Thermal stability of phases in a NiCoCrAlY coating alloy

The temperature dependence of the thermal stability in a NiCoCrAlY coating alloy was examined by experimental observation and thermodynamic modeling in the 400–1200 °C temperature range. The results indicated that the thermal stabilities of primary β–NiAl, β–NiAl/α–Cr eutectic, and γ–Ni were slightly temperature dependent, but those of γ′–Ni₃Al, σ–(Cr,Co,Ni), and α–Cr were strongly temperature dependent in the annealed NiCoCrAlY specimens. The temperature dependence of the thermal stabilities among γ′–Ni₃Al, σ–(Cr,Co,Ni), and α–Cr might be ascribed to the σ → α transformation at ∼1100 °C and the γ′ → γ transformation at ∼800 °C. Further, using Thermocalc associated with TTNi7 database, thermodynamic equilibria were calculated. The modeling results were compared with the experimental results and found to be in reasonable agreement with the experimental observations of β–NiAl, σ–(Cr,Co,Ni), and γ′–Ni₃Al. Some deviations observed are discussed in the light of both limited availability of thermodynamic data and experimental problems.