Abstract
In this study, intermittent creep tests were conducted on GH4169 (IN718) at 800°C to elucidate the degradation mechanism of the grain boundary strength accompanied by microstructural changes under mechanical loading at elevated temperature. The microstructural degradation process was visualized by EBSD (Electron Back-Scatter Diffraction) method and the degradation of the strength of grain boundaries was verified by a micro tensile test in a scanning electron microscope using FIB (Focused-Ion Beam) apparatus. Under the creep loading, the precipitation of δ-phase (Ni3Nb) was found to grow around grain boundaries. The δ-phase precipitates grew around grain boundaries by the coarsening of the solute strengthened γ″ phase in this alloy. Thus, the growth of the δ-phase caused the disappearance of the solute strengthened phase in the surrounded grains. As a result, the effective strength of grains decreased, and the effective strength of grain boundaries increased by the precipitation strengthening around the grain boundaries. The effective strength of grain boundaries, however, started to decrease with time due to the acceleration of the growth of vacancies and dislocations around the interfaces between the precipitates and the matrix. This accumulation of defects was attributed to the large lattice mismatch between the precipitate and the matrix. As a result, intergranular cracking was accelerated. The change of the effective strength of a grain and a grain boundary was validated by the micro tensile test of bicrystal structures cut from the damaged polycrystalline alloy during the intermittent test.
