Strain-induced Precipitation Behavior of δ Phase in Inconel 718 Alloy

To study the precipitation dynamics of δ phase in Inconel 718 alloy, two-stage interrupted compression method was used in the region of cold deformation temperatures and the temperatures range from 875 to 975 °C. The precipitation-time-temperature (PTT) curve of δ phase was obtained by analyzing the softening kinetics curves. For verifying the type of the precipitates and confirming the validity of the test, the transmission electron microscopy (TEM), scanning transmission electron microscopy (STEM) and energy dispersion spectrum (EDS) were employed. Experimental results indicated that the PTT curve for δ precipitation exhibited a typical “C” shape and the nose points of start and finish precipitation were about 5 s at 920 °C and 2815 s at 940 °C, respectively. In addition, the nucleation of δ was heterogeneous. The nucleation sites varied with temperatures, including dislocation, grain boundary and stacking fault within γ′ phase. And δ particles grew quickly at higher temperature with lower density. Moreover, the driving force of nucleation was mainly including chemical free energy, interfacial energy and dislocation distorted energy. And the dislocation distorted energy could decide the density of nucleation in the strain-induced process.