Phase formation in the Ni-enriched zone below the surface oxide on NiTi

Upon exposure to oxygen, NiTi forms a Ti-rich surface oxide layer, and mass balance requires that a Ni-enriched zone forms below the oxide. Both the surface oxide layer and the Ni-enriched zone are discussed to affect key properties of NiTi as material for minimally invasive medical application or for actuators, e.g., the release of Ni and the resistance to initiation and propagation of cracks when exploiting the shape memory effect/pseudoelasticity. However, owing to the small extension of the Ni-enriched zone of a few nanometers, little is known about its crystallinity, phase and evolution of composition. We study the formation of the surface layers during annealing at similar to 500 degrees C for up to 10 min, as routinely applied for shape-setting. With an approach employing nano beam electron diffraction in a transmission electron microscope and an in-house software package for phase analysis it is shown for the first time that phase transformations in the Ni-enriched zone, specifically from NiTi to Ni4Ti3, occur already after 2 min of annealing, much earlier than documented in the literature. Furthermore, the phase transformation to the thermodynamically stable Ni3Ti in the Ni-enriched zone is shown to be mediated by metastable Ni4Ti3. In contrast to Ni4Ti3 precipitates in NiTi bulk, the morphology of the crystalline phases is granular. Considering the swift formation of Ni-rich phases below the surface oxide and the observation of a band-like diffraction contrast in the TEM images originating from crystal interfaces, a phase transformation mechanism in the Ni-enriched zone is suggested consisting of short range order rearrangements of atoms.