Unusual composition dependence of transformation temperatures in Ti-Ta-X shape memory alloys

Abstract

Ti-Ta-X (X = Al, Sn, Zr) compounds are emerging candidates as high-temperature shape memory alloys (HTSMAs). The stability of the one-way shape memory effect (1WE), the exploitable pseudoelastic (PE) strain intervals as well as the transformation temperature in these alloys depend strongly on composition, resulting in a trade-off between a stable shape memory effect and a high transformation temperature. In this work, experimental measurements and first-principles calculations are combined to rationalize the effect of alloying a third component to Ti-Ta based HTSMAs. Most notably, an increase in the transformation temperature with increasing Al content is detected experimentally in Ti-Ta-Al for low Ta concentrations, in contrast to the generally observed dependence of the transformation temperature on composition in Ti-Ta-X. This inversion of trend is confirmed by the ab-initio calculations. Furthermore, a simple analytical model based on the ab-initio data is derived. The model can not only explain the unusual composition dependence of the transformation temperature in Ti-Ta-Al, but also provide a fast and elegant tool for a qualitative evaluation of other ternary systems. This is exemplified by predicting the trend of the transformation temperature of Ti-Ta-Sn and Ti-Ta-Zr alloys, yielding a remarkable agreement with available experimental data.

Structure and Properties of Ti-Al-Ta and Ti-Al-Cr Cladding Layers Fabricated on Titanium

“…Despite the lack of information in the literature on the formation of the ω-related phases in ternary Ti-Al-Ta alloys (to the best of our knowledge, the ω phase in alloys with a composition similar to that discussed in our study was mentioned only in [39]), it can be expected that the effect of tantalum and niobium being both refractory β-stabilizing elements, is similar. At least, binary Ti-Ta alloys [40], and Ti-Ta alloys with minor additions of aluminum [41,42], are prone to form the ω phase. According to Rietveld analysis, the ω volume fraction in the Ti-41Al-7Ta sample was low (about 1.1%).…”

Unexpected variations in the kinetics of solid solution alloys due to local interactions

“…We exemplify this here for Ti-Ta alloys that have recently been suggested a promising materials for high-temperature shape memory applications [17,56]. In particular, it was shown that the functional properties are strongly composition dependent [17,[57][58][59], and at higher temperature diffusion and phase separation can play an important role in the transformation between different phases and, correspondingly, impact the functional properties.…”

Laboratory-Scale Processing and Performance Assessment of Ti–Ta High-Temperature Shape Memory Spring Actuators

“…The influence of oxidation on the martensitic transformation in the surface region was investigated [27]. Atomistic calculations on the stability of phases, especially the x-phase, were performed [28][29][30]. Special emphasis was placed on the role of the x-phase during functional fatigue of Ti-Ta alloys [31][32][33][34] and it has been shown that by controlling the temperature during heating and cooling one can counteract fast functional degradation by x-phase precipitation.…”