High-resolution deformation mapping of martensitic transformation and plasticity in superelastic Nickel-Titanium

Shape-memory alloys (SMAs) such as nitinol (NiTi) can recover large strains through a reversible stress- or temperature-induced martensitic transformation, but cyclic transformation degrades reversibility. Recent experimental evidence has linked this functional fatigue to the emission of dislocations from the fine martensitic microstructure that forms near the phase boundary, but the precise coupling mechanism between dislocation slip and martensitic microstructure is still widely debated. This creates a mesoscale gap in the understanding of SMAs and their fatigue: multiscale simulation is prohibitively expensive, while experimental methods that can spatially resolve fine microstructure and individual dislocations (e.g., transmission electron microscopy) cannot capture bulk mechanical behavior. In biomedical applications, understanding and modeling the mechanisms of slip localization and functional fatigue will be particularly crucial for the newest generations of ultra-high-purity nitinol, with flaw sizes below the theoretical crack length threshold. In the work to which this dataset is linked, we develop a new framework (MMICROSTRUCTURE) to reconstruct the geometrically necessary martensitic transformation and plastic slip in polycrystalline SMAs from high-resolution, full-field deformation maps. Using digital image correlation in a scanning electron microscope (SEM-DIC), we experimentally measure deformation with approximately 200 nanometer spatial resolution over a 0.5-millimeter field of view. We align this deformation data to austenite grain structure mapped via electron backscatter diffraction (EBSD). Using the MMICROSTRUCTURE framework, we quantitatively map the activity of individual slip systems and martensitic variants in each DIC subset. We show that localized networks of coupled slip and martensitic reorientation form microstructural "bridges" that propagate transformation through clusters of poorly oriented grains. The energy dissipated during bridging may be the origin of the unusual prestrain effects in nitinol, where higher prestrains (for example, crimping of a medical device) have been correlated to increased fatigue strength. Contrary to recent theories of functional fatigue focusing on a Type II twin interface, we observe that the most intense slip localization events are coupled to the development of Type I twins with a finite width.