Electrical resistivity of NiTi with high transformation temperature

For a better understanding of the transformation behaviour on NiTi, the electrical resistivity of Ni49.5Ti50.5 was precisely measured by a four-probe potentiometric method. The temperature dependence of the electrical resistivity is linear in the regions of the high- and low-temperature phases, and a drastic change in electrical resistivity is observed in the temperature range of the transformation. The transformation remains of the one-stage between the high- and low-temperature phase even with thermal cycling, although with increasing the number of thermal cycles the electrical resistivity versus temperature curve shifts to the low temperature side and the electrical resistivity increases in the single phase temperature region which is attributable to the generation of transformation-induced defects. The one-stage transformation changes to a two-stage transformation due to the appearance of an intermediate phase after an incomplete thermal cycle, and with subsequent thermal cycling the peak in the electrical resistivity is enhanced to stabilise the intermediate phase. The as-cut Ni49.5Ti50.5, of which the surface is damaged, shows a two-stage transformation, even during the first cooling after cutting, and a resistivity-peak due to such a two-stage transformation is also enhanced with thermal cycling. It is thought that the amount of defects plays important roles in the shifts of transformation temperatures and in the transfer from one-stage to the two-stage transformation, and contributes to the stability of each phase. (C) 2003 Elsevier B.V All rights reserved.