  Deformation heterogeneities within microstructures of polycrystalline shape
memory alloys (SMAs) during superelastic stressing are studied using both
experiments and simulations. In situ X-ray diffraction, specifically the
far-field high energy diffraction microscopy (ff-HEDM) technique was used to
non-destructively measure the grain-averaged statistics of position, crystal
orientation, elastic strain tensor and volume for hundreds of austenite grains
in a superelastically loaded nickel-titanium (NiTi) SMA. This experimental data
were also used to create a synthetic microstructure within a finite element
model. The development of intragranular stresses were then simulated during
tensile loading of the model using anisotropic elasticity. Driving forces for
phase transformation and slip were calculated from these stresses. The
grain-average responses of individual austenite crystals examined before and
after multiple stress-induced transformation events showed that grains at the
specimen interior carry more stress and plastically deform less than the
surface grains as the superelastic response shakes down. Examination of the
heterogeneity within individual grains showed that regions near grain
boundaries exhibit larger stress variation compared to the grain interiors.
This intragranular heterogeneity is more strongly driven by the constraints of
neighboring grains than the initial stress state and orientation of the
individual grains.
