High-temperature shape memory alloys (HTSMAs) with phase transformation temperatures higher than 100 °C received considerable attention for technical applications, e.g. in the aerospace and automotive sectors. HTSMAs based on the Titanium-Tantalum system have been proposed as attractive candidates because they show martensite start temperatures higher than 200°C. In addition, Ti-Ta alloys exhibit a good workability and the costs of alloying elements are reasonably low. In the range of 20 to 40 at.-% Ta the austenitic β-phase (bcc) transforms on cooling to the martensitic αˮ-phase (orthorhombic). However, in the binary Ti-Ta system the formation of a nano-scaled hexagonal ω-phase in the temperature regime between 100 and 400 °C can be observed. The ω-phase suppresses the martensitic αˮ-phase associated with a loss of functional properties.
In the present study the temperature dependent formation and dissolution behavior of the ω-phase in a Ti-rich alloy are investigated by synchrotron radiation and in situ transmission electron microscopy. Synchrotron heating and cooling experiment with a temperature rate of 20°C/min was performed on an initial martensitic Ti75Ta25 alloy heating from ambient temperature to 650°C. On heating at about 300°C synchrotron data reveals a transformation from the martensitic αˮ-phase to austenitic β-phase. On further heating in the austenitic temperature regime at about 400°C faint reflections of the ω-phase appear, increasing in intensity by increasing of temperature and time, respectively. At about 500°C the reflections of the ω-phase disappear and at temperatures up to 650°C only reflections of the austenitic β-phase are present. On cooling of the austenitic state from 650°C at temperatures of about 400°C again reflections of the ω-phase appear beside that of the austenitic β-matrix. On further cooling down to ambient temperatures only the reflections of the ω-phase and β-phase can be detected revealing the suppression of the martensitic αˮ-phase. In agreement post mortem transmission electron microscopy investigations on the Ti75Ta25 sample after thermal cycling during the synchrotron experiment were performed showing an austenitic matrix with ω-phase. In addition, in situ TEM investigations on a Ti75Ta25 sample containing a larger volume fraction of the ω-phase confirm the dissolution at temperatures of about 550°C.
Financial support by Deutsche Forschungsgemeinschaft within the Research Unit Program “Hochtemperatur-Formgedächtnislegierungen” (Contract nos. MA 1175/34-2, EG 101/22-2, FR 2675/4-2 and SO 505/2-2) is gratefully acknowledged.
Dennis Langenkämper1, Alexander Paulsen1, Christoph Somsen1*, Jan Frenzel1, Elvira Karsten2, Hans Jürgen Maier2 and Gunther Eggeler1
1 Institut für Werkstoffe, Ruhr-Universität Bochum, Universitätsstr. 150, 44801 Bochum, Germany
*corresponding author: firstname.lastname@example.org
2 Institut für Werkstoffkunde (Materials Science), Leibniz Universität Hannover, 30823 Garbsen, Germany