Microstructural deterioration mechanism of directionally solidified Ni-based superalloy used in first-stage industrial gas turbine blades after ultra long-term service

The microstructural degradation of the substrate and coating restricts service lives of gas turbine blades. In this study, microstructure evolution of DS Ni-based superalloy, which were cut off from 1st -stage industrial gas turbine blades, were measured by field emission scanning electron microscope (FESEM) after exposure for nearly 50,000 engine operating hours (EOH). gamma' precipitates and carbides at several locations in substrate, as well as beta-NiAl phases in bond coating were investigated to evaluate degradation behavior. The formation mechanism of microcracks on the internal cooling channel surface were emphasized. The mechanical properties of the blades were also investigated by hardness test. The results showed that gamma' precipitates degradation was influenced by thermal gradients, surface treatment conditions, service temperature and stress fields. The relatively severe deterioration area occurred at leading edge (LE) and trailing edge (TE), followed by the pressure side (PS). The diffusion of Al element dominated the formation of oxides near the interface, inner bond coating, and cooling channel surface. Multiple microstructures degradation phenomena occurred near interface between the substrate and the bond coating. The cracks initiated from cooling channel surface were induced by thermal gradients and brittle oxide scale. The crack propagation were driven by creep stress assisted grain boundary oxidation.