Structural and Functional Stability of Ti-30Ta High Temperature Shape Memory Alloys

Ti-Ta high temperature shape memory alloys (HT-SMA) exhibit martensite transformation temperatures (M_s) up to 200°C as well as excellent workability at ambient temperatures, which renders them promising candidates for shape memory applications in automotive and aerospace industries.

We investigated the thermal stability of a Ti-30Ta alloy (M_s ~ 180 °C), which was produced by arc-melting and rolled to sheets. Thermal ageing of Ti-30Ta at intermediate temperatures (200-400°C) leads to formation of thermal ω phase within the β matrix. Ageing is accompanied by a diffusional decomposition of the β phase into a Ti-rich ω-phase and a Ta-rich β phase. The enrichment of the β matrix with Ta as a β stabilizing element results in a decrease of M_s. Therefore, formation of thermal ω phase causes austenite stabilization in Ti-Ta alloys and has a detrimental effect on the SME.

A better understanding of structural, thermodynamic and kinetic mechanisms is of utmost importance for the development of new Ti-Ta-based alloys with high structural and functional stability. We investigated microstructures and atomic structures using TEM, XRD and synchrotron radiation in order to shed light on structural relations of α”, β and ω phase in Ti-Ta alloys as well as thermodynamic and kinetic processes, which bias the SM-effect.