Detrimental Effect of Aluminum on the Oxidation Behavior of Co–20Re–25Cr–2Si Superalloy at Low Oxygen Pressures

The effect of Al on the high‐temperature oxidation resistance of Co–Re–Cr–Si alloys remains poorly understood, restricting their development as multicomponent materials. To address this knowledge gap, the oxidation behavior of Co–20Re–25Cr–2Si–1Al and Co–20Re–25Cr–2Si–3Al alloys was investigated at 1000–1300 °C under an oxygen partial pressure of 3.0 × 10 ⁻⁵ Pa for 24 h, and compared with that of the base Co–20Re–25Cr–2Si alloy. Both Al‐containing alloys followed parabolic oxidation kinetics. The oxide scales on these alloys exhibited a similar two‐layer structure, consisting of an outer mixed Co + Cr oxide layer and an inner continuous Cr ₂ O ₃ layer, along with internal oxidation zones of Si and Al. The oxidation activation energies of the 1 and 3Al alloys were determined to be 134.6 kJ·mol and 92.8 kJ·mol, respectively, both significantly lower than that of the base alloy (262.7 kJ·mol), indicating that a minor Al addition deteriorates oxidation resistance. This deterioration is attributed to two mechanisms: (i) Al has a higher oxygen affinity than Cr, which reduces the effective Cr concentration and delays the formation of a dense Cr ₂ O ₃ scale; (ii) the Al internal oxidation zone hinders the outward diffusion of Cr, thereby inhibiting the formation and self‐healing of the protective Cr ₂ O ₃ scale.