Eutectic materials, naturally occurring composites of two or more crystals, are used in engine blocks, solder and 3D printing. Often, such applications involve heating the materials, which leads to changes in their microstructure that can affect their mechanical properties, such as strength. Using the Advanced Photon Source, a U.S. Department of Energy (DOE) Office of Science user facility at DOE’s Argonne National Laboratory, a team of researchers has learned how the microstructure evolves upon heating, which may allow them to change the synthesis of eutectics to improve those mechanical properties.
They studied a model silver-aluminum-copper alloy composed of three phases, one silver-rich, one aluminum-rich and one copper-rich. They heated the material to 773K and annealed it for four hours at that temperature. The material started out as three crystals that were interwoven in a structure resembling a ladder. When heated, the material tries to find its lowest energy state by lengthening the interfaces between the crystals. The microstructure coarsens, with some of the crystals becoming larger at the expense of others.
Much of the theory about eutectics is based on microstructures that have a small volume of one phase embedded in another. That theory predicts that the material would be self-similar, appearing identical at different size scales. In the model system, with three phases making up equal fractions of the volume, researchers were surprised to find no self-similarity. Instead, the microstructure evolved in part by coalescence. Rods of the silver-rich phase, for instance, would grow and become thicker until they touched each other, then they would merge into one rod. That evolution was irreversible. Such a change in the microstructure alters the mechanical properties of the material.
Additionally, the three phases did not coarsen independently of each other, but rather affected how the others evolved. When neighboring silver-aluminum rods coalesce, they pushed out the copper-rich channels that had existed between them. That is one reason for the lack of self-similarity in the evolving material.
Read more on Argonne website
Image: A eutectic material (left) contains three phases—silver-aluminum (red), aluminum-copper (blue) and aluminum (green). At an elevated temperature, the Ag2Al rods coarsen over time from zero hours (center) to 4 hours (right), where they have coalesced.
Credit: the University of Michigan