Impact of Heating–Cooling Rates on the Functional Properties of Ti–20Ta–5Al High-Temperature Shape Memory Alloys
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Due to their ability to provide a shape memory effect at elevated temperatures, high-temperature shape memory alloys (HT-SMAs) came into focus of academia and industry in the last decades. Ternary and quaternary Ni–Ti-based HT-SMAs have been in focus of a large number of studies so far. Ti–Ta HT-SMAs feature attractive shape memory properties along with significantly higher ductility and lower costs for alloying elements compared to conventional Ni–Ti-based HT-SMAs, which qualifies them as promising candidate alloys for high-temperature applications. Unfortunately, precipitation of undesired phases, e.g., the ω-phase, leads to significant functional degradation upon cyclic loading in binary Ti–Ta. Therefore, additions of ternary elements, such as Al, which suppress the ω-phase formation, are important. In the present study, the influence of different heating–cooling rates on the cyclic functional properties of a Ti–20Ta–5Al HT-SMA is investigated. Transmission electron microscopy as well as in situ synchrotron analysis revealed unexpected degradation mechanisms in the novel alloy studied. Elementary microstructural mechanisms leading to a degradation of the functional properties were identified, and the ramifications with respect to application of Ti–Ta–Al HT-SMAs are discussed.
KeywordsShape memory alloy (SMA) Martensitic transformation Functional degradation Martensite stabilization Precipitation
Financial support by the German Research Foundation (DFG) within the Research Unit Program “Hochtemperatur-Formgedächtnislegierungen” (Project Number 200999873; Contract Nos. FR2675/3-2; NI1327/3-2; MA1175/34-2; SCHM 930/13-2; SO505/2-2 and EG101/22-2) is gratefully acknowledged. DESY (Hamburg, Germany), a member of the Helmholtz Association HGF, is thanked for the provision of experimental facilities. Parts of this research were carried out at PETRA III. Jozef Bednarcik is thanked for assistance in using the photon beamline P02.1 and the support laboratory.
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