  Near-field and far-field high-energy diffraction microscopy and microcomputed
tomography X-ray techniques were used to study a bulk single crystal
nickel-titanium shape memory alloy sample subjected to thermal cycling under a
constant applied load. Three-dimensional in situ reconstructions of the
austenite microstructure are presented, including the structure and
distribution of emergent grain boundaries. After one cycle, the subgrain
structure is significantly refined, and heterogeneous {\Sigma}3 and {\Sigma}9
grain boundaries emerge. The low volume and uneven dispersion of the emergent
{\Sigma} boundaries across the volume show why previous transmission electron
microscopy investigations of {\Sigma} grain boundary formation were
inconsistent.
