Constitutional Phases and Transformation Characteristics of Ni41Ti39Ta20 Alloy

The constitutional phases and transformation characteristics of the Ni41Ti39Ta20 alloy were carefully studied by means of optical microscopy, SEM, XRD, and DSC. The results showed that the microstructure of as-cast samples is a fully eutectic structure, which consists of the NiTi (B19-prime) phase and the Ta-rich phase. The homogenized and aged Ni41Ti39Ta20 alloy contains three phases: the NiTi phase, which is responsible for the shape memory effect, the Ta-rich phase, and a 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), and A(f) were 75, 50, 87, and 115 C, respectively. The results of the microstructure studies using SEM and XRD are presented and discussed. The change of microstructure and the distribution of the constitutional phases under different heat-treatment processes is reported. (Author)