Clarifying the effects of composition and phase stabilization on the oxidation resistance in AlCoCrFeNiTi series high-entropy alloys at 1100 °C

The application of high entropy alloys in thermal barrier coating (TBC) systems offers a viable solution for mitigating interfacial instability arising from the elevated-temperature interdiffusion within the bonding layer/matrix. Nevertheless, the elucidation of the phase stability under elevated temperatures and the intricate interplay among thermal-mechanical-chemical factors in HEAs remain lacks clarity. Herein, a set of Al _xCo _10.5Cr ₁₀Fe ₂₅Ni _(50-x)Ti _4.5 HEAs featuring desirable high-temperature phase stability, oxidation resistance, and spalling resistance at 1100 °C were meticulously designed. And the effects of phase composition and thermodynamics on the evolution of surface oxides in these HEAs were investigated. Specifically, the formation of the unfavorable σ phase between 860 and 1200 °C is avoided through the composition adjustment. The oxidation resistance is governed by the presence of large-size columnar grains and the spalling resistance of the Al ₂O ₃ oxide film. The spalling resistance is mainly linked to the configuration sequence (Spinel→FeAlTiO ₅→TiO ₂) of the outer oxide layer and the phase transition of the matrix either to FCC or BCC. Meanwhile, the formation of FeAlTiO ₅ assumes a crucial role in the optimization of spalling (>770 h) and oxidation resistance (2.6×10 ⁻¹² g ²·cm ^{− 4}·s ⁻¹) within an Al ₂₀Co _10.5Cr ₁₀Fe ₂₅Ni ₃₀Ti _4.5 alloy. These findings provide guidance for the development of TBC systems endowed with exceptional properties tailored for engineering applications.