Suffolk Reporter

By Patricia DeLacey

A recent study led by researchers from the University of Michigan (UMich) has shed light on the fundamental principles of eutectic solidification using real-time, nanoscale imaging. Beamline scientist Xianghui Xiao collaborated with UMich researchers to investigate the solidification mechanism of an aluminum-nickel alloy. Utilizing a state-of-the-art transmission X-ray microscope at the Full Field X-ray Imaging (FXI) beamline at the National Synchrotron Light Source II, a U.S. Department of Energy (DOE) Office of Science User Facility at DOE’s Brookhaven National Laboratory, the team combined real-time radiographic imaging within a limited angle range with high-resolution tomographic imaging innovatively.

The data collected helped unveil mechanisms controlling the formation of different microstructures as solids self-assemble. This understanding could lead to tuning material properties such as strength and flexibility for various applications. Xiao hosted this student project with sponsorship from DOE’s Science Graduate Student Research program.

During eutectic solidification, a mixture of two or more solids self-assembles into composite microstructures ranging from ordered layers to intricate maze-like patterns that influence properties like tensile strength or ductility. Researchers have struggled to understand what conditions drive eutectics to form specific patterns, which is crucial for designing reproducible next-generation eutectic composites.

Capturing real-time solidification of an aluminum-nickel eutectic alloy (Al-Al3Ni) in nanometer resolution revealed that increasing the solidification velocity shifts microstructure from irregular and faceted to regular and rounded, according to a study published in Acta Materialia by UMich researchers.

Leveraging this new understanding will help tune microstructure for materials used in high-temperature components in turbines or reactors.

“I have always been captivated by patterns in nature—like snowflakes, where no two are ever identical. This fascination with how such seemingly simple processes can give rise to endlessly rich, complex, and unique structures drives me to explore the underlying principles behind them,” said Ashwin Shahani, an associate professor of materials science and engineering and chemical engineering at UMich and senior author of the study.

“In materials science, the same kind of wonder applies: how do small changes in conditions lead to dramatically different microstructures?”

To better understand how eutectic microstructures form, the research team designed a new in-situ furnace at the synchrotron beamline for directional solidification—a technique where crystal growth from liquid to solid phase is oriented in a specific direction. The apparatus allows precise control over the solidification processing space, enabling detailed study of pattern formation during eutectic solidification.

To correlate nanoscale observations with microscale phenomena, researchers combined optical microscopy for large space- and time-scale observation with synchrotron transmission X-ray microscopy for nanoscale insights. The latter was conducted at Brookhaven National Laboratory’s full-field X-ray imaging beamline 18-ID.

This approach allowed direct observation of interactions between liquid aluminum (Al) and nickel aluminide (Al3Ni) crystals during solidification under different conditions. The growth rate comparison between Al3Ni and Al determined the shape of resulting solid microstructures. At lower velocities, Al3Ni grows ahead causing irregular growth; at higher velocities, both grow simultaneously resulting in rounded growth.

In casting processes, several factors including thermal conductivity and heat extraction rate influence solidification velocity.

“Our first-of-its-kind experiments and real-time observations help explain the diversity of patterns produced by eutectics containing stiff intermetallic phases,” said Paul Chao, doctoral graduate at UMich who spent 2022 as a resident researcher at Brookhaven's synchrotron beamline and first author of the study.

“Our experiments serve as an example of how excellent mentorship at UMich, partnership with Brookhaven National Laboratory, and international collaboration are critical to cutting-edge research resolving fundamental scientific enigmas.”

This finding holds broad relevance across various eutectic systems including metallic, semi-metallic, and organic types.

“Manipulating these patterns is more than just a technical pursuit—it is a way to unravel fundamental principles and apply them meaningfully,” added Shahani.

The research received support from multiple sources including NSF CAREER program (1847855), Air Force Office of Scientific Research United States (FA9550-21-1-0260), and DOE (DE-SC0012704; 2021 Office of Science Graduate Student Research Award).