On the oxidation and interdiffusion behavior of an AlCoCrFeNiY high-entropy alloy bond coat on a directionally solidified Ni-based superalloy

The oxidation and interdiffusion behavior of a novel AlCoCrFeNiY bond coat deposited on a directionally solidified Ni-based superalloy were systematically studied at 1050, 1100 and 1150°C, and compared with a conventional NiCoCrAlY coating deposited on the same substrate. The AlCoCrFeNiY bond coat exhibits lower oxide growth rates due to its large columnar grains and low Al activity at the oxide scale/bond coat interface. Meanwhile, AlCoCrFeNiY has higher resistance to oxide spallation than NiCoCrAlY, which is attributed to the formation of a clean and defect-free metal/oxide interface. Significant interdiffusion occurs across the AlCoCrFeNiY/superalloy substrate interface. Our experimental evidence and thermodynamic modelling suggest that Fe accelerates interdiffusion and destabilizes the γ' phase, thereby causing the formation of a thick and γ'-depleted interdiffusion zone. In addition, the AlCoCrFeNiY bond coat undergoes more Al depletion and subsequent β to γ transformation compared with NiCoCrAlY. Based on the findings in this work, a novel AlCoCrFeNiY/NiCoCrAlY double-layer bond coat was designed, tested and validated to achieve optimal balance between oxidation and interdiffusion.