Abstract
NiTi shape memory alloys (SMA) are attractive functional materials that are widely used in different fields of engineering and medicine for production of various shape-memory devices. The severe plastic deformation (SPD) is one of the most effective methods for the formation of an ultrafine-grained structure (UFG) in NiTi SMA, which is characterized by the best combination of functional properties. The development of SPD methods continues in the direction of the investigation of new approaches allowing to produce bulk samples with a nanocrystalline structure. Severe torsion deformation (STD) is a promising SPD method that allows for high plastic strain accumulation without significant dimensional changes. It was established that application of STD provides simultaneous improvement of strength and ductility of studied alloys. However, STD has never been applied to NiTi SMA. Therefore, in this study, STD was applied to NiTi SMA at various deformation temperatures from 350 to 600°C in order to maximize the number of turns and accumulate significant strain. Deformation in this temperature range leads to the formation of a dynamically polygonised dislocation substructure, that must provide conduction of the highest number of turns and facilitate the formation of an UFG structure. The goal of this work is to evaluate the possibility of applying the torsional deformation to bulk NiTi samples at low temperatures (in the range of dynamic polygonization) to accumulate high strains and refine the structure.
