Influence of δ phase intergranular precipitation on the embrittlement phenomenon in alloy 718.

The Alloy 718 holds the largest market share among the nickel-base superalloys. Due to the significance of this category in high-temperature and high-pressure applications, impairments to mechanical properties under these conditions are highly undesirable. The scope of this work aimed to contribute to the understanding of the mechanism that limits this material’s operating temperature to 650^oC, usually known in the literature as OAIC (Oxidation-Assisted Intergranular Cracking). Above this temperature, the material experiences a pronounced loss of ductility and intergranular brittle fracture, with minimum ductility observed between 800^oC and 850^oC. The explanation of this mechanism from the scientific literature is permeated with uncertainties regarding the formation of the brittle oxide Nb₂O₅ at the grain boundaries through the dissolution of NbC carbides in the presence of oxygen, and the rapid kinetics of the phenomenon remains unjustified. This study presents a TEM microstructural characterization of samples (FIB lamellas) after high temperature short duration tensile tests (strain rates ranging from 10^-5s^-1 to 10^-3s¹) and accelerated creep tests (ranking from 40 to 3000 min) in the temperature range of the embrittlement phenomenon. The results show a typical microstructural morphology for both the solution annealed (1050^oC/1h) and aged for 32 hours (800^oC) conditions and after tensile or creep tests. With the increase of temperature and stress, γ” phase (TCC Ni₃Nb) loses its stability, locally increasing Nb concentration, accelerating the δ phase precipitation (Orthorhombic Ni₃Nb), resulting in δ-decorated grain boundaries, with adjacent precipitation-free γ” regions. This suggests that the stress concentration in δ phase vicinity due to the grain boundary sliding or to dislocation pile-ups, contributes to the brittle intergranular fracture. The results consistently demonstrated that factors such as oxygen diffusion, NbC dissolution, and oxide formation are less decisive in the occurrence of the embrittlement phenomenon.