Deformation‑Induced Intergranular δ‑Phase Precipitation and Its Impact on High‑Temperature Embrittlement in Alloy 718

Alloy 718 holds the largest market share among nickel-based superalloys used in industrial applications, particularly under high-temperature and high-pressure conditions, where the preservation of mechanical integrity is critical. This study offers additional experimental evidence relevant to the mechanisms governing the practical operating temperature of alloy 718, which in industrial applications is typically limited to approximately 650 °C and is commonly described in the literature as oxidation-assisted intergranular cracking (OAIC). Beyond this temperature, the alloy exhibits a pronounced loss of ductility and fails by intergranular brittle fracture, with the minimum ductility consistently observed in the temperature range between 800 and 850 °C. The prevailing interpretation of OAIC, widely acknowledged as the fundamental mechanism governing this phenomenon, attributes this behavior to the formation of brittle Nb2O5 films at grain boundaries, resulting from the dissolution of NbC carbides in the presence of oxygen. However, direct experimental evidence of Nb2O5 at grain boundaries remains scarce, and the rapid kinetics required to account for the observed embrittlement have not yet been satisfactorily explained. In this study, high-temperature tensile testing combined with detailed microstructural characterization using transmission electron microscopy and elemental mapping demonstrates that intergranular embrittlement in alloy 718 is governed by deformation-induced precipitation of the δ-phase at grain boundaries, accompanied by the development of a precipitation-free zone (PFZ) during plastic deformation.