The reversible deformation-induced martensitic transformation in NiTi gives rise to superelasticity. NiTi is used as structural material for stents in biomedical applications. The slender and intricate structure of a stent causes the material to experience locally multiaxial loading conditions in vivo. However, until now most of the knowledge of the degradation mechanisms of NiTi by functional fatigue is based on monotonic loading experiments. The effect of multiaxial loading or changes in load path on the behavior of the material remains unclear.
In this study, the deformation behavior and degradation of commercial superelastic NiTi is investigated with in situ synchrotron X-ray diffraction at the MS beamline of the Swiss Light Source and complemented by EBSD and HRDIC measurements. The initial microstructure consists of large austenitic grains subdivided in bands of nanoscaled sub-grains . Cruciform shaped samples are subjected to various cyclic loading conditions including uniaxial tension, equibiaxial tension and load paths that involve a sudden change in loading direction. The transformation behavior and martensitic variant selection is discussed in terms of the initial microstructure and the different loading paths. The degradation of the material after multiple load-unload cycles is found to depend strongly on the loading path.
 Acta Materialia 144 (2018) 874.