From corrosion-induced microcracking to bulk stress-rupture failure: Multiscale investigation of microstructural evolution in salt-coated GH3625 superalloy

The long-term operational reliability of key components in concentrated solar power (CSP) plants is highly dependent on the mechanical stability of GH3625 superalloy under molten salt environments. This study, through stress-rupture tests under various conditions, reveals that the molten salt significantly reduces the alloy’s stress-rupture life and markedly decreases the stress-rupture elongation. Corrosion layers detected in the microcrack present a four-tier structure: an outer NiO layer, an intermediate NiO + Cr 2 O 3 mixed layer, an inner Cr 2 O 3 layer, and a nearby Cr-depleted zone adjacent to the matrix. Microstructural characterization combined with thermodynamic calculations reveals that, under molten-salt-free conditions, deformation-induced nanotwins serve as barriers to dislocation movement, substantially improving the alloy’s stress-rupture resistance. By contrast, the salt-coated counterpart exhibits an intergranular fracture mode: oxidation-triggered Cr depletion induces severe grain-boundary embrittlement, leading to a drastic reduction in stress-rupture ductility. Notably, no deformation nanotwins are observed in the deformed microstructure of the salt-coated sample, where only stacking faults are identified, further undermining the material’s intrinsic deformation resistance. Additionally, during testing, δ-Ni 3 Nb phases preferentially nucleate at grain boundaries due to Nb segregation at grain boundaries. This work elucidates the degradation mechanism of the GH3625 superalloy in molten-salt conditions, providing essential data and a theoretical foundation to ensure the long-term operational safety of CSP plants.