The Precipitation Behavior of Superalloy ATI Allvac 718Plus

ATI Allvac 718Plus is a novel nickel‐based superalloy, which was designed for heavy‐duty applications in aerospace gas turbines. The precipitation kinetics of the intermetallic δ (Ni3Nb) and γ′ (Ni3(Al,Ti)) phases in this alloy are of scientific as well as technological interest because of their significant influence on the mechanical properties. Important parameters like grain size are controlled by coarse δ precipitates located at grain boundaries, whereas small γ′ precipitates are responsible for strengthening by precipitation hardening. In the present study, the microstructure is investigated by three‐dimensional atom probe tomography and simulated by computer modeling using the thermo‐kinetic software MatCalc. The results of numerical simulations and experimental data are compared and critically discussed. It is shown that the chemical compositions of the phases change during isothermal aging, and the precipitation kinetics of δ and γ′ phases interact with each other as shown in a time temperature precipitation (TTP) plot. The TTP plot shows C‐shaped curves with characteristic discontinuities in the temperature range, where simultaneous and concurrent precipitation of the δ and γ′ phases occurs. This leads to a competition in the diffusion of Nb and Al, which are partly present in both phases. Thus, the present study gives important information on heat treatments for ATI Allvac 718Plus in order to achieve the desired microstructure and mechanical properties. The precipitation kinetics of intermetallic δ (Ni3Nb) and γ′ (Ni3(Al,Ti)) phases in the nickel‐based superalloy ATI Allvac 718Plus are of high interest due to their significant influence on the mechanical properties. Therefore, the microstructure is investigated by APT and simulated by computer modeling. It is shown that chemical compositions of the phases change during isothermal aging, and the precipitation kinetics interact with each other.