Shearing Mechanisms of Co-Precipitates in IN718

Abstract: The Ni-base superalloy 718 is the most widely used material for turbomachinery in the aerospace industry and land-based turbines. Although the relationship between processing and the resulting properties is well known, an understanding of the specific deformation mechanisms activated across its application temperature range is required to create more mechanistically accurate property models. Direct atomic-scale imaging observations with high angle annular dark-field scanning transmission electron microscopy, complemented by phase-field modeling informed by generalized stacking fault surface calculations using density functional theory, were employed to understand the shear process of ${\gamma}''$ and ${\gamma}'/{\gamma}''$ co-precipitates after 1 \% macroscopic strain at lower temperature (ambient and $427 {\deg}C$). Experimentally, intrinsic stacking faults were observed in the ${\gamma}''$, whereas the ${\gamma}'$ was found to exhibit anti-phase boundaries or superlattice intrinsic stacking faults. Additionally, the atomically flat ${\gamma}'/{\gamma}''$ interfaces in the co-precipitates were found to exhibit offsets after shearing, which can be used as tracers for the deformation events. Phase-field modeling shows that the developing fault-structure is dependent on the direction of the Burgers vector of the $a/2 \langle110\rangle$ matrix dislocation (or dislocation group) due to the lower crystal symmetry of the ${\gamma''}$ phase. The interplay between ${\gamma}'$ and ${\gamma}''$ phases results in unique deformation pathways of the co-precipitate and increases the shear resistance. Consistent with the experimental observations, the simulation results indicate that complex shearing mechanisms are active in the low-temperature deformation regime and that multiple $a/2 \langle110\rangle$ dislocations of non-parallel Burgers vectors may be active on the same slip plane.