Study on γ′ precipitation behavior in a nickel-based PM superalloy during interrupted continuous cooling

Abstract

The multi-stage nucleation phenomenon and the subsequent evolution of γ′ 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 γ′ phases precipitated preferentially at a relatively high temperature during the cooling process. As the temperature decreased with a slow rate, the secondary γ′ phases grew up and changed shape gradually and parts of the tertiary γ′ phases precipitated, resulting in a multimodal size distribution of γ′ phases. While, when the cooling rate is relatively fast, the γ′ 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 γ′ and the γ matrix, while Al and Ti elements is enriched in γ matrix away from the secondary γ′ precipitates. The distribution of alloying elements in the γ′ phases and γ matrix change with the decrease of the temperature, and the phase interface is a favorable place for the dynamic exchange of alloying elements.