Effects of 800°C NaCl salt-spray corrosion on fracture behavior and deformation mechanism in a NiCoCr-based superalloy

Abstract

NiCoCr-based superalloys operating in marine environments are subjected to severe corrosion, which can significantly affect their mechanical performance. In this study, a systematic investigation is carried out to elucidate the effects of NaCl salt-spray exposure at 800 °C on fracture behavior and deformation mechanisms of a newly developed γ′-strengthened NiCoCr-based superalloy, K439B. The tensile tests reveal a degradation of mechanical performance after NaCl salt-spray exposure at 800 °C for 70 h: the ultimate tensile strength (UTS) decreases from 1113 MPa to 816 MPa, and the elongation at break (EL) drops from 6.0 % to 1.2 %. Fracture morphologies show that the uncorroded specimen exhibits mixed ductile-brittle fracture features with quasi-cleavage facets, ductile dimples, and slip-band evidence, whereas the corroded specimen develops a corrosion-affected zone at the fracture edge, containing a high density of corrosion-induced voids. These voids are considered as crack initiation sites and stress concentrators, promoting brittle failure. Elemental distribution within the corrosion-affected zone reveals the depletion of Cr and γ′-forming elements (e.g., Al, Ti), and the local stress concentrations adjacent to corrosion voids are observed. In addition, the uncorroded specimen deforms primarily by planar slip and stacking fault cutting γ′ phase, while the corroded specimen shows dense dislocation tangling around corrosion voids and pile-up at the grain boundary and around the Al oxides, leading to the significant decrease in ductility. The results show mechanical degradation and altered deformation mechanisms in K439B alloy after NaCl salt-spray exposure at 800 °C, offering critical insights for evaluating its service performance in high-temperature marine environments.