A microstructure-based constitutive model for the pseudoelastic behavior of NiTi SMAs

Experiments showed that the pseudoelastic behavior of NiTi shape memory alloys (SMAs) subjected to pure tension and pure torsion is distinctly different, which can be attributed to the different deformation-induced-transformation microstructures. SEM observation shows uniformly distributed parallel texture on the surface of the NiTi SMA microtubes subjected to pure torsion, indicating alternatively arranged parallel lamellas of austenite and martensite phases. During pure tension, it can be observed that martensite initiate from the parent phase and grow to macroscopic bands, and, correspondingly, a typical stress drop can be observed in the tensile stress–strain curve. A microstructure-based constitutive model is proposed for the NiTi SMAs by three stages: (1) the constitutive relationships for the individual martensite phase and austenite phase, respectively; (2) the model for a representative volume element (RVE) composed of alternatively arranged parallel lamellas of austenite and martensite; and (3) the description for the NiTi SMAs by considering the materials as aggregates of numerous cells with different orientations and making use of Hill’s self-consistent scheme. The proposed model can satisfactorily describe the main characteristics of the pseudoelastic behavior of NiTi shape memory alloys under pure tension and pure torsion. The capability for the model to be applied to the pseudoelastic responses of NiTi SMAs under complex triaxial deformation is also discussed.