Mechanical and corrosion behavior of additively manufactured NiTi shape memory alloys

Abstract NiTi shape memory alloy (SMA) serves as an excellent seismic and vibration-damping material, frequently used in marine engineering to resist the impact of ocean waves and submarine earthquakes. However, research on its corrosion resistance in seawater has been limited to nonloaded states, with minimal reporting on its corrosion behavior under varying strain levels. Utilizing selective laser melting (SLM) technology, this study successfully fabricated SMA with harmonic structures. Multiscale characterization demonstrates that in the SLM state, it predominantly comprises B2 phase with a minor presence of B19’ phase, while in the initial plastic deformation (IPT) stage, amorphous lamellae and B2 phases coexist, and after fracture (FS), nanocrystalline B2 grains along with fragmented amorphous phases are observed. The corrosion resistance properties of these three states were meticulously investigated through the application of Tafel analysis and electrochemical impedance spectroscopy (EIS) testing. Exhibiting the optimal corrosion resistance, IPT samples were followed by SLM samples, whereas FS samples demonstrated the least favorable performance. Theoretical calculations yielded passivation film thicknesses of 0.55, 0.86 and 0.82 nm for the three states, respectively. X-ray photoelectron spectroscopy analysis revealed that the passivation film of the SLM samples not only exhibits the highest density but also contains a substantially larger proportion of TiO 2 and NiO, whereas the passivation film of the FS samples predominantly consists of a substantial amount of less dense Ni(OH) 2 and Ti 2 O 3 , as evidenced by the n value obtained from EIS fitting, which further suggests inferior homogeneity of the passivation film in FS samples. By employing first-principles calculations, the density of states and work functions for each phase were successfully determined, thereby shedding light on their corrosion resistance properties as well as on the microgalvanic reactions occurring between the B2 and B19’ phases.