Creep Cavitation in Nickel-Based-Alloys

In high efficiency and low emissions thermal power plants, advanced materials are needed to withstand the high temperatures and pressures therein. These metals and alloys are put into service for many decades, where they experience the slow plastic deformation mechanism known as creep. During high temperature creep, at which diffusion is greatly enhanced, vacancies move from areas of compression to areas under tensile stress. This gives rise to the nucleation of nanoscopic cavities, especially at grain boundaries due to the competition between their surface energies to reduce the total free energy. These cavities grow and coalesce during creep and ultimately lead to microscopic fractures in the microstructure and macroscopic rupture of the component. We have used advanced micro- and nano-Computed Tomography (CT) as well as high resolution Scanning Electron Microscopy (SEM) to observe these cavities in nickel-based Alloy 625 which was stressed to 165 MPa at 700°C for up to 15000 hours. We present the results of our findings concerning the prevalence and morphology of cavities and chains of connected cavities and discuss the implications for ongoing and future efforts to simulate their nucleation and growth with physically based models.