The environmental effect of SO₂-bearing atmosphere on the creep fatigue failure of aluminide-coated MM-002 nickel-base superalloy at 870 °C

High-temperature low-cycle fatigue (HTLCF) failure mechanisms of aluminide-coated MAR-M002 nickel-base superalloy in air, argon and Ar+5% SO₂ atmospheres were investigated at 870 °C. The loading conditions were constant and consisted of creep tension and plastic compression according to the creep plasticity mode of the strain-range partitioning method. The resultant failure mechanisms were investigated using scanning electron microscopy, energy dispersive spectroscopy and X-ray diffraction analysis. The results obtained indicated that the combination of environmental effect and cyclic loading resulted in an early failure of the coating. This failure acted as the incubation and initiation stages for the HTLCF failure of the coated alloy. However, it was found that under an SO₂-containing environment, an accelerated and premature failure was obtained compared to that obtained in argon and air environments. Hence, it is evident that although the aluminide coating provides relatively adequate resistance towards oxidation environment, this protection was ineffective under the SO₂-bearing atmosphere.