Two-stage transformation of aluminum-containing NiTi

Abstract

Measurements of electrical resistivity, ultrasonic velocity and attenuation for equiatomic NiTi and Ni 50Ti 49Al 1 were performed in order to characterize the thermoelastic martensitic transformation during cooling. The NiTi shows a one-stage transformation, essentially, because a premartensitic phenomenon is negligibly faint. The ultrasonic anomalies in NiTi appear near the temperature at a faint resistivity-peak, which correspond to lattice softening at the start temperature of martensitic transformation from the high-temperature phase to the low-temperature phase. The resistivity of Ni 50Ti 49Al 1 has a negative temperature coefficient over the wide temperature range from room temperature to154 K, and indicates a large peak. Such an anomaly in resistivity corresponds to an enhanced premartensitic phenomenon, where ultrasonic anomalies caused by a two-stage transformation are observed which accompany a large change in electrical resistivity. However, a resistivity peak in the aluminum-containing NiTi is formed at lower temperature than those at ultrasonic anomalies by over 40 K, and the temperature at the resistivity peak is not one characterizing the transformation behavior. It proved helpful to measure the ultrasonic properties in comparison with the electrical resistivity in order to characterize successive transformations in the temperature range of a premartensitic phenomenon of NiTi enhanced by addition of aluminum.