Study On Gamma ' Precipitation Behavior In A Nickel-Based Pm Superalloy During Interrupted Continuous Cooling

The multi-stage nucleation phenomenon and the subsequent evolution of gamma' precipitation during cooling process of super-solvus heat treatment for a nickel-based powder metallurgy (PM) superalloy are studied by means of Gleeble thermal simulation, field emission scanning electron microscope (FE-SEM), energy dispersive spectrometer (EDS) and three-dimensional atom probe tomography (3D-APT). The results show that the secondary gamma' phases precipitated preferentially at a relatively high temperature during the cooling process. As the temperature decreased with a slow rate, the secondary gamma' phases grew up and changed shape gradually and parts of the tertiary gamma' phases precipitated, resulting in a multimodal size distribution of gamma' phases. While, when the cooling rate is relatively fast, the gamma' phase exhibits a monomodal size distribution. The results of EDS and 3D-APT show that there is a depleted zone of Al and Ti elements near the interface between secondary gamma' and they matrix, while Al and Ti elements is enriched in gamma matrix away from the secondary gamma' precipitates. The distribution of alloying elements in the gamma' phases and y matrix change with the decrease of the temperature, and the phase interface is a favorable place for the dynamic exchange of alloying elements.