Characterization of Gamma Prime (γ′) Precipitates in a Polycrystalline Nickel-Base Superalloy Using Small-Angle Neutron Scattering

Small angle neutron scattering (SANS) has been used to evaluate the temporal evolution of the secondary and tertiary gamma' precipitates in the nickel-base superalloy, RR1000, in situ during an aging heat treatment at 1033 K (760 degrees C) following a supersolvus heat treatment and oil quench. The bimodal distribution of secondary and tertiary gamma' was analyzed using a specially developed polydispersive model capable of evaluating the scattering curves to obtain precipitate size distributions (PSDs) and volume fractions as a function of time. The model was designed to be suitable for high volume fractions of gamma' and takes into account the scattering interaction between precipitates. The results show an increase in the volume fraction and the mean particle size of both the secondary gamma' and tertiary gamma' during aging. The initial and final precipitate distributions have been characterized using transmission electron microscopy (TEM) and show satisfactory correlation with the SANS data across the scattering vector range.