Development of Ni3Ta-based High Temperature Shape Memory Alloys

The development of shape memory alloys operating at temperatures above 373K is a research topic relevant for designing new actuators/sensors able to exploit exhaustive gases and, in general, to increase the working temperature range of the conventional NiTi alloys. The Ni 3 Ta binary system can be considered a promising intermetallic compound because it offers transformation temperatures above 200°C and appears stable under thermal cycling. Like several HTSMAs, this alloy suffers from a poor plastic deformation workability, caused by its coarse microstructure and intrinsic brittleness. In this work, an alloying approach was evaluated with the goal of refining the microstructure without altering the transformation temperatures. Therefore, the alloy chemistry of Ni 3 Ta was modified by the addition of Cu or Co. Furthermore, the effect of heat treatments to tailor the microstructure was explored. The transformation temperatures of samples were characterized by Differential Scanning Calorimetry, the microstructure by Scanning Electron Microscopy and X-Ray Diffraction, and the mechanical properties by microhardness testing. It was observed that Co does not significantly affect the transformation temperatures, while the addition Cu decreases them slightly. Heat treatments were used to affect hardness and transformation temperatures; however, the alloy functional properties proved to be rather stable.