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.