In Situ Micromechanical Study of Bimodal γ′–γ″ Precipitate Assemblies in Ni–Cr–Al–Nb Superalloy

Inconel 718 is a Ni-base superalloy widely used in turbine disk components due to its excellent strength derived from the high volume fractions of γ″ precipitates. However, a transformation of γ″ to δ phase at elevated temperatures (700°C–900°C) restricts its use. To address this, a polycrystalline Ni–Cr–Al–Nb alloy was developed by optimizing Al:Nb ratio to increase the precipitate volume fraction. In this study, a bimodal microstructure was developed through controlled annealing, in which γ″ precipitates heterogeneously nucleated on γ′ precipitates, forming composite precipitate assemblies with coarse and fine distributions, as revealed by atom probe tomography (APT). In situ scanning electron microscope (SEM) micropillar compression tests exhibited a remarkable room-temperature strength of 1370 MPa and retained a strength of 1150 MPa at 773 K, indicating that the bimodal distribution effectively impedes dislocation motion. Precipitate shearing was found to be the dominant strengthening mechanism, irrespective of precipitate size, as identified by postmortem transmission electron microscope (TEM) analysis. The deformation pathway, determined using atomic-resolution high-angle annular dark-field (HAADF)–scanning transmission electron microscopy (STEM), was an intrinsic stacking fault (ISF) in γ″ coupled with a superlattice ISF (SISF) in the γ′ phase.