The commercial IN718 superalloy contains mainly coherent tetragonal γ''-precipitation (Ni3Nb), coherent cubic γ'-phase [Ni3(Ti, Al)] and fcc γ-matrix. It has extraordinary high temperature properties, which are achieved mainly by γ''-precipitation strengthening in the γ-matrix. The metastable γ''-phase has ordered tetragonal crystal structure and disc-shaped forms in three orthogonal directions. It should be noticed that the stable phase of intermetallic Ni3Nb is incoherent orthorhombic δ phase, which tremendously weaken the mechanical properties of the superalloy. Therefore, consideration of the kinetics of microstructure evolution is important. Moreover, the growth and ripening behavior of γ'' is diffusion-controlled. In order to investigate the γ''-precipitation strengthening, as an example-mechanism involving a strong coupling between thermo-chemical and thermo-mechanical driving forces, we perform both experiments as well as simulations.
In the experimental part, we produce samples, in which γ' forming elements, Al and Ti, are removed, allowing us to concentrate on the γ''-precipitation alone. Furthermore, because of the longer δ-phase precipitation times and avoidance of the heterogeneous precipitation, single crystal casts are prepared, which are free of grain boundaries and advantageous for us to further focus on the physical mechanism of intrinsic particle strengthening by coherent γ''-precipitations. The anisotropic lattice misfits and the elastic properties are then measured.
With regard to the simulations, a phase-field model for the simulation of diffusion-limited precipitation in Nickel-based superalloys is developed. It accounts for multi-component chemical diffusion as well as elastic effects, which result from the lattice misfit and elastic inhomogeneities between the γ and the γ'’-phase. This project was recently started as part of the second phase of the DFG priority program 1713. We aim to study the growth and ripening of γ''- precipitations under external thermo-mechanical loads. The respective γ/γ'' microstructure evolution will be investigated in a joint approach using phase field simulations together with customized metallurgic experiments.