Netsu Sokutei, 28 (1), p. 2, (2001)



Anomaly in Electrical Resistivity with Thermoelastic Martensitic Transformation of Shape Memory NiTi Alloy

For a better understanding of the thermoelastic martensitic transformation in near equiatomic NiTi, the measurement of electrical resistivity is precisely performed by a four-probe potentiometric method. The transformation behavior of NiTi depends on the martensitic transformation start temperature (Ms) and thermal cycling with a repetition of transformation. The NiTi with a higher Ms shows a one-stage transformation, of which the Ms decreases with increasing the number of complete thermal cycles. Moreover, a resistivity hump appears in the vicinity of Ms after an incomplete thermal cycle, which correspond to the formation of the R-phase. On the other hand, The NiTi with a lower Ms has a tendency to show a two-stage transformation and its peak in resistivity is enhanced by complete thermal cycles to result in further enlargement of the temperature range of R-phase. The resistivity in the high-temperature phase is nonlinear to temperature, which imply that a pre-martensitic phenomenon extend over a wide range of temperature. The shift of the transformation temperature and the enhancement of the R-phase with increasing thermal cycling are taken to be attributable to the accumulation of transformation-induced defects.