Dynamic recrystallisation in Inconel®718 at creep conditions

Inconel®718 is a nickel-based superalloy primarily used in applications at high temperatures and loads. Its main characteristic is the high creep resistance due to a combination of microstructural features, mainly the presence of coherent intermetallic phases. Although the design of this type of alloy relies on controlling the distribution, nature and stability of the intermetallic phases, dislocations may play a determinant role on the creep resistance of nickel-based superalloys. This work investigates the creep resistance of Inconel®718 after two different heat treatments between 590 and 650 °C at 550 and 830 MPa stresses. After analysing the microstructure, we observe that dynamic recrystallisation happens at high temperatures and stresses, softening the material considerably. Therefore, we further develop mean-field models that predict the strain and microstructure evolutions during creep considering the size and fraction of precipitates, the dislocation densities, grain sizes and recrystallisation grade. Finally, we simulate the creep behaviour at different testing conditions and the initial microstructures and explore the robustness of the model beyond the measurement capabilities. The main conclusion is that an initial large amount of dislocations accelerates the nucleation and recrystallisation rates, decreasing the creep resistance of Inconel®718. In consequence, the material aged after forging presents lower creep resistance than the standard aged because of the larger initial dislocation density. On the contrary, the standard aged material undergo a strong reduction of the remaining dislocation density from forging during the solution treatment.