Abstract
Ti75Ta25 high-temperature shape memory alloys exhibit a number of features which make it difficult to use them as spring actuators. These include the high melting point of Ta (close to 3000 °C), the affinity of Ti to oxygen which leads to the formation of brittle α-case layers and the tendency to precipitate the ω-phase, which suppresses the martensitic transformation. The present work represents a case study which shows how one can overcome these issues and manufacture high quality Ti75Ta25 tensile spring actuators. The work focusses on processing (arc melting, arc welding, wire drawing, surface treatments and actuator spring geometry setting) and on cyclic actuator testing. It is shown how one can minimize the detrimental effect of ω-phase formation and ensure stable high-temperature actuation by fast heating and cooling and by intermediate rejuvenation anneals. The results are discussed on the basis of fundamental Ti–Ta metallurgy and in the light of Ni–Ti spring actuator performance.
Highlights
Towards the turn of the millennium materials science and technology of shape memory alloys (SMAs) had reached a level, which allowed to realize many interesting shape memory applications in different fields of engineering and in medical technology [1,2,3,4,5]
Ti75Ta25 high-temperature shape memory alloys exhibit a number of features which make it difficult to use them as spring actuators
From the results obtained in the present work the following conclusions can be drawn: One can successfully develop an experimental procedure which allows to produce Ti75Ta25 spring actuators on the lab scale
Summary
Towards the turn of the millennium materials science and technology of shape memory alloys (SMAs) had reached a level, which allowed to realize many interesting shape memory applications in different fields of engineering and in medical technology [1,2,3,4,5]. At the time Ni–Ti based SMAs were in the focus of interest, because their structural and functional properties made them the commercially most successful SMAs. In the last two decades traditional and new processing techniques were analyzed from a fundamental and technological point of view [6,7,8,9,10]. New alloy systems received considerable attention, including Ni-free shape memory alloys [11], highentropy SMAs [12,13,14,15] and high-temperature SMAs (HTSMAs) [16,17,18,19]. Ma et al [18] have pointed out that SMAs with high transformation temperatures can enable simplifications and improvements in operating efficiency of many functional components designed to operate at temperatures above 100 °C in the automotive, aerospace, manufacturing and energy exploration industries. Special emphasis was placed on the role of the x-phase during functional fatigue of Ti–Ta
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