Comparative performance of turbine blades used in power generation: Damage vs. microstructure and superalloy composition selected for the application

First-stage turbine blades made of different grades of Ni-base superalloys and environmentally protected by the same Cr-modified aluminide coating are examined after exposure to the same service conditions in an electric power plant. Emphasis is placed upon microstructural changes, which can degrade the blade performance. Various electron-optical techniques are used to characterize the microstructures of unused and used blades. Three types of microstructural changes, which can lead to intergranular creep failure, are identified. These changes include: (i) coarsening, agglomeration, and rafting of the strengthening γ′-phase, (ii) formation of γ′-denuded zones alongside grain boundaries, and (iii) precipitation of intermetallic compounds. However, fatigue failure is also observed particularly in cases where higher than normal temperature is encountered. Although the same microstructural changes are found to occur in the blades included in the study, the respective kinetics appear to be influenced by at least two parameters: (i) exact superalloy composition and (ii) actual operating temperature. It is concluded that the life expectancy of blades used in such applications can be realized by appropriate selection of superalloy composition and adherence to design specifications.