Abstract: Restoring the deteriorated nickel-based blades is of great significance for the durability of turbines. This study proposes a revised heat treatment (HT) for the crept CM247LC to restore the structure and tensile properties by designing the trimodal γ' microstructure. The trimodal structure includes the primary γ' phase ($\gamma_{\mathrm{I}}^{\prime}$, 1084.9 nm), the secondary γ' phase ($\gamma_{\mathrm{II}}^{\prime}$, 299.3 nm) and the tertiary γ' phase ($\gamma_{\mathrm{III}}^{\prime}$, 65.0 nm). The large size of the $\gamma_{\mathrm{III}}^{\prime}$ contributes to the stability of the trimodal structure after deformation at both room temperature and 900 °C. The trimodal structure combines various mechanisms, such as the dislocation network, Orowan bypassing and shearing, leading to a better performance than samples after the standard HT. It leads to a significant increase in the elongation (21.5 %) by 30.3 % with no sacrifice of the yield strength (832.6 MPa) at room temperature. And the yield strength, ultimate tensile strength and elongation increase by 20.0 %, 22.7 % and 16.8 % at 900 °C, respectively. These findings suggest a simple method to restore the microstructure and properties at the same time.

Key words: Nickel-based superalloy, Heat treatment, Trimodal structure, Restoration, Yield strength