  We investigate solidification of an Al-Al2Cu as a model system to understand
the emergence of patterns (such as lamellar, rod and maze-like) within eutectic
colonies. To uncover the morphological transitions in-situ and in 3D, we
introduce here a new synchrotron-based, X-ray imaging procedure. Our method
simultaneously maximizes the temporal (200 ms) and spatial resolution ($0.69^2
\mu m^2/pixel$) over that of traditional imaging approaches. The wealth of
information obtained from this procedure enables us to visualize the
development of a crystallographically `locked' eutectic microstructure in the
presence of thermosolutal convection. This data provides direct insight into
the mechanism of the lamella-to-rod transition as the eutectic accommodates
fluctuations in interfacial composition and growth velocity. We find that this
transition is brought about by impurity-driven forces acting on the
solid-solid-liquid trijunction that must overcome the stiffness of the
solid-solid interfaces. Our pseudo-4D imaging strategy holds broad appeal to
the solidification science community, as it can overcome the space-time
trade-off in conventional in situ X-ray microtomography.
