Growth and ripening kinetics of γ'' precipitates in Nb containing nickel-base superalloys: Experimental investigation
IN718 is a widely used nickel-base wrought alloy with good mechanical properties at elevated temperature. The major strengthening phase in this alloy is the metastable γ''-phase (Ni3Nb), with ordered tetragonal crystal structure. In this work, we consider the influence of different aging treatment on aspects of the γ-γ'' microstructure formation.
We prepare undeformed single crystalline as well as polycrystalline samples of modified IN718, i.e. IN718M where all the γ' forming elements, Al and Ti, have been replaced by Ni. This allows us to focus on the homogeneous γ'' precipitation, without the additional influences from γ' co-precipitation. Homogenization has been performed at 1150°C for 24 h, followed by various aging-treatment from 700°C to 760°C. The resulting γ-γ'' microstructures are investigated using scanning-electron and transmission-electron-microscope (SEM and TEM).
The γ'' precipitation microstructure after aging has been found to be related to the dislocation microstructure before aging. In the case of the single crystalline samples, dislocations have been generated during the dendritic solidification and arranged within the slip systems of the fcc matrix during the solution heat treatment. After aging, γ'' precipitates are preferentially found to be arranged in the slip plains, which can be seen as a basket weave structure with respect to the view of  direction. The similar phenomenon has also been found in the undeformed polycrystalline samples, which include the influences of the grain boundaries. The γ''-coarsening kinetics are analyzed and compared to results reported in the literature as well as phase field simulations.