Elementary Transformation and Deformation Processes and the Cyclic Stability of NiTi and NiTiCu Shape Memory Spring Actu
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INTRODUCTION
SINCE the discovery of shape memory effects (SMEs),[1,2] the possibility of deforming a metal to strains significantly higher than the elastic strain and to reverse the associated shape changes by simply heating or unloading (one-way SME (1WE), and pseudoelasticity (PE)) has fascinated scientists and engineers.[3–16] A number of shape memory alloys (SMAs), including NiAl, CuZnAl, CuAlNi, AuCd, AgCd, CuZnGa, NiTi, and others[4,8] show this behavior. It is well known that the SMEs rely on the martensitic phase transformation. On cooling from the high-temperature phase austenite, an SMA starts to transform into martensite at a temperature MS (martensite start temperature). At MF (martensite finish temperature), the transformation is completed. During subsequent heating, the reverse transformation starts and finishes at AS and AF (austenite start and finish temperatures, respectively). Martensitic transformations are typically associated
Ch. GROSSMANN and T. DEPKA, Ph.D. Students, J. FRENZEL, Research Associate, and G. EGGELER, Professor, are with the Institute for Materials, Ruhr University Bochum, 44801 Bochum, Germany. Contact e-mail: [email protected] V. SAMPATH, Professor, is with the Department of Metallurgical and Materials Engineering, Indian Institute of Technology Madras, Chennai600 036, India. Manuscript submitted November 25, 2008. Article published online September 14, 2009 2530—VOLUME 40A, NOVEMBER 2009
with thermal or mechanical hysteresis, i.e., the forward transformation starts at a lower temperature (thermal transformation) or at a higher stress (stress-induced transformation) and the reverse transformation begins at a higher temperature (heating of martensite) or a lower stress (unloading of stress-induced martensite).[3–8] In the present study, we focus on the functional properties of NiTi-based spring actuators that exploit the 1WE. In NiTi-based SMAs, phase transition temperatures can be adjusted through the Ni content,[17–19] although a typical binary NiTi-alloy actuator material will contain close to 50.0 at. pct Ni. The NiTi-based SMAs outperform other SMAs (including CuZnAl and CuAlNi) in terms of their thermomechanical performance.[9,14,20] Actuators typically have fully martensitic microstructures at ambient temperature. The Cu additions to NiTi (up to 10 at. pct, replacing Ni in NiTi) improve several properties that are important for actuators. They cause a decrease in the width of the thermal hysteresis and a reduction in the strong sensitivity of the transformation temperatures to small variations in the Ni content; in addition, they suppress resistivity anomalies, thus allowing for better actuator control.[21–27] Today’s shape memory (SM) technology is strongly driven by PE applications in the medical field,[14,28] and even though the 1WE appears more fascinating and has found highly successful niche applications (in the areas of fashion, decoration, gadgets, couplings, fasteners, microactuators, adaptive materials, and hybrid composites[9]), no real METALLURGICAL AND MATERIAL
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