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

Abstract 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 Nb 2 O 5 films at grain boundaries, resulting from the dissolution of NbC carbides in the presence of oxygen. However, direct experimental evidence of Nb 2 O 5 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. Complementary four-dimensional scanning transmission electron microscopy (4D-STEM) enables direct mapping of local elastic strain components (εxx, εyy, εxy) and lattice rotation fields (θ) in the vicinity of grain boundaries, revealing pronounced strain localization at the δ phase/PFZ interface. Finite element simulations (2D FEA) of the grain boundary region further confirm the presence of significant stress concentrations at this interface, supporting a microstructurally driven mechanism for intergranular brittle fracture. These findings indicate that δ-phase precipitation and the associated local mechanical heterogeneity play a dominant role in high-temperature embrittlement, while the contributions of NbC dissolution, oxygen diffusion, and Nb 2 O 5 formation appear to be secondary. The results challenge the conventional OAIC framework and provide new insights into the microstructural origins of high-temperature embrittlement in alloy 718.