Analysis of Microstructural Evolution in Alloy 718 Nickel-Based Superalloy During Long-Term Annealing at 760 °C

Abstract Alloy 718 (UNS N077718) is commonly used in the aerospace industry, particularly in aircraft engines. In this study, a test piece made of Alloy 718 with the chemical composition Ni19Cr18.5Fe3Mo0.9Ti0.6Al0.04C (wt pct) was subjected to annealing at 760 °C for durations up to 1000 hours. The observed microstructural changes were compared with numerical simulations based on the CALPHAD methodology. Increasing the annealing time at 760 °C leads to an enlargement of the γ″ phase, which consists of disk-shaped precipitates, and their subsequent dissolution into the γ matrix. The matrix becomes enriched in niobium; as a result, the δ phase, coherent with the matrix, precipitates. This phase exhibits a plate-like morphology. An annealing leads to significant degradation of the microstructure. After 1000 hours, the hardness decreased from the initial state of 479 to 345 HV0.5. CALPHAD simulations are valuable input, but the precipitation simulation model underestimates the amount of γ′ and incorrectly predicts the presence of γ″. A comparison of microstructural examinations with CALPHAD simulations indicates that 1000 hours at 760 °C is insufficient to achieve thermodynamic equilibrium. The volume fraction of phases after annealing at 760 °C was determined on 3D reconstructed volumes obtained using FIB-SEM tomography methods. A γ″ volume fraction was 12.8 pct with a mean equivalent diameter <D> = 15 nm after 50 hours, and the volume fraction increased to 13.5 pct after 250 hours. After 250, the volume fraction of δ phase was 11.6 pct. Spheroidal γ′ precipitates with a mean diameter <D> = 53.4 nm were found in places where γ″ dissolved. Longer annealing times, up to 1000 hours resulted in the replacement of γ″ phase by spheroidal γ′ phase precipitates. Transmission Electron Microscopy methods enabled the identification of phases and the determination of crystallographic orientation between phases present in the alloy’s microstructure. HRSTEM measurements indicate that the γ phase creates a local compressive stress field in the matrix perpendicular to the γ/γ″ interface, specifically along the (002) γ matrix plane. Analysis of the deformation field around the γ″ phase precipitate resulting from lattice mismatch reveals its anisotropic nature. The stress level generated by precipitates was estimated using the Eshelby theory for elastic disk-shaped inclusions. The normal stress components in the matrix are as follows: σ 11 = 7.882 GPa, σ 22 = 7.882 GPa, σ 33 = 13.306 GPa. The maximum stress develops along the direction normal to the γ/γ″ interface, restricting plate growth in this direction (i.e., along the plate thickness). In contrast, growth in the two orthogonal directions is thermodynamically favorable, leading to the formation of disk-shaped precipitates.