Many established, but also potential future applications of NiTi-based shape memory alloys (SMA) in biomedical devices and solid-state refrigeration require long fatigue life with 10 7 -10 9 duty cycles 1,2 . However, improving the fatigue resistance of NiTi often compromises other mechanical and functional properties 3,4 . Existing efforts to improve the fatigue resistance of SMA include composition control for coherent phase boundaries 5-7 and microstructure control such as precipitation 8,9 and grain-size reduction 3,4 . Here, we extend the strategy to the nanoscale and improve fatigue resistance of NiTi via a hybrid heterogenous nanostructure. We produced a superelastic NiTi nanocomposite with crystalline and amorphous phases via severe plastic deformation and low-temperature annealing. The as-produced nanocomposite possesses a recoverable strain of 4.3% and a yield strength of 2.3 GPa. In cyclic compression experiments, the nanostructured NiTi micropillars endure over 10 8 reversible-phase-transition cycles under a stress of 1.8 GPa. We attribute the enhanced properties to the mutual strengthening of nanosized amorphous and crystalline phases where the amorphous phase suppresses dislocation slip in the crystalline phase while the crystalline phase hinders shear band propagation in the amorphous phase. The synergy of the properties of crystalline and amorphous phases at the nanoscale could be an effective method to improve fatigue resistance and strength of SMA.The fatigue of NiTi SMA is reflected in both decreases in the functional properties such as martensitic transformation stress and strain (that is, functional fatigue), and structural damage via cracking (that is, structural fatigue) 10 . The functional fatigue of NiTi is attributed to transformation-induced dislocations that arise from microscopic strain incompatibility at the moving phase boundaries (habit planes) between austenite (B2 cubic) and martensite (B19′ monoclinic) phases 6,11,12 , and is coupled with structural fatigue via crack nucleation and propagation that eventually lead to catastrophic fracture of the material 10,13,14 . In contrast to the large recoverable strain and limited resistance to dislocations of crystalline NiTi, amorphous bulk metallic glasses have high strength, but show limited recoverable strain and are very susceptible to deformation via localized shear bands 15 . Yet, nanosized metallic glasses can show a large recoverable strain of roughly 4.4% and enhanced resistance to shear bands that require a critical length scale (approximately 100 nm) to nucleate 16 . Similarly, transformation-induced dislocations in the crystalline NiTi can be substantially suppressed via reducing the grain or crystal size to the nanoscale 3,4 . Therefore, one possible strategy to obtain high-performance NiTi is to create unique heterogenous multi-phase nanostructures of specifically controlled critical length scales in the material, where the proper-

The dynamic response of nonlinear torsional vibration with phase transformable NiTi Shape Memory Alloy (SMA) wire is investigated by experiment in this paper. The thermomechanical responses of NiTi wire as a softening nonlinear damping spring in the torsional vibration system are measured by synchronized acquisition of the rotational angle and temperature under external excitation. Frequency Response Curves (FRCs) at fixed excitation amplitude and Amplitude Response Curves (ARCs) at fixed frequency are obtained in the frequency and amplitude domains respectively. It is found that, as the deformation of NiTi wire goes into the softening nonlinear phase transition region, the smooth and stable dynamic responses along one branch of FRC or ARC will gradually enter into metastable region and eventually become unstable and drastically switch to a new contrasting alternative stable state along the other branch. The jump phenomenon between the alternative stable states on the lower and upper branches of the FRC or ARC and the hysteresis between the jump-up and jump-down are identified in experiments. In addition, the effects of internal and external disturbance (both magnitude and direction) on triggering the jumps between the alternative stable states along the two metastable branches are examined in the time domain. The stability of the nonlinear dynamic response is analyzed by the Duffing oscillator model and interpreted via the stability landscape. For the first time, we directly reveal the alternative stable states and jump phenomena of thermomechanical responses by experiments in the frequency, amplitude and time domains. The results not only showed the important roles of phase transition nonlinearity in bringing multiple equilibrium states and their fast switches, but also provided a solid experimental base for the identification of metastable regions as well as further management of the undesired dynamic responses of vibration system where NiTi is used as a nonlinear damping spring.