An Investigation of High-Temperature Precipitation in Powder-Metallurgy, Gamma/Gamma-Prime Nickel-Base Superalloys

The high-temperature-precipitation behavior of a typical powder-metallurgy, gamma-gamma-prime, nickel-base superalloy (LSHR) was determined to develop and validate a quantitative fast-acting model. A series of experiments including supersolvus solution treatment followed by continuous cooling at rates typical of those experienced during the manufacturing of full-scale components was conducted for LSHR. The nucleation and growth of secondary-gamma-prime precipitates were analyzed via metallography on samples water quenched at various temperatures during the cooling cycle. Further insights into nucleation and the extent of retained supersaturation during cooling were obtained through in situ synchrotron X-ray diffraction experiments that involved cooling LSHR samples at identical rates with or without a hold time at an intermediate temperature. The data were interpreted using a fast-acting spreadsheet model incorporating classical, homogeneous-nucleation theory and growth by bulk diffusion. Special attention was focused on determining model input parameters such as composition, free energy of formation, surface energy of precipitates, and effective diffusivity, contrasting these values with those from existing databases. It was demonstrated that calculations using a nickel-chromium pseudo-binary system closely matched measurements of precipitate volume fraction, number density, and size evolution during continuous cooling.