Role of microstructural alterations caused by heat treatment on passivity of additively manufactured Ni-Fe-Cr-based alloy 718

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Although the mechanical behaviour of additively manufactured alloy 718 has been widely investigated, the mechanistic link between heat treatment, microstructure, plastic deformation, and passive film stability remains poorly understood. In particular, how crystallographic features, precipitation state, and dislocation density influence passivation before and after mechanical loading has received limited attention. In this study, a systematic investigation is presented to elucidate the coupled effects of thermal treatment and microstructural evolution on the electrochemical behaviour of laser powder bed fused alloy 718. As built and double aged conditions, including solution annealing at 965 °C and 1050 °C, were examined using a combination of SEM, TEM, EBSD, XPS, tensile testing, and electrochemical techniques. The results show that passive current density, oxide defect density, and charge transfer resistance are strongly governed by microstructural state. Subgrain scale chemical heterogeneity and coarse δ phase precipitation might disrupt chromium rich passivation, while a homogeneous matrix with nanoscale γ″ precipitation could promote the formation of a compact, low defect passive film. Furthermore, plastic deformation generally improved passivation, which may be associated with the increased dislocation density providing fast diffusion pathways for Cr oxidation, although excessive dislocation accumulation could adversely affect long term stability.