A correlative multidimensional study of γ′ precipitates with Ta addition in Re-containing Ni-based single crystal superalloys

The microstructural evolution in Re-containing Ni-based single crystal superalloys with different Tantalum (Ta) content (2Ta, 5Ta and 8Ta in wt %) was investigated. Ta addition significantly affected the γ′ precipitate morphology, γ/γ′ lattice misfit and microstructural stability during long-term aging. Results showed that the partitioning behaviors of solutes were enhanced by Ta addition, meanwhile, the reversal partitioning behavior of W was triggered which partitioned from γ′ precipitate to γ matrix. The elemental concentration redistribution caused variations in lattice misfit from positive to negative, the values of lattice misfit were measured to be 0.16 % for 2Ta alloy, then decreased to −0.07 % for 5Ta alloy and negatively increased to −0.23 % for 8Ta alloy. These variations in the lattice misfit were reflected on the transition of γ′ morphology from round-cornered cuboidal shape to cuboidal with sharp corners, accomplished with increasing shape parameter ratio η. Consequently, the optimal γ′ shape could be obtained at lattice misfit of approximately 0.3 %. The γ′ coarsening investigation at 900 °C (up to 2000 h) indicated that Ta addition was beneficial for improving the microstructural stability by reducing the coarsening rate and interfacial energy, accompanied by the enhanced capability of resisting γ′ coalescence. By incorporating the calculated interfacial energy, computational modeling, Thermo-Calc and PrecipiCalc, were employed to elucidate the γ′ kinetic pathways, the simulation results agreed with experiments, indicating that the model and parameters were reasonable. Additionally, it was found that there was no overlap between γ′ nucleation and coarsening when the γ/γ′ interfacial energy increased to a critical value.