| Abstract Scope |
In this study, intermittent creep tests and EBSD (Electron Backscatter diffraction) analyses were conducted on GH4169 (IN718) at 800°C to elucidate the change mechanism of its micro texture and the degradation mechanism of the strength of grain boundaries under mechanical loading at elevated temperature. Under creep loading, the growth of δ-phase (Ni3Nb) precipitates was found to be accelerated around grain boundaries. This growth of δ-phase precipitates caused the disappearance of the solute strengthened phase in the surrounded grains and, thus, the effective strength of grains decreased. Even though the precipitation around grain boundaries 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 precipitates and the matrix. This acceleration was attributed to the large lattice mismatch between the precipitate and the matrix, and the structural singularity around the edges of the needle-shape precipitates. Finally, intergranular 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 diffusion, by applying the modified Arrhenius equation. The growth time of δ-phase precipitates around grain boundaries was also predicted quantitatively. |