Microstructural evolution and functional fatigue of a Ti-25Ta high-temperature shape memory alloy
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xander Paulsen, Dennis Langenkämper, Peer Decker, Jan Frenzel, Christoph Somsen, Alfred Ludwig, and Gunther Eggelerc) Institut für Werkstoffe, Ruhr-Universität Bochum, Bochum 44780, Germany
Thomas Niendorfb) Institut für Werkstofftechnik, Universität Kassel, Kassel 34125, Germany (Received 5 May 2017; accepted 14 July 2017)
Titanium–tantalum based alloys can demonstrate a martensitic transformation well above 100 °C, which makes them attractive for shape memory applications at elevated temperatures. In addition, they provide for good workability and contain only reasonably priced constituents. The current study presents results from functional fatigue experiments on a binary Ti–25Ta high-temperature shape memory alloy. This material shows a martensitic transformation at about 350 °C along with a transformation strain of 2 pct at a bias stress of 100 MPa. The success of most of the envisaged applications will, however, hinge on the microstructural stability under thermomechanical loading. Thus, light and electron optical microscopy as well X-ray diffraction were used to uncover the mechanisms that dominate functional degradation in different temperature regimes. It is demonstrated the maximum test temperature is the key parameter that governs functional degradation in the thermomechanical fatigue tests. Specifically, x-phase formation and local decomposition in Ti-rich and Ta-rich areas dominate when Tmax does not exceed 430 °C. As Tmax is increased, the detrimental phases start to dissolve and functional fatigue can be suppressed. However, when Tmax reaches 620 °C, structural fatigue sets in, and fatigue life is again deteriorated by oxygen-induced crack formation.
I. INTRODUCTION
There is substantial interest in shape memory alloys (SMAs) that can be operated at elevated temperatures, and SMAs with phase transformation temperatures higher than 100 °C are referred to as high-temperature shape memory alloys (HTSMAs). Still, these materials are only used for niche applications in industry. Today, most of the HTSMAs are based on the nickel–titanium system, where the phase transformation temperature has been shifted up by alloying with ternary elements, such as Pt,1–5 Pd,1–4,6 and Hf.7–11 The current approaches either call for high amounts of expensive noble elements or often result in systems with poor workability.7,8 It should be noted, however, that for Ni–Ti–Hf HTSMAs the workability
Contributing Editor: Yuntian Zhu Address all correspondence to these authors. a) e-mail: [email protected] b) e-mail: [email protected] c) This author was an editor of this journal during the review and decision stage. For the JMR policy on review and publication of manuscripts authored by editors, please refer to http://www.mrs. org/editor-manuscripts/. DOI: 10.1557/jmr.2017.319
issue can be overcome by appropriate processing, and even thin wires can be produced as successfully demonstrated lately.11 Lately, titanium–tantalum has been identified as a system that features substantial transformation strains from a marten
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