Characterization of microstructural fluctuations in Waspaloy exposed to 760 °C for times up to 2500 h

The microstructure of Waspaloy, a nickel-base superalloy, was investigated as a function of thermal exposure to 760 °C for times up to 2502 h. The primary strengthening phase in this material is the γ′ precipitate phase—Ni3(Al, Ti). The microstructure of the specimens was monitored using impedance spectroscopy (IS), X-ray diffraction (XRD), optical and scanning electron microscopy (SEM). The average grain size varied significantly during thermal exposure. SEM analysis indicated that the precipitates had a bimodal size distribution, with morphologies ranging from spherical to cubic. High-resolution XRD experiments indicated that the γ′ lattice parameter is a non-linear function of the exposure time. Energy dispersive (EDS) analysis of Al and Ti composition in the matrix surrounding the primary γ′ precipitates also varied. The normalized admittance maximum displayed a partial inverse correlation with the volume fraction of the primary γ′ precipitates, and a partial direct correlation with the matrix grain size as a function of thermal exposure time. It is hypothesized that the fluctuations detected by the above techniques are indicative of repetitive nucleation, coarsening and dissolution events occurring during the course of the extended thermal exposure. These results suggest that IS may be useful for monitoring the microstructure of these alloys non-destructively.