| Shape Memory Alloys (SMAs) undergo a reversible martensitic transformation (MT) that may be thermal- or stress-induced enabling the recovery of large deformations, up to 12% strain, can be fully recovered, albeit a hysteresis exists. Characterizing the hysteresis for classes of SMAs is essential for their practical application. It is well known that the hysteresis is due to energy dissipative mechanisms, with the primary mechanisms being frictional resistance to interfacial motion and partial plastic accommodation of shape and volume changes. In previous work, however, the origins of potential dissipative mechanisms have not been clarified, and the factors that influence these mechanisms (i.e. external stress fields, matrix strength properties, precipitate microstructure, plastic accommodation, detwinning, etc.) have not been investigated with rigorous experimentation. In this study, an experimental program is designed to address this issue. This comprehensive experimental program, including stress-free thermal cycling, constant load thermal cycling, and constant temperature stress/strain cycling, is undertaken for NiTi, CoNiAl, NiFeGa, and NiMnGa SMAs in the aged and unaged states. The NiFeGa and NiMnGa classes of SMAs undergo martensite to martensite inter-martensitic transformations. The experimental findings characterize the effect of second-phase particles and inter-martensitic transformations on the level of hysteresis.;To ascertain physical mechanisms responsible for the hysteresis, in-situ transmission electron microscopy (TEM) analysis provides insight into microstructure features. The results expose the role of frictional resistance and partial plastic accommodation on the observed differential magnitudes of thermal and stress hysteresis. In particular, larger hysteresis magnitudes are mainly attributed to dislocation emissions at the austenite/martensite phase boundary, designated micro-scale plasticity, that lower coherent interface strains, and thus, relax the stored elastic strain energy. Remarkably, the effect of relaxation of coherent interface strains can be seen without significant levels of remnant macro-scale strain.;Influences of energy dissipative mechanisms are rationalized using a theoretical approach which combines micro-mechanics and thermodynamics into a thermo-mechanical framework. Models are formulated for the thermal and stress hysteresis based on a thermo-mechanical framework so as to shed light into differential hysteresis magnitudes. Current theoretical formulations consider energy dissipation from a phenomenological point view and satisfy the second law of thermodynamics by considering a dissipative potential. In the current study, an earlier thermo-mechanical formulation is advanced beyond phenomenological considerations to include the effects of micro-scale plasticity. With this, the hysteresis models can account for energy dissipation resulting from strain energy relaxation. The current modeling approach provides intuitive predictions of the experimentally observed behavior particularly a growing hysteresis with increasing stress for NiTi alloys. |
