Electrical Resistivity Changes during Transformations in Ti-Ni Alloy under Constrained Tensile Stress

In Ti-Ni shape memory alloys annealed below the recrystallization temperature after cold working, successive transformations from the parent B2 phase to the trigonal R phase and subsequently to the monoclinic B19' phase during cooling have been confirmed via electrical resistivity measurements. However, the behavior of resistivity during successive transformations under tensile stress remains unclear.

This study investigates the effect of tensile stress on resistivity changes during the successive forward and reverse transformations during cooling and heating, respectively. An electrical resistivity measurement system equipped with stress-loading and temperature-control mechanisms was used. The experiments were conducted on a Ti-50at% Ni alloy wire, 0.075 mm in diameter, cold-drawn and subsequently heat-treated at 460℃.

Under stress-free conditions, the forward transformation process results in an increase in resistivity from B2 to R, followed by a decrease from R to B19'. During the reverse transformation process, resistivity increases from B19' to R and decreases from R to B2. At stresses exceeding even 100 MPa, resistivity changes during the forward transformation process still follow the B2 to R to B19' sequence. However, during the reverse transformation, no increase in resistivity corresponding to B19' to R is observed. Instead, resistivity decreases sharply due to the reverse transformation directly from B19' to B2.

Ti-Ni alloy wire under 400 MPa stress exhibits a change in resistivity of -16%/℃ during heating. This value is significantly larger compared to the change of thermistors, which is -2 to -3%/℃.