  Thermomechanical processing such as annealing is one of the main methods to
tailor the mechanical properties of materials, however, much is unknown about
the reorganization of dislocation structures deep inside macroscopic crystals
that give rise to those changes. Here, we demonstrate the self-organization of
dislocation structures upon high-temperature annealing in a mm-sized single
crystal of aluminum. We map a large embedded 3D volume ($100\times300\times300$
$\mu $m$^3$) of dislocation structures using dark field x-ray microscopy
(DFXM), a diffraction-based imaging technique. Over the wide field of view,
DFXM's high angular resolution allows us to identify subgrains, separated by
dislocation boundaries, which we identify and characterize down to the
single-dislocation level using computer-vision methods. We demonstrate how even
after long annealing times at high temperatures, the remaining low density of
dislocations still pack into well-defined, straight dislocation boundaries
(DBs) that lie on specific crystallographic planes. In contrast to conventional
grain growth models, our results show that the dihedral angles at the triple
junctions are not the predicted 120$\degree$, suggesting additional
complexities in the boundary stabilization mechanisms. Mapping the local
misorientation and lattice strain around these boundaries shows that the
observed strain is shear, imparting an average misorientation around the DB of
$\approx 0.003-0.006 \degree{}$
