Characteristics of cracks formed at the surface of NiTi during a single cycle of pseudoelastic deformation

Pseudoelastic deformation of NiTi usually results in cracks at the surface. Cracking is promoted by surface oxide layers that form, e.g., during heat treatments required for shape-setting of minimally invasive implants. In connection with the advancing miniaturization of minimally invasive implants, the question arises whether their mechanical integrity may get impaired by such cracks. In the present work, the characteristics of the cracks was investigated in cross section with the help of targeted preparation using focused ion-beams. SEM and TEM on samples after a single cycle of pseudoelastic deformation revealed that cracks extend perpendicular to the loading direction in the surface oxide layer and change to angles between 90° and 45° in the Ni-rich layer below. Pores observed in the surface oxide close to the Ni-rich layer did not prevent the extension of cracks towards the NiTi bulk. When the cracks reach the NiTi, blunting of the crack tip was observed. The crack length essentially corresponds to the thickness of the surface oxide layer and the Ni-rich layer. The findings provide data for estimating crack propagation in according implants in the future.