THERMOMECHANICAL FATIGUE DEGRADATION OF A NICKEL-ALUMINIDE COATING ONA SINGLE-CRYSTAL NICKEL-BASED ALLOY

A microstructural study of the degradation of an NiAl coating on a nic kel-base single crystal subjected to thermo-mechanical (TMF) and low-c ycle fatigue (LCF) has been made, using scanning electron microscopy a nd energy and wavelength dispersive spectrometry. Various thermal hist ories were investigated, and the associated coating degradation mechan isms identified. Under TMF conditions the hot part of the cycle (950-1 050 degrees C) helps to stabilize the beta-phase of the coating. Surfa ce oxidation proceeds slowly and does not affect the bulk of the coati ng. Evidence of enhanced outward aluminium diffusion and inward oxygen diffusion was found at long exposure times. Fatigue cracks initiate a t the surface, grow intergranularly through the coating and are freque ntly arrested at the coating-substrate interface. In LCF conditions, m assive oxidation of the surface and interdiffusion at the coating-subs trate interface cause NiAl decomposition into first gamma' and later g amma phases. Cracks initiate and grow through the coating in a mode si milar to the TMF case.