Microstructure evolution and superelastic fatigue of NiTi

In spite of its key importance for superelastic NiTi technologies, the accumulation of damage in cyclically deformed superelastic NiTi belongs among the puzzles in the field. It is generally accepted that the alloy microstructure and surface finishing (grain size, precipitates, inclusions, surface oxide quality and thickness) play key role in fatigue life of NiTi. But even if these effects are minimized, significant fatigue damage accumulates during the superelastic cycling and ultimately leads to the fatigue failure, particularly for strain amplitudes larger than 1%. In this respect, evolution of the alloy microstructure during the cycling (dislocation defects, incremental plastic deformation, residual stress, residual martensite) is believed to promote the fatigue degradation but detailed mechanism remains unclear, since it is not easy to gather relevant experimental information on it. In this work, results of dedicated in-situ synchrotron X-ray diffraction experiments and ex-situ TEM studies during cyclic superelastic deformation of thin NiTi wires having various starting microstructures due various heat treatments, are overviewed.  A mechanism of the instability of cyclic superelastic deformation of NiTi originating from the experimentally observed microstructure evolution is proposed and it is discussed how it might be related to the fatigue damage and failure.