Abstract
Phase separation of gamma' precipitates determines the microstructure and mechanical properties of nickel-based superalloys. In the course of ageing, disordered gamma spheres form inside ordered (L1(2)) gamma' precipitates, undergo a morphological change to plates and finally split the gamma' precipitates. The presence of gamma particles inside gamma' affects coarsening kinetics and increases alloy hardness. Here we use atom probe tomography to visualize phase separation in a Ni86.1Al8.5Ti5.4 alloy in three dimensions and to quantify the composition of all the phases with near-atomic resolution. We find that gamma' precipitates are supersaturated in nickel, thereby driving the formation of gamma particles and observe a compositional evolution of the gamma particles, which accompanies their morphological change. Our results suggest that by controlling nickel supersaturation we can tailor the phase separation and thereby the properties of nickel-based superalloys.