In-Situ γ-γ′ Lattice Parameter Evolution and Tertiary Burst Phenomena During Controlled Cooling of Commercial PM Nickel-Base Superalloys

The γ and γ′ lattice parameter evolution of two commercial powder metallurgy (PM) nickel-base superalloys, ME3 and Rene’88DT, during cooling from above the γ′ prime solvus temperature is characterized using in-situ synchrotron X-ray Diffraction (XRD). The peak intensity deconvolution necessary for quantifying misfit between the two phases from XRD is accomplished by combining direct observation of several superlattice peak positions with thermodynamic modeling to quantify the intensity relationship between the overlapping phases. The misfit values obtained from the XRD measurements are compared to Scanning Electron Microscopy (SEM) observations of γ′ precipitate shapes for a subset of the experimental conditions where it can be observed that the exposures that result in cuboidal precipitate shapes are associated with the highest degrees of relative misfit. Time-resolved observations of the on-cooling lattice parameter evolution suggest a potential direct observation of the tertiary γ′ burst events in the two compositions within both the (100) superlattice peak and the (311) fundamental peak. The onset temperatures for the tertiary γ′ burst events for ME3 and Rene’88DT compositions for the cooling rates examined were found to be approximately 925 °C and 815 °C, respectively.