Multimodal characterization of deformation and damage in Ni superalloy

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Abstract: 

Inconel 718, a nickel-based superalloy, is widely used in extreme environments such as jet engines and nuclear reactors because of its excellent mechanical strength and corrosion resistance. However, in reactor conditions ( 350 ◦C, high pressure, oxidizing media, and irradiation), it is susceptible to stress corrosion cracking (SCC), a degradation mechanism facilitated by the coupling of mechanical stress and chemical environment. A similar phenomenon, stress-assisted grain boundary oxidation (SAGBO), occurs at higher temperatures ( 650 ◦C) and low strain rates in air. Both mechanisms promote crack propagation along grain boundaries through oxidation-assisted damage. This study proposes the combined use of diffraction contrast tomography (DCT) and phase contrast tomography (PCT) to investigate SCC and SAGBO in alloy 718. Using in-situ tensile tests and digital volume correlation (DVC), the evolution of internal strain fields and crack formation was tracked under both room and high temperature conditions. Data processing strategies including projection alignment, robust flat-field normalization, and ring artifact removal were combined in a pipeline to improve the quality of PCT reconstructions, enabling strain mapping at the grain scale. The results reveal early strain localization near grain boundaries, slip band activity, and crack nucleation and growth correlated with micro-structural characteristics. This integrated approach provides the tools required for future studies combining experimental characterization with numerical simulations, carried out the digital twin of the material, and with the ultimate goal to enhance predictive capabilities of models describing the progression of grain boundary damage in this industrially relevant alloy system.