High temperature shape memory alloys (HT-SMAs) based on Ti-Ta show potential for actuator applications. Their use gets especially interesting in cases, where transformation temperatures, above the range achievable by conventional NiTi-based SMAs, are required. At these temperatures, there are a number of processes affecting the martensitic transformation, which governs all shape memory effects. These can include short range diffusion processes, precipitation processes and oxidation, which will be focused on in the present work.
In the present study, the oxidation behavior of a Ti80Ta20 alloy is investigated at temperatures of 330°C (< Mf), 450°C (> Af) and up to 850°C by using thermogravimetric analysis (TGA) for 100 hours. Oxide layers and microstructures are analyzed by X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Special emphasis is placed on the effect of oxidation on the martensitic transformation behavior.
At low temperatures of 330°C the oxidation’s effect is comparable weak, while formation of the omega phase governs the material’s functional degradation. Oxidation just above Af leads to a different degradation mechanism due to segregation into the alpha and beta phases. This effect is especially pronounced near the sample surface and grain boundaries, as oxygen diffuses into the sample. After high temperature oxidation strong internal oxidation and therefore strong segregation can be observed.
This shows the importance of studies to enhance the phase stability of HT-SMAs like Ti-Ta, as ways to compensate other degradation mechanisms like the formation of the omega phase for example via heat treatment recovery  are reported.