On the Oxidation Behavior and Its Influence on the Martensitic Transformation of Ti–Ta High-Temperature Shape Memory Alloys

In the present work, the influence of oxidation on the martensitic transformation in Ti–Ta high-temperature shape memory alloys is investigated. Thermogravimetric analysis in combination with microstructural investigations by scanning electron microscopy and transmission electron microscopy were performed after oxidation at 850 °C and at temperatures in the application regime of 450 °C and 330 °C for 100 h, respectively. At 850 °C, internal oxidation results in the formation of a mixed layered scale of TiO₂ and β-Ta₂O₅, associated with decomposition into Ta-rich bcc β-phase and Ti-rich hexagonal α-phase in the alloy. This leads to a suppression of the martensitic phase transformation. In addition, energy dispersive X-ray analysis suggests an oxygen stabilization of the α-phase. At 450 °C, a slow decomposition into Ta-rich β-phase and Ti-rich α-phase is observed. After oxidation at 330 °C, the austenitic matrix shows strong precipitation of the ω-phase that suppresses the martensitic transformation on cooling.