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 gamma' 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 gamma phase field, continuous cooling at a very high rate results in a monomodal size distribution of gamma' precipitates with a high nucleation density and non-equilibrium compositions. In contrast, a relatively slower cooling rate (similar to 24 degrees C/min) results in a multi-modal size distribution of gamma' precipitates with he larger first generation primary precipitates exhibiting close to equilibrium composition, along with the smaller scale secondary gamma' precipitates, exhibiting non-equilibrium composition (excess of Co and Cr, depleted in Al and Ti). The composition of the gamma 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. (c) 2011 Elsevier Inc. All rights reserved.