Comparing the oxidation behaviour of three generations of single crystal nickel-based superalloys at an intermediate service temperature

The oxidation behaviours of three different generations of single crystal Ni-based superalloys (SRR-99 – 1st generation, CMSX-4 – 2nd generation, and CMSX-10N – 3rd generation) have been investigated and compared at 550 °C. Isothermal oxidation tests were carried out at various exposure times ranging from 2 h up to 1000 h and it was found that oxide morphology and development showed noticeable differences between the alloys. Scanning electron microscopy (SEM), 3D imaging reconstruction, energy-dispersive X-ray spectroscopy (EDS) and X-ray diffraction (XRD) were employed to characterise the oxide scales. The external and internal oxides were studied through high-resolution imaging, measurements from which revealed the oxidation kinetics of CMSX-4 and CMSX-10N followed a near-parabolic law whilst a logarithmic law better described the kinetics behaviour of SRR-99. Thermodynamic modelling was used to predict the species and composition of oxides formed in each alloy and compared to measured EDS and XRD results. From the experimental results and modelling, it was found that the mechanism and oxide products formed for CMSX-4 and CMSX-10N are very similar, as NiO initially formed externally over the γ channels and the γ’ precipitates were preferentially oxidised internally at the γ/γ’ interface.