Quantification of the coarsening kinetics of γ′ precipitates in Waspaloy microstructures with different prior homogenizing treatments

We report on quantification of the γ′ precipitate population, and its coarsening behavior, in controlled Waspaloy microstructures synthesized to possess γ matrix grain sizes ranging from 13 to 89 μm. The grain microstructures were produced by initial solution-treatments at 1045, 1090 and 1145 °C. The γ′ precipitates were obtained by aging at 779 and 796 °C for times ranging from 0.1 to 263.5 h. Specimen characterization was conducted via optical microscopy and scanning electron microscopy, DC four-point probe resistivity and ex situ ultra-small-angle X-ray scattering (USAXS) experiments at each aging time. The γ′ size distribution, obtained from the USAXS analysis, transformed from an initial unimodal to an eventual bimodal distribution with continued aging. The overall coarsening kinetics, although non-steady state, followed t1/3 behavior, when the primary γ′ radius was used as the quantifying precipitate dimension. The coarsening rate constants were primarily determined by the aging temperature used, while the influence of prior homogenizing treatments was minimal to non-existent. A generic correlation was found to exist between a newly proposed figure-of-merit of scattering, η based on the USAXS-derived γ′ precipitate distribution(s) and the measured electrical resistivity.