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 nicke l-based superalloys UDIMET 720 (U720) and UDIMET 720Li (U720Li) have been c haracterized using a combination of electrolytic extraction and quantitativ e X-ray diffraction (XRD) involving the Rietveld method. In order to perfor m the analysis, a database of crystallographic parameters is required and d etails of this are presented. It is shown that the numerical data generated using this technique are consistent with observations made using conventio nal 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 tha n U720. The presence of a tensile stress accelerates the reaction, but the kinetics are very slow at temperatures lower than 700 degrees C. Thermodyna mic calculations have been used in order to infer that the formation of sig ma is associated predominantly with the dissolution of the gamma matrix. Th eoretical modeling of sigma formation is carried out using a coupled thermo dynamic/kinetic analysis assuming multicomponent diffusion-controlled growt h. The numerical results are broadly consistent with the experimental data; in the U720Li alloy, there appears to be an incubation period associated w ith the onset of reaction, but this is negligible in U720. In principle, th e method could be adapted for the quantification of microstructural instabi lities in other superalloys. If a maximum tolerable amount of microstructur al degradation is defined, the method provides a rational basis for the est imation of the limiting combinations of temperature, time, and stress that any given alloy can withstand.