Characterization of the microstructure evolution in a nickel base superalloy during continuous cooling conditions

The solidification characteristics of the γ phase from the liquid and the subsequent decomposition of the γ phase control the evolution of the microstructure in nickel–base superalloy welds. The precipitation of the γ′ phase from the γ phase during continuous cooling conditions (0.17–75 K s−1) from the solutionizing temperature was characterized in a directionally solidified CM247DS alloy with thermomechanical simulator, and by transmission electron microscopy, atom probe field ion microscopy and atom probe tomography. The number density increased; size decreased and morphology of the γ′ precipitates changed with an increase in cooling rate. Under rapid water-quenched conditions, complex partitioning of the alloying elements between γ and γ′ phases was observed. Atom probe tomography on samples subjected to slower cooling rates showed different partitioning behavior compared to that of water-quenched samples and the presence of secondary γ′ precipitates in the samples subjected to a cooling rate of 1 K s−1.