Surprising results from looking under the surface of nickel-titanium single crystals with 3D X-ray diffraction

It has become increasingly clear over the past decade that local microstructure effects dictate the overall performance of SMAs, and researchers are either directly or indirectly using this information to make monumental advances in SMA design. The ability to spatially resolve and quantify these local microstructure effects at the nano- and microscale will be essential for continuing these advances. We present on a number of surprising findings recently made by probing the interior nano- and microstructure of bulk single crystal nickel-titanium samples using modern 3D X-ray diffraction techniques High-Energy Diffraction Microscopy (HEDM) and Dark-Field X-Ray Microscopy (DFXM). First, we show that the widely accepted maximum work criterion for predicting martensite microstructures does not work with materials that have defects (e.g., precipitates, inclusions, subgrains). Second, we show that the kinematic constraints at the austenite-martensite interface have both short-range and long-range effects on the austenite phase, including interfacial strain fields that extend orders of magnitude farther into the microstructure than was originally assumed.