Relaxation behavior of martensitic NiTi SMA wires in tension

Martensitic NiTi shape memory alloys are increasingly used as adaptive materials as well as sensors and actuators in micro-technology. While stress relaxation phenomena (e.g., a time-dependent stress drop at constant strain) are of practical importance because they can affect the efficiency of simple control algorithms, relaxation at ambient temperature has hardly been analysed from a fundamental point of view before. In this study, we characterize the relaxation behavior of martensitic actuator wires under uniaxial tension. Our results show that the magnitude of the stress drop (which can be as high as 110 MPa) strongly depends on the total strain – as well as, surprisingly, on the strain rate during prior loading. Evaluating data from experiments covering a wide range of strain rates, we discuss our observations and we demonstrate that two different deformation mechanisms occur during prior loading (dominated by rate-independent twin boundary motion) vs. subsequent relaxation (driven by thermally activated dislocation motion). This view is supported by additional analyses of the experimental rate sensitivities in Haasen-like λ plots. Clearly, relaxation phenomena can be understood by quantifying the underlying microstructural deformation mechanisms. Most importantly, dislocation-twin boundary interactions determine the amount of relaxation in our NiTi wires.