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In ultra-supercritical power plants, Ni-base alloys are candidate materials for long-term, high-temperature applications, operating at temperatures and pressures as high as 750°C and 35 MPa. Alloy IN740 and its modification, alloy IN740H, are considered for such applications. Their microstructural evolution, at 750°C for times ranging between 3000 and 5000 hours, has been investigated by means of scanning electron microscopy, electron back-scattered diffraction, energy dispersive X-ray analysis and phase quantification. All phases were identified and quantified allowing comparison between the two microstructures, their evolution and stability. Particular attention was paid to γ′, η and G phases. The results are used within a broader investigation aimed at improving and further developing a predictive creep model based on continuous damage mechanics. [ABSTRACT FROM AUTHOR]

In the context of ultrasupercritical power plants, Ni base alloys are prime candidate materials for long term, high temperature applications, such as boilers, operating at temperatures and pressures as high as 750°C and 35 MPa. This necessitates the investigation of their microstructural evolution as a function of thermal treatment and simulated service conditions out to longer times at forecasted service temperatures, coupled with modelling activities able to predict the microstructural evolution under these new conditions. The lack of widespread microstructural data for most commercial nickel base alloys in these time-temperature regimes makes this type of investigation even more important. In this study, the microstructural evolution of IN740, a Ni-Cr-Co-Mo-Nb-Ti-Al superalloy, is characterised as a function of aging conditions. A method for phase identification is described that can be confidently used to gather relevant information for modelling activity, such as: phase identity, volume fraction, size distribution and interparticle spacing. The microstructural evolution of IN740 is investigated at temperatures of 700 and 750°C for aging times up to 10 000 h. The data obtained experimentally from the aged specimens as well as from literature are used both as input for and validation of a microstructurally based continuum damage mechanics (CDM) model for forecasting creep properties. Using this predictive model, discussion is made of possible approaches needed to optimize creep performance. [ABSTRACT FROM AUTHOR]