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This paper presents an investigation of the influence of strain rate on the functional behaviour of TiNi alloy. Specimens were deformed in compression and tension modes using the Kolsky method for ...

Cyclic life under loading-unloading cycles of a precipitation hardening austenitic steel Fe – 0.40 С – 18 Mn – 2 Si – 2 V steel (mass percent) specimens was studied experimentally and simulated by means of a microstructural model. This steel with the shape memory possessing rather high strength around 1000 MPa demonstrates also rather good fatigue properties withstanding 20000 cycles at the maximum stress of the loading-unloading cycle 800 MPa. The microstructural model of the deformation of FeMnSi-based SMA, which takes into account the specific features of the martensitic transformation and the micro plastic deformation, and supplemented with a deformation-and-stress criterion of fracture proved to be an adequate tool for describing the fatigue fracture of steels undergoing fcc-hcp martensitic transformation. A good agreement between the calculation and the experimental data was obtained. Thus, this model can be expected to be suitable for estimations of the cyclic life of SMA working elements under various thermal and mechanical loadings.

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.

This study makes use of the magnetic pulse method for providing the uniaxial tension of TiNi shape memory alloy specimens. Finite element simulations demonstrate good agreement between the evaluated residual strains and experimental values. The evaluated average strain rates are ~ 4000–5000 s−1 and in local areas, they reach 10,000–12,000 s−1. The functional properties of the alloy after magnetic pulse tension are shown and compared with the results after quasistatic tension. The values of the shape memory effect after magnetic pulse tension decrease by 15–20%. Magnetic field simulation shows that induced currents are negligible and do not lead to heating in the working part of the specimens. It is concluded that the reason for the decrease in the shape memory effect is the high pre-strain rate. Reorientation processes must be sensitive to the strain rate, so the proportion of the oriented martensite decreases with increasing strain rate.

The influence of long-term storage of TiNi-based and CuZnAl alloys on their functional properties was investigated. It was established that during 30 years’ storage of TiNiFe thermomechanical couplings, the stresses in them practically did not decrease. Couplings with CuZnAl alloy sleeves during storage weakened slightly. The one-way shape memory effect in equiatomic TiNi alloy did not change after 25 years. It was found that the value of two-way shape memory effect in the first cycle after storage at room temperature in martensitic state for more than 15 years was higher than before storage.

The paper presents data on the mechanical testing of argon-arc weld joints of the Ti–55.42 wt% Ni wires diameter of 2 mm. The mathematical modeling results of the temperature fields evolution of the weld zone during cooling are given. For this calculations residual stress mechanism of shape memory effect was used. The microstructure images of the weld zone were done.

The influence of prestrained material storage in the martensitic state on the two-way shape memory effect was investigated. The study was conducted on cylindrical specimens of equiatomic TiNi alloy (M s =74°C), prestrained by compression: with high strain rate from 750 to 1000 s -1 and quasi-statically with strain rate 10 -3 s -1 . The storage time was almost 17 years. The dynamic and quasi-static case were compared. It was found that after the long storage time, the value of the two-way shape memory effect increases. Increase of two-way shape memory could be associated with relaxation of the internal stresses during storage.