3D in-situ characterization of dislocation density in nickel-titanium shape memory alloys using high-energy diffraction microscopy

Functional fatigue—changes to the material response during cyclic loading—is a major barrier to the cycle lifetime demands of shape memory alloy technologies. Functional fatigue is caused by permanent changes to the microstructure such as the generation of dislocations during the forward and reverse martensitic phase transformation. In this work, far-field and near-field high-energy diffraction microscopy (ff- and nf-HEDM) are used to characterize the local accumulation of geometrically necessary dislocation (GND) density in the austenite phase in situ and in 3D across a bulk Ni49.9Ti50.1 polycrystalline shape memory alloy during load-biased thermal cycling. A custom nf-HEDM data analysis procedure is used to reconstruct spatially-resolved intragranular misorientation maps that are then converted to spatially-resolved GND density maps. In this way, GND density is tracked in individual grains across cycles. We find that neither Schmid factor nor the maximum transformation work correlates strongly with GND density evolution during load-biased thermal cycling. The results show that the spatially-resolved GND density is distributed heterogeneously, but GND density increases faster near grain boundaries, in grains at the sample surface, and in grains with large volumes, indicating that these regions/types of grains will undergo different functional fatigue behaviors. Finally, the effect of grain neighborhood and grain boundaries on GND evolution are investigated, highlighting the role played by grain boundaries and the grain neighborhood in the evolution of GND density. This work demonstrates the utility of nf-HEDM for understanding the evolution of subgrain-scale plastic deformation, including materials that undergo a martensitic phase transformation.