Characterization and modeling of the precipitation of the sigma phase in UDIMET 720 and UDIMET 720LI

The kinetics of the formation of the sigma phase in the high strength nickel-based superalloys UDIMET 720 (U720) and UDIMET 720Li (U720Li) have been characterized using a combination of electrolytic extraction and quantitative X-ray diffraction (XRD) involving the Rietveld method. In order to perform the analysis, a database of crystallographic parameters is required and details of this are presented. It is shown that the numerical data generated using this technique are consistent with observations made using conventional optical, scanning, and transmission electron microscopies. The results are presented in the form of temperature-time-transformation (TTT) diagrams. It appears that U720Li is very much less prone to sigma precipitation than U720. The presence of a tensile stress accelerates the reaction, but the kinetics are very slow at temperatures lower than 700 degrees C. Thermodynamic calculations have been used in order to infer that the formation of sigma is associated predominantly with the dissolution of the gamma matrix. Theoretical modeling of sigma formation is carried out using a coupled thermodynamic/kinetic analysis assuming multicomponent diffusion-controlled growth. The numerical results are broadly consistent with the experimental data; in the U720Li alloy, there appears to be an incubation period associated with the onset of reaction, but this is negligible in U720. In principle, the method could be adapted for the quantification of microstructural instabilities in other superalloys. If a maximum tolerable amount of microstructural degradation is defined, the method provides a rational basis for the estimation of the limiting combinations of temperature, time, and stress that any given alloy can withstand.