EUROPEAN CONGRESS AND EXHIBITION ON ADVANCED MATERIALS AND PROCESSES - EUROMAT 2021

The strength of superalloys is strongly influenced by gamma prime precipitates, whose size and volume fraction can be adjusted by heat treatments. According to classical precipitation strengthening models, an increasing precipitate diameter should lead to a transition from weak to strong coupling of the dislocation pairs that form superdislocations in the gamma prime phase. We show that long-term annealing of the Ni-base superalloy A718Plus at 670 and 680 °C increases the alloy’s strength without significantly changing the grain size and η fraction. To understand the effect of the slight increase in gamma prime size, investigations on multiple size scales were done. SEM and TEM images were used to analyze the gamma prime size and morphology as well as other microstructural properties. Following this, detailed atom probe tomography (APT) was performed. Here, different field evaporation rates of the phases strongly affect the determination of the gamma prime volume fraction when using the usual isosurface construction. This can be mitigated by considering the number density of atoms inside and outside the gamma prime precipitates. Using an approximation of the precipitate shapes and arrangements from the APT data, atomistic simulations revealed that precipitate shearing by both, weakly and strongly coupled dislocations can occur in the same specimen due to the wide distribution of precipitate sizes. These results highlight the need for advanced strengthening models that take into account the gamma prime size dis-tribution.