Thermo-Mechanical Aspects Of Short- and Long-Term Ni-Release From NiTi

The release of nickel is a permanent concern in the medical application of NiTi. Contradictory reports stating negligible or critical release of Nickel, respectively reflect the enormous variability of the material and indicate the need for identifying mechanisms and relevant parameters with respect to material composition and pre-treatment. Research focusing on the role of annealing accompanied by the formation of an oxide layer and the enrichment of Nickel inside and/or below the oxide found increased Ni-release compared to polished or electro-polished material. However, the release of Nickel from annealed NiTi varies significantly, raising the question for sources and kinetics of Ni-release. Recently, single-cycle pseudo-elastic deformation of annealed NiTi was shown to result in cracking and flaking of the surface oxide layer. It was suggested that flaking exposes the Ni-enriched layer below the oxide and thus contributes to enhanced nickel release. However, this assumption is in contradiction to previously presented Ni-release rates of annealed NiTi. In the present work we investigate the effect of pseudo-elastic pre-deformation on the surface oxide layer in detail using Transmission Electron Microscopy and Nano Beam Diffraction. The sites where oxide particles flaked off the oxide layer are clearly visible, and the correlation of flaking and microstructure is identified. It is shown that cracks can propagate into the Ni-enriched layer below the oxide and thus provide access of the immersion liquid to these zones. Based on these thermo-mechanical aspects, the short-term and long-term Ni-release from annealed NiTi are discussed consistently.