Element accumulation beneath the scale/alloy interface of a CoNiCrAlReY alloy

Highlights: • We report two forms of accumulations at scale/alloy interface of Co alloy. • Cr and Re were found enriched beneath the alumina scale. • Cr and Re were enriched as a layer in one alloy and as σ particles in another. • Thermochemical reasoning behind the two accumulation forms are discussed. - Abstract: In the present study, two types of subscale Cr and Re accumulation found at scale/alloy interface of two batches of Co₃₂Ni₂₁Cr₁₀Al_3.5ReY alloys are described. During oxidation, one alloy developed a supersaturated Cr–Re rich layer and another formed Cr–Re precipitates beneath the oxide in the subscale region. Diffusional and thermodynamic modelling showed the accumulation is a result of decreasing Cr and Re chemical potentials with Al depletion. The observed difference in accumulation forms is caused by the different phases in two alloys, with the former being γ, β, α and σ and the latter having only γ, β and σ