Stress-Strain-Temperature Behavior Due to B2-R-B19′ Transformation in NiTi Polycrystals

Abstract

Thermomechanical behavior of superelastic NiTi wires undergoing sequential B2-R-B19′ martensitic transformation was investigated by two recently developed in-situ experimental methods (in-situ neutron diffraction and combined ultrasonic and electric resistance measurements) capable of detecting and recognizing the activity of various deformation/transformation processes in NiTi and theoretically by micromechanical modeling. An earlier model of SMA polycrystal transformation is further developed, so it accounts for the strains due to the R-phase related deformation processes in activated NiTi. A continuous variation of the rhombohedral distortion angle α of the R-phase structure with temperature and stress is newly introduced as a legitimate deformation mechanism. Simulation results for NiTi bars and wires exposed to three types of thermomechanical tests—mechanical loading at constant temperature, cooling under constant stress, and recovery stress tests are presented and confronted with results.