Effect of plastic slip on thermomechanical behavior of NiTi polycrystals investigated by micromechanics modelling

Earlier developed micromechanics crystallographic model of SMA polycrystals has been modified by incorporating plastic deformation as additional deformation mechanism. It is assumed that dislocation slip proceeds in austenite and B19' martensite phases in parallel with the processes derived from B2-B19' martensitic transformation in NiTi alloy. The model was used to simulate the responses of NiTi polycrystal in three thermomechanical cycles typically carried out in the SMA research. Based on the simulation results, it is proposed that, due to the dislocation slip occurring simultaneously with transformation related processes, the stresses and strains in the transforming polycrystal are significantly redistributed which in turn modifies the macroscopic thermomechanical responses. In tensile tests at low temperature, the plastic deformation occurs in martensite phase after the stress induced transformation or martensite reorientation, while at medium and high temperatures both deformation mechanisms proceed in parallel. Depending on whether the plastic deformation in tensile tests had taken place in the austenite (typically at high temperatures) or in the martensite (typically at low temperatures) phase, distribution of internal stresses is quite different. Surprisingly, significant plastic deformation activity is predicted for the cooling/heating tests under constant tensile stress. Plastic deformation occurs in this case mainly in the martensite state and redistribution of stress plays significant role. Results of the simulation of the cyclic tensile test at constant temperature explain qualitatively most of the features of the unstable superelastic stress-strain curves observed in cyclic loading experiments. It is proposed that the actual reason for accumulation of unrecovered strain during cyclic loading is in fact the continuously evolving distribution of internal stresses, strains and phase fractions - it causes the plastic deformation to progress even if the macroscopic maximum strain remains constant and peak stress decreases upon cyclic loading.