Understanding the solidification of microstructure is the key to controlling it.  This holds especially true for eutectics, which exhibit outstanding mechanical or electrical properties because their polyphase microstructures act as natural composite materials.  It remains to be determined how the eutectic pattern develops in strongly anisotropic (or ‘irregular') systems, in which one of the solid phases grows faceted from the liquid. There exist conflicting accounts in the literature on the degree of diffusive-coupling between the solid phases at the growth front, and the shapes of the crystals left frozen in the bulk solid.  For this reason, irregular eutectics have persisted as an enigma for the past half-century.  

Here, we obtain first-ever nanoscopic and 3D insights on irregular eutectic solidification enabled by new developments in synchrotron-based x-ray nanotomography by transmission x-ray microscopy (TXM). We study via TXM the irregular Al-Al₃Ni eutectic as a model system.  At low velocity (V) in directional solidification, we observe both coupling and decoupling of the two solid phases. At higher V, the lead distance of the faceted Al₃Ni phase is reduced or even eliminated, and its shape ceases to be governed by faceted growth in a liquid. We observe a faceted to rod-like transition upon increasing V.  In this talk, we will discuss the interrelationships between coupling, velocity, alloy composition, and interfacial morphology of broad relevance to irregular eutectic alloys.  The results enable us to test the predictions of simulations and theory and suggest new avenues for the expansion of theory.  They also provide a first glimpse into the origin of anisotropic patterns, with implications to the design of novel microstructures.

Ashwin J. Shahani is an Associate Professor of Materials Science and Engineering at the University of Michigan. His group specializes in the application of advanced characterization methods for the study of phase and structural transformations in metallic materials. Shahani’s research has broad, technical impact on the manufacturing of tailored microstructures via solidification and thermomechanical processing. In support of his research pursuits, Shahani has won the AFOSR YIP award in 2017, ARO YIP in 2018, and NSF CAREER in 2019.  Recently, he won PECASE, the highest honor bestowed by the federal government to young scientists beginning their academic careers.  His citation reads: “for his development of state of the art characterization tools for capturing materials evolution during processing in situ, his tenacious outreach efforts to underrepresented students in the state of Michigan, and his leadership in the materials science community.”