Ti-Ta-X shape memory alloys are emerging candidates for high-temperature applications. The shape memory effect in these alloys is based on the martensitic transformation between the low-temperature α'' phase (orthorhombic) and the high-temperature β phase (body-centered cubic); however, the shape memory behaviour can be damaged by the formation of the hexagonal ω phase.
The stability and the martensitic temperature of Ti-Ta-X alloys are found to depend strongly on the alloy composition. Experimentally, it has been found that alloying p-valent elements, such as Al and Sn, or hcp d-valent elements, such as Zr, stabilizes the shape memory effect, but in most cases decreases the transformation temperature; this often results in the need to compromise between a stable martensitic transformation and a high transition temperature.
In this contribution, electronic structure calculations are used to explore how the alloying of a third element to Ti-Ta affects the stability of the shape memory effect as well as the martensitic transformation temperature. In particular, it will be analyzed how different chemical compositions change the relative stability of the α'', β and ω phases; based on the first-principles data, an analytical model that describes the compositional dependence of the transformation temperature can be eventually derived and used to guide experimental alloy development. This work shows how first principle calculations can in fact be exploited to propose new candidates to design novel stable and high-temperature shape memory alloys.