Martensitic transformation and anomalies in resistivity of (Ti-50Ni)1-xCx (x=0.1, 0.5 at.%) shape memory alloys

Phase transformation of solid solution (Ti-50M)(1-x)C-x (x = 0.1, 0.5 at. %) alloys have been studied by using differential scanning calorimetry, physical property measurement system and optical microscope. The transformation temperature decreases due to the existence of titanium carbide (TiC) particles compared with that of near-equiatomic Ti-Ni shape memory alloy. The resistivity vs. temperature curves show hysteresis. Thermoelastic martensitic transformation occurred in two alloys despite the difference in TiC content. Nevertheless, the resistivity results show different martensitic transformation routes. A one-step B2 -> B19' transformation occurred in the low TiC content alloy and an R transformation appeared in another alloy, suggesting that the martensitic trans formation routes depended on the TiC content. The cumulative effect of the TiC particles causes the local stress field and lattice distortion to restrain the transformation of the B19'. On the other hand. the TiC content has an effect on the temperature coefficient of electrical resistivity (TCR) of alloys. The Ti-Ni-0.5C alloy shows a negative TCR in the range 100-300 K during which transformation Occurs. Another alloy shows the opposite result. The cause of the negative TCR is briefly discussed. (C) 2008 Published by Elsevier B.V.