Heterogeneity and inelasticity of deformation in a notched martensitic NiTi shape memory alloy specimen

The low symmetry martensite phase in Nickel-Titanium (NiTi) shape memory alloy (SMA) has a hierarchical microstructure with micro-scale crystallites, which themselves consist of nano-laminates with an internal twin structure. The martensite phase deforms inelastically though reorientation of the twin structure. Furthermore, structural features in the specimens such as cracks or notches interact with the microstructure, influencing the deformation of martensite. In this work, using multi-scale experiments and macro-scale modeling, we demonstrate that the deformation of martensite in NiTi at the macro scale in a notched specimen is largely determined by the structural features, and the hierarchical nature of the microstructure plays a smaller role. At the micro-scale, in-situ measurements indicate that the martensite microstructure continuously evolves during tensile loading. However, the nano-scale twin laminate structure persists despite the large imposed deformation and the presence of stress concentrating features such as a notch. This observation is counter to the intuition of martensite twins reorienting to large domains of the most favored crystallographic variant at larger loads. This persistence of a nano-laminate structure rather than coarsening into large domains of individual variants indicates that the deformation field around cracks/notches in martensite may be modeled for fracture-related phenomena with adequate accuracy using coarse-grained models, without a compelling need for martensite single-variant-scale models.