Phases and Phase Transformation of Ni-rich Sublayers Below the Surface Oxide on NiTi

Manufacturing of medical devices from NiTi semifinished products often alters the surface of the material by the formation of a surface oxide layer that is rich in Ti. The accompanying enrichment in Ni below the surface oxide layer is understood to affect the subsequent Ni release characteristics. Although possible consequences of such sublayers are widely discussed, little is known regarding phase state and phase transformations below the oxide layer, mainly owing to the nanometer length scale of the layers. It is reasonable to assume that not only the thickness of the sublayer and the content of Ni, but also the phase state affect the release characteristics of a device. Established approaches for phase identification, e.g. gracing incidence X-ray diffraction, are not applicable in the present case due to the small dimension of the sublayer.

In the present work, the evolution of the Ni content below the surface oxide is explored quantitatively duringthe early stages of oxidation up to 30min. Transmission electron microscopy equipped with energy dispersive X-ray spectroscopy reveals an increase up to 75at.% Ni during annealing for less than 10min. By means of nanobeam electron diffraction, the formation of nanocrystalline metastable and stable intermetallic phases rich in Ni is observed in less than 2min of annealing. Such swift phase formation is striking due to the expected incubation time that is usually linked to nucleation. High-resolution transmission electron microscopy images indicate a phase transformation mechanism based on gradual short-order rearrangements of the crystal lattice from NiTi to Ni₃Ti.