Impact of γ’’ and δ Phase Precipitation on Intergranular Embrittlement Under Constant Loading in Nickel-Based Superalloy 718

The nickel-based superalloy 718 has a strong market presence due to its high mechanical strength and exceptional corrosion resistance, and is commonly used in aerospace, power, and chemical industries. However, its application is restricted to temperatures up to 650°C due to a significant loss of ductility caused by the brittle intergranular fracture. The mechanisms behind this phenomenon are still under investigation, particularly regarding its rapid progression under constant loading. To mitigate these mechanisms, tensile and creep tests were performed at temperatures of minimum ductility, extending to fracture and interrupted at various stages of deformation. The tests were conducted on specimens with both annealed and aged microstructures. A comprehensive microstructural characterization was performed using SEM and TEM, including electron diffraction and EDS elemental mapping.
The results consistently show a progressive precipitation of the δ phase with a continuous morphology at the grain boundaries, while a γ’’-free zone forms in this area, leading to a localized stress concentration. The rapid kinetics of this transformation occur within this temperature range as γ'' loses stability, resulting in an increased Nb concentration near the boundaries, which drives the stress-enhanced diffusion precipitation of δ phase. No significant difference was observed between the solution-treated and aged conditions.