Microstructure of surface and subsurface layers of a Ni-Ti shape memory microwire

DFT investigation of the dissolution trends of NiTi alloys with the B2 and B19' phases during the initial oxidation stage

The selective corrosion of NiTi alloys was studied using density functional theory calculations, and the dissolution trends of the NiTi-B₂ and NiTi-B19' phases in the initial oxidation stage were compared to predict their corrosion difference. The dissolution process of Ni and Ti was simulated by creating Ni or Ti vacancies on the unoxidized and oxidized NiTi alloy surfaces. The results show that the surface vacancy formation energy of Ti vacancies is higher than that of Ni vacancies, indicating that Ti is more difficult to dissolve than Ni. Furthermore, oxidation promotes and impedes the dissolution of Ni and Ti, respectively. This study improves the fundamental understanding of the corrosion mechanism of NiTi alloys.

Ion Release and Surface Characterization of Nanostructured Nitinol during Long-Term Testing

The corrosion resistance of nanostructured nitinol (NiTi) was investigated using long-term tests in solutions simulating physiological fluids at static conditions, reflecting the material structure and metal concentration in the solutions. Mechanical polishing reduced the ion release by a factor of two to three, whereas annealing deteriorated the corrosion resistance. The depassivation and repassivation of nitinol surfaces were considered. We found that nanostructured nitinol might increase the corrosion leaching of titanium into solutions, although the nickel release decreased. Metal dissolution did not occur in the alkaline environment or artificial plasma. A Ni-free surface with a protective 25 nm-thick titanium oxide film resulted from soaking mechanically treated samples of the NiTi wire in a saline solution for two years under static conditions. Hence, the medical application of nanostructured NiTi, such as for the production of medical devices and implants such as stents, shows potential compared with microstructured NiTi.

Effect of thermomechanical pre-treatment on short- and long-term Ni release from biomedical NiTi

The effect of annealing and deformation on short-term (21days) and long-term (8months) Ni release from biomedical NiTi wires is studied. The deformation of annealed NiTi wires causes cracking and flaking of the surface oxide layer. Flaking of oxide particles does not uncover the Ni-rich layer underneath the surface oxide layer, since at sites where flaking occurs, a thin (∼25nm) layer of oxide remains on top of this Ni-rich layer. The number of cracks in the oxide and Ni-rich layer, respectively, increases with deformation, and intercrystalline crack propagation into the Ni-rich layer and the NiTi bulk is observed. In plastically deformed wires, the cracks may remain opened, providing access of immersion liquid to these zones. Characteristics and quantity of short-term Ni release are significantly affected by the pre-deformation, resulting in an up to 2 times higher total Ni release within the first 21days of deformed compared to annealed wires. Pre-deformation does not significantly influence long-term Ni release; all annealed and deformed samples exhibit similar long-term Ni release rates. The source of Ni during short-term release is the Ni contained in the surface zone of the oxide layer. For high pre-deformation, the Ni-rich layer is a second source for Ni. This second source is also the cause for Ni release in long-term immersion experiments.

Stability of Ni in nitinol oxide surfaces

The stability of Ni in titanium oxide surface layers on nitinol wires known to release certain amounts of Ni was investigated by first principles density functional theory and transmission electron microscopy. The oxides were identified as a combination of TiO and TiO(2) depending on the thickness of the layer. The calculations indicate that free Ni atoms can exist in TiO at ambient temperature while Ni particles form in TiO(2), which was confirmed by the transmission electron microscopy observations. The results are discussed with respect to surface stability and Ni release due to free Ni atoms and Ni particles.