Compositional Variations between Different Generations of γ′ Precipitates Forming during continuous cooling of a Commercial Nickel-Base Superalloy

The compositional and microstructural evolution of different generations of precipitates of the ordered gamma' phase during the continuous cooling, followed by isothermal aging, of a commercial nickel-base superalloy, Rene 88DT, has been characterized by three-dimensional atom probe (3DAP) tomography coupled with energy-filtered transmission electron microscopy (EFTEM) Studies. After solutionizing in the single gamma-phase field, during continuous cooling at a relatively slow rate (-24 degrees C/min), the first-generation primary gamma' precipitates, Forming at relatively higher temperatures, exhibit near-equilibrium compositions, while the smaller-scale secondary gamma' precipitates, forming at lower temperatures, exhibit nonequilibrium compositions often containing an excess of Co and Cr while being depleted in Al and Ti content. The compositions of the gamma matrix near these precipitates also exhibit similar trends, with the composition being closer to equilibrium near the primary precipitates as compared to the secondary precipitates. Subsequent isothermal aging at 760 degrees C leads to coarsening of the primary gamma' precipitates without affecting their composition significantly. In contrast, the composition of the secondary gamma' precipitates is driven toward equilibrium during the isothermal aging process.