (x1 + x2 + x3 + x4 + x5 = 1, (5RE_xi)₂Si₂O₇) as environmental barrier coating (EBC) materials. Precise tuning of the average RE³⁺ ionic radius directly can govern the phase component and lattice distortion. Increasing the average ionic radius can induce a β→γ→δ phase transformation in high-entropy RE disilicates, with critical transition radii at the β/γ and γ/δ phase boundaries of 0.886 and 0.9 Å, respectively, while simultaneously reducing lattice distortion. All (5RE_xi)₂Si₂O₇ compositions exhibited superior CMAS corrosion resistance at 1300 °C. This resistance stems from a dense cyclosilicate barrier layer formed at the corrosion front via a reaction-precipitation mechanism, effectively suppressing CMAS attack. The efficacy of barrier layer formation is intrinsically linked to the radius-controlled microstructures, including phase stability, RE-O bond length, and lattice distortion. However, at ultrahigh-temperature of 1500 °C, CMAS corrosion involved both grain boundary corrosion and grain dissolution. The dominant corrosion mode shifted gradually from grain boundary corrosion to grain dissolution as the average RE³⁺ ionic radius increased. Thus, the average RE³⁺ ionic radius served as a fundamental descriptor linking microstructure to CMAS corrosion behavior. These findings establish a vital theoretical framework for designing the composition of high-entropy RE disilicates as next-generation EBC materials.