2026 Cempura Alloy 718 Long Term Annealing

Alloy 718 (UNS N077718) is commonly used in the aerospace industry, particularly in aircraft engines. This study investigates the microstructural evolution of Alloy 718 subjected to long-term annealing at 760 °C for up to 1000 hours. The objective was to analyze the phase transformations and microstructural changes through both experimental methods and numerical simulations using the CALPHAD methodology. The results demonstrate that prolonged annealing leads to significant degradation of the microstructure, indicated by a decrease in hardness from 479 to 345 HV0.5. Notably, the c'' phase experiences enlargement and subsequent dissolution into the c matrix, with the matrix becoming enriched in niobium, resulting in the formation of a d phase with a plate-like morphology. Microstructural examinations utilizing FIB-SEM tomography revealed volume fractions of 12.8 pct for c'' after 50 hours, increasing to 13.5 pct after 250 hours, while the d phase reached 11.6 pct after 250 hours. Transmission Electron Microscopy highlighted the crystal orientation and local compressive stress fields created by c phase precipitates. Ultimately, the findings suggest that 1000 hours of annealing is insufficient to attain thermodynamic equilibrium.