Constitutional Phases and Transformation Characteristics of Ni<sub>41</sub>Ti<sub>39</sub>Ta<sub>20</sub> Alloy

In this paper, the constitutional phases and transformation characteristics of the Ni(41)Ti(39)Ta(20)alloy have been carefully studied by means of optical microscopy, SEM, X-ray diffraction, and DSC. The results showed that (1) the microstructure of as-cast samples is a fully eutectic structure, which consists of the NiTi (B-19) phase and the Ta-rich phase; (2) The homogenized and aged Ni41Ti39Ta20 alloy contains three phases: the NiTi phase, which is responsible for the shape memory effect, Ta-rich phase and Ti-rich compound. The EDS study indicates that the Ta-rich phase is BCC beta-Ta with a minor amount of Ni and Ti. The transformation temperatures (as-cast alloy) M-s, M-f, A(s), A(f) were 75 degrees C, 50 degrees C, 87 degrees C and 115 degrees C, respectively. The results of the microstructure studies using SEM and X-ray diffraction will be presented and discussed. The change of microstructure and the distribution of the constitutional phases under different heat-treatment processes will be reported.