Thermal degradation of alloy CCA617 in pure steam at 750 and 850°C

Abstract

Oxidation and oxidation-induced microstructure evolution of CCA617 alloy was studied in pure steam at 750 and 850°C. Results showed that the alloy was oxidised approximately following a parabolic law. Temperature increase greatly promoted the external and internal oxidation. A single (Cr, Mn)₂O₃ layer with distribution of minor TiO₂ and MnCr₂O₄ formed at 750°C whereas a duplex oxide scale formed at 850°C consisting of a thin MnTiO₃ outer layer and a thicker (Cr, Mn)₂O₃ inner layer. TiO₂ and Al₂O₃ were formed internally especially along grain boundaries. Intragranular Mo-rich M₂₃C₆ type carbides were also observed. The carbides dissolved into the matrix as oxidation progressed. Based on detailed compositional and microstructural characterisation of the oxidised alloy, the mechanisms of external and internal oxidation as well as dissolution of the intragranular carbides are presented.

Keywords:

• Nickel-based superalloy
• Steam oxidation
• Microstructure
• Degradation
• XRD
• SEM
• TEM

Acknowledgements

The work is supported by National Natural Science Foundation of China (NSFC, project Grant No. 51401163 and 51301130) and by China Huaneng Group, Huaneng Power International, Inc.