Pseudo-4D view of the growth and form of crystallographically ‘locked’ eutectic colonies

Abstract

A number of fundamental questions remain unanswered regarding the interior structure of eutectic cells or colonies growing into a liquid. Here, we observe experimentally the solidification behaviour of an Al-Al2Cu eutectic as a model system using a novel imaging approach that combines in situ X-radiography and ex situ X-ray tomography. The former allows us to trace the solid-liquid interfaces while the latter enables us to visualize the solid-solid interfaces. When put together, we can visualize in 4D (i.e., three-dimensional space plus time) the development of a crystallographically ‘locked’ eutectic microstructure in the presence of an oscillatory eutectic-liquid velocity driven by thermosolutal convection. These details provide 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 morphological transition reflects a delicate balance between the motion of trijunctions and the stiffness of low-energy, solid-solid interfaces. Our pseudo-4D tomography approach holds broad appeal to the solidification science community, as it can temporally resolve the solidification process on the order of seconds and spatially resolves individual lamellae on the order of micrometres, thereby overcoming the space-time trade-off in conventional in situ X-ray tomography.