Mechanism of oxide layer growth during annealing of NiTi.

After annealing at 540°C, NiTi is covered by a characteristic oxide layer with an Ni-containing outer and an Ni-free inner titanium oxide region. To elucidate details of the yet unclear formation process, samples were annealed in an atmosphere containing different oxygen isotopes at a time and analyzed by nondestructive ion beam techniques at different stages of the oxidation. During the heating stage, an oxygen permeable "low Ni" titanium oxide forms, and the oxide layer grows inward. Subsequently, when the annealing temperature of 540°C is reached, Ni-free stoichiometric titanium oxide forms and inhibits the transport of oxygen toward the bulk. Thus, the oxide layer growth changes to outward, and the final location of the reaction front between O and Ti is inside the oxide layer at the transition of "low Ni" oxide to "Ni-free" oxide. Consequently, the annealing conditions during inward oxide layer growth govern the surface properties, whereas the conditions during outward oxide layer growth are uncritical with respect to the surface properties. The findings are directly applicable to set the amount of surface Ni of NiTi devices, provide basis for detailed interpretation of experimental results involving annealing of NiTi, and can further respective modeling.