High temperature shape memory alloys (HT-SMAs) based on Ti-Ta show potential for actuator applications, for example in the fields automotive, aerospace and energy applications, where transformation temperatures are required, which are higher than what can be achieved with conventional NiTi-based SMAs. TiTa HT-SMAs show phase transitions at temperatures well above 100°C, the martensite finish (Mf) and austenite finish temperatures (Af) can be 365°C and 425°C, respectively. At these temperatures, there are a number of processes which affect the martensitic transformation, which governs all shape memory effects. These can include short range diffusion processes, rearrangements of dislocation substructures, precipitation processes and oxidation. In the present work we focus on oxidation. Up to now, the oxidation behavior of Ti-Ta alloys has received limited attention. Parabolic rate laws were observed in short term experiments (up to 16 hours) which were performed at temperatures between 800°C and 1400°C .
In the present study, the oxidation behavior of a Ti80Ta20 alloy is investigated at temperatures between 335 (< Mf) and 455°C (> Af). In addition one test was performed at 850°C to study oxidation in the absence of α-phase and to link our work to the previous investigation . Oxidation was investigated using thermogravimetric analysis (TGA) up to 100 hours. The formation of the oxide layer is associated with a change of alloy chemistry at the metal/oxide-interface.
Oxide layers and microstructures in the alloy/oxide layer region are analyzed by X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Preliminary TGA measurements at 850°C suggest a linear reaction kinetics. The oxide layer consists of two oxide phases based on TiO2 (tetragonal) and Ta2O5 (orthorombic). Special emphasis is placed on the effect of oxidation on the martensitic transformation processes near the oxide scale.
 Y. Park, D. Butt, Composition Dependence of the Kinetics and Mechanisms of Thermal Oxidation of Titanium- Tantalum Alloys, Oxid. Met. 51 (1999) 383–402.