Elementary Transformation and Deformation Processes and the Cyclic Stability of NiTi and NiTiCu Shape Memory Spring Actuators |
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Authors: | Ch Grossmann J Frenzel V Sampath T Depka G Eggeler |
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Affiliation: | (1) Institute for Materials, Ruhr University Bochum, 44801 Bochum, Germany;(2) Department of Metallurgical and Materials Engineering, Indian Institute of Technology Madras, Chennai, 600 036, India;; |
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Abstract: | The present work addresses functional fatigue of binary NiTi and ternary NiTiCu (with 5, 7.5, and 10 at. pct Cu) shape memory
(SM) spring actuators. We study how the alloy composition and processing affect the actuator stability during thermomechanical
cycling. Spring lengths and temperatures were monitored and it was found that functional fatigue results in an accumulation
of irreversible strain (in austenite and martensite) and in increasing martensite start temperatures. We present phenomenological
equations that quantify both phenomena. We show that cyclic actuator stability can be improved by using precycling, subjecting
the material to cold work, and adding copper. Adding copper is more attractive than cold work, because it improves cyclic
stability without sacrificing the exploitable actuator stroke. Copper reduces the width of the thermal hysteresis and improves
geometrical and thermal actuator stability, because it results in a better crystallographic compatibility between the parent
and the product phase. There is a good correlation between the width of the thermal hysteresis and the intensity of irrecoverable
deformation associated with thermomechanical cycling. We interpret this finding on the basis of a scenario in which dislocations
are created during the phase transformations that remain in the microstructure during subsequent cycling. These dislocations
facilitate the formation of martensite (increasing martensite start (M
S
) temperatures) and account for the accumulation of irreversible strain in martensite and austenite. |
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