Influence of cooling rate on the development of multiple generations of γ′ precipitates in a commercial nickel base superalloy

The compositional and microstructural evolution of different generations of γ′ precipitates during the continuous cooling of a commercial nickel base superalloy, Rene88DT, has been characterized by three dimensional atom probe tomography coupled with energy-filtered transmission electron microscopy studies. After solutionizing in the single γ phase field, continuous cooling at a very high rate results in a monomodal size distribution of γ′ precipitates with a high nucleation density and non-equilibrium compositions. In contrast, a relatively slower cooling rate (~ 24 °C/min) results in a multi-modal size distribution of γ′ precipitates with the larger first generation primary precipitates exhibiting close to equilibrium composition, along with the smaller scale secondary γ′ precipitates, exhibiting non-equilibrium composition (excess of Co and Cr, depleted in Al and Ti). The composition of the γ matrix near these precipitates also exhibits similar trends with the composition being closer to equilibrium near the primary precipitates as compared to the secondary precipitates.