Effect of cooling rate in aging treatment on secondary γ′ precipitation: Experiments and phase-field simulation

Since quantitative descriptions of secondary γ′ phase precipitation evolution with cooling rate are rarely reported, this study investigated the precipitation behavior of secondary γ′ phase during aging processs in DD5 Ni-based single-crystal superalloy through experiments at different cooling rates (30 to 3,000°C·min -1 ), combined with a multi-component phase-field method coupled with a nucleation model. Experimental results show that secondary γ′ precipitates was observed at 240 °C·min -1 but not at 60 °C·min -1 or lower, and the nucleation amount increases with rising cooling rate within 240 to 3,000 °C·min -1 . This precipitation behavior is consistent with phase-field simulation results. Further simulations at higher cooling rates (up to 300,000 °C·min -1 ) reveal a significant nonlinear effect of cooling rate on the number of secondary γ′ precipitates: the number first increases and then decreases with increasing cooling rate, peaking at approximately 3,000 °C·min -1 . A high cooling rate promotes the early initiation of secondary nucleation. However, excessively high cooling rate (>3,000 °C·min -1 ) restricts elemental diffusion due to rapid temperature drop. This significantly shortens nucleation duration and inhibits further precipitation of secondary γ′ phase. The established non-isothermal phase-field model can quantitatively describe the experimentally observed cooling-rate dependence of secondary γ′ phase precipitation. It provides theoretical support for optimizing the cooling path during aging heat treatment of Ni-based superalloys.