Differentiation of γ′- and γ″- precipitates in Inconel 718 by a complementary study with small-angle neutron scattering and analytical microscopy

We present an experimental method to distinguish and quantify the two strengthening phases γ′ and γ″ in the nickel-based superalloy Inconel 718. So far, this was only achieved by techniques that evaluated sample volumes in the nano-to micrometer range. In this study, reliable volume fractions of the precipitates were obtained and calculated from ex-situ small-angle neutron scattering (SANS) on differently heat treated specimens. For interpretation of the SANS curves, a structural model was set up, using complementary information from measurements by transmission electron microscopy (TEM) and atom probe tomography (APT). Phase identification, as well as size distributions, morphologies and crystallographic information of the precipitates were obtained by TEM. APT provided compositional information, which is necessary to calculate the scattering contrast of each phase. As a benefit of using bulk neutron diffraction for quantification, volumes of a few tens of cubic millimeters are analyzed and thus, significantly better statistics are obtained. The measured γ″ volume fractions are remarkably lower than stated in previous works, but now well fulfill the chemical mass balance.