Localized deformation processes in NiTi based shape memory alloys


NiTi shape memory alloys (SMAs) in form of wires and ribbons are being used in medical devices and technical applications as they exhibit large reversible deformations (e. g. vascular stents) and can actuate cyclically upon heating and cooling (e. g. switches, actuators, thermal regulators). These functional properties stem from thermally and stress induced changes in microstructure (crystal lattice) called martensitic transformations, which can proceed reversibly in SMAs unlike in steels, where they are accompanied with plastic deformation resulting in the hardening effect. The stress-induce martensitic transformation in NiTi is localized; it nucleates and proceeds in shear bands of transformation deformation (http://ofm.fzu.cz/localized-deformation-of-niti-in-tension). The shear band fronts act as stress concentrators and, hence, contribute to the overall low fatigue performance of SMAs being the key obstacle to development of novel NiTi-based applications. The subject of the PhD dissertation is analysis of origins, mechanisms, and morphology of shear banding in NiTi with regard to loading modes, sample shapes, and microstructure parameters. Localized plastic deformation accompanying the martensitic transformation is yet one another aspect of the subject. The ultimate goal of the dissertation is a better understanding of localized deformation processes in NiTi based alloys on the level of continuum mechanics.