Aging Effect on the One-Way and Two-Way Shape Memory in TiNi-Based Alloys

The influence of aging of TiNi-based shape memory alloy (SMA) specimens on the one-way and two-way shape memory (TWSM) is studied experimentally and simulated with the use of a microstructural model. Experiments show that a long-time storage of preliminarily strained TiNi specimens does not change the strain recovery on heating. At the same time, the strain stroke due to the realization of the TWSM is bigger than before aging. Computer modeling of these phenomena is done within the frames of a microstructural model, in which the macroscopic strain is considered to be the average of the micro-strains-strains of micro-volumes. The irreversible strains are described from the standpoint of the plastic flow theory. Isotropic hardening and kinematic hardening are taken into account and are related to scattered and oriented deformation defects. To describe the effects of aging, special terms are introduced into the evolution equations for the defect densities. Thus, modified equations are able to account for a decrease of the yield limit due to a decrease of the deformation defect densities. The results of modeling show a good qualitative agreement with the experimental data, particularly an increase of the TWSM deformation after aging.