Influence of the mechanical loading history on the stress assisted two way memory effect in a Ti-Ni-Cu alloy

Abstract

In this work, several tests of thermal cycling under constant load are carried out on Ti-45.0Ni-5.0Cu (at%) shape memory wires. The properties related to the Stress Assisted Two Way Memory Effect (SATWME) of the material are investigated as a function of the mechanical loading history for the same temperature range during cooling and heating. For this reason, two thermomechanical tests have been employed: tests I, where one sample is used for just one constant stress level test and tests II, where only one sample is employed for several constant stress level tests in sequence. The results obtained show that for loads applied below 150 MPa, the transformation temperatures and the thermal hysteresis associated with the transformation of the material are the same during the two tests. However, above 150 MPa in tests II transformation temperatures and thermal hysteresis are respectively higher and smaller than the ones obtained in tests I. On the other hand, transformation temperatures obtained from both tests are in good agreement with the ones measured by DSC and electrical resistance measurements. It is also observed that the SATWME obtained by tests II is smaller than the one measured during tests I. It is shown that these different behaviors are induced by accumulation of plastic strain in the sample during tests II.

On two-step strain–temperature behavior in Cu–Zn–Al shape memory alloy wire actuators

“…As indicated in figure 9(d), for stresses as high as 100 MPa, a relative stability of SME was observed, but accompanied by an important accumulation of plastic strain under load. These behaviors are similar to those in Cu-25.9Zn-4.0Al (wt%) ribbons by Benchiheub et al [21] and in Ti-Ni based thin wires [6,22].…”

Coupled stress-strain and electrical resistivity measurements on copper based shape memory single crystals

“…Electrical resistivity measurements coupled with thermomechanical tests in Ti-Ni polycrystalline alloys have confirmed a linear variation of ER as a function of strain 11,12 . The ER was also used to study the Two Way Shape Memory Effect (TWSME) phenomenon as a function of stress, strain and number of cycles during training process 9,[12][13][14] .…”

“…The ER was also used to study the Two Way Shape Memory Effect (TWSME) phenomenon as a function of stress, strain and number of cycles during training process 9,[12][13][14] .…”

Analysis of the transformations temperatures of helicoidal Ti-Ni actuators using computational numerical methods

“…The evaluation of the mechanical behavior and the shape memory effect of the actuator was performed by repeating 40 times the heating and cooling cycle for each applied load (stress) of 35, 70, 105, 135, 170 and 200 MPa 15,16 . Figure 2 shows the main parameters from a generic thermo-elastic deformation curve.…”

“…During the training process, M s temperature decreases with number of cycles for small stress value, while for stress above 135 MPa the evolution is the inverse. In the first case, the M s temperature decreases because of dislocation reorientation which modifies the internal stress and points the martensitic variant in the preferential direction 16,24 . In the second case, the M s temperature increases for high stress due to fast martensitic reconfiguration and exhaustion (practically all martensitic variants were immediately activated by stress) 23 .…”