Stress-Induced Acceleration of the Growth of δ-Phase Precipitates in Ni-Based Superalloy GH4169 (IN718) and Its Effect on the Acceleration of Intergranular Cracking Under Creep Loading at Elevated Temperature

In this study, intermittent creepCreep tests and Electron Backscatter Diffraction (EBSD) analyses were conducted on GH4169GH4169 (IN718) at 800 °C to elucidate the change mechanism of its micro texture and the degradation mechanism of the strength of grain boundariesGrain boundary under mechanical loading at elevated temperature. Under creepCreep loading, the growth of δ-phase (Ni3Nb) precipitatesPrecipitates was found to be accelerated around grain boundariesGrain boundary. This growth of δ-phase precipitatesΔ-phase Precipitate caused the disappearance of the solute strengthened phase in the surrounding grains and, thus, the effective strength of grains decreased. Even though the precipitationPrecipitation around grain boundariesGrain boundary increased their effective strength at first, however, it started to decrease with time due to the acceleration of the growth and accumulation of vacancies and dislocations around the interface between the precipitatesPrecipitates and the matrix. This acceleration was attributed to the large lattice mismatch between the precipitatePrecipitates and the matrix, and the structural singularity around the edges of the needle-shape precipitatesPrecipitates. Finally, intergranular crackingIntergranular cracking was accelerated. The degradation of the strength of the material was validated by using a micro tensile test using a scanning electron microscope. The acceleration process was successfully explained by the concept of the stress-induced acceleration of diffusionStress-induced Acceleration of Diffusion, by applying the modified Arrhenius equation. The growth time of δ-phase precipitatesΔ-phase Precipitate around grain boundariesGrain boundary was also predicted quantitatively.