Termomechanics of laser annealed 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 thermomechanical treatment of semi-finished products from NiTi using laser annealing mechanical constrains enables to set the shape and functional properties in a wide range. The subject of the PhD dissertation is a general analysis of how the laser treatment can be used to modify the martensitic transformation proceeding in NiTi wires and ribbons in terms of transformation stresses and strains, localization, accompanying plasticity and fatigue properties. In more detail the laser surface annealing under stress is to be studied with the aim to introduce compressive stresses in order to improve the fatigue performance. The aim of the PhD dissertation is a better understanding the relation between the thermomechanical behaviour of NiTi wires or ribbons and parameters of laser annealing under mechanical constraints.