Study of the stress-assisted two-way memory effect of a Ti–Ni–Cu alloy using resistivity and thermoelectric power techniques

The stress-assisted two-way memory effect has been studied for successive cycles in a Ti–Ni–Cu alloy for two different thermo-mechanical treatments: recovery annealed (TT425) and fully recrystallized. Simultaneous tensile strain and electrical resistance measurements have been performed as a function of temperature and for different applied stresses. The thermoelectric power (TEP) in the stressed martensitic state has also been measured. For the treatment TT425 corresponding to 30% cold drawing followed by 425 °C annealing, a stable reversible strain εm-β of 4.5% at maximum is found. A relatively low plastic strain εp occurs for stresses above 125 MPa, which increases with the number of thermal cycles. For the recrystallized treatment, the maximum amplitude of the reversible strain is almost the same but a quite large plastic strain occurs even at 50 MPa and strongly increases with the number of cycles, reaching 12% for 20 cycles at 200 MPa. It is shown that the electrical resistivity change, associated with the reverse transformation, increases linearly with the reversible strain εm-β. This effect is related to the electrical resistivity anisotropy of the martensite, with the slope being slightly different for the two treatments. In addition, the resistivity is relatively insensitive to the plastic strain. The TEP and the resistivity are sensitive to the orientation of the martensite, but the TEP also strongly depends on the plastic strain.