2, Northwestern University, Evanston, Illinois, United States
Shape memory alloys (SMAs) – a class of functional materials with commercial applications in the biomedical industry – deform primarily by the mechanisms of reversible martensitic phase transformation. The elements of microstructure in SMAs such as nano-scale precipitates, impurity inclusions, and grain boundaries constrain their deformation in various ways. We combined in-situ high-energy X-ray diffraction microscopy (HEDM), a synchrotron-based 3D, non-destructive technique to obtain grain-scale lattice strains and crystal orientations with microstructural modeling to study the influence of these microstructural elements on specific deformation phenomena. First we will present our general methodology for combining data from HEDM to inform and then validate a microstructural model for phase transformation. Then we will present mechanisms for two phenomena in single and polycrystalline NiTi SMAs. In a single crystal, transformation strain produced was lower than theoretically possible due to constraint from inclusions. In a polycrystal, cyclic loading produced spatially heterogeneous residual strains due to constraint from neighboring grains. This combination of HEDM and modeling can also be applied to study the deformation mechanics of SMA devices such as cardiovascular stents.