Thermomechanical fatigue of a coated directionally solidified nickel-base superalloy

The termomechanical fatigue (TMF) behaviour of the directionally solidified (DS) nickel-base superalloy CM 247 LC, coated with a plasma-sprayed NiCrAlY-coating (PCA-1), was investigated experimentally; The tests were carried out, using a particular out-of-phase cycle, at different total strain ranges in the temperature range between the lower temperature of 400 degrees C and the upper temperature of 1000 degrees C. For a better fundamental understanding, corresponding tests were performed not only on the uncoated substrate material but also on bulk specimens of the coating material. Detailed microstructural studies were carried out in order to characterize the microstructural changes occurring in the substrate and in the coating material during a TMF cycle and in order to elucidate the basic damage mechanisms. It was concluded that fatigue cracks initiate in the coating as a consequence of the high tensile stresses prevailing at the lower temperature and that the fatigue life of the coated alloy is governed by the mechanical strain amplitude experienced by the coating.