Carnegie Mellon Professor's Research Illuminates Why Some Metal Mixtures Don’t Quite Hold Together
By Ben Panko
If you zoomed in far enough on many solid objects, you would see countless microscopic crystals linked together. While these crystals, called grains, can be beautiful to behold, it's the space between them that plays a critical role in determining the strength and durability of manufactured materials.
In a paper published in the Oct. 6 issue of Science, Carnegie Mellon University Professor of Physics Michael Widom and his colleagues showed what happens at the grain boundaries of one particular alloy of the metals nickel and bismuth that makes it brittle.
The boundaries between the grains are what hold bulk polycrystalline materials, such as metals, together. The adhesion of the grain boundaries determine the material’s strength. While most metals are known for their strength, some mixtures, or alloys, do not hold together so well, making them unsuitable or dangerous to use.
Bismuth-nickel alloys, for example, can tend to be brittle. This can be a concern as the shiny, dense bismuth is often used in solder and fuses, while lightweight nickel is used in materials such as airplane wings.
"It's a very bizarre material," Widom said of bismuth. When bismuth and other metals are heated up and mixed together to make an alloy, bismuth "likes to work its way into the space between the grains where the grains meet at those boundaries."
Using advanced electron microscopes, Widom’s collaborators at Lehigh University scrutinized these microscopic grain boundaries at an atomic level. In a "very heroic experimental program" they discovered that when grains met, the bismuth and nickel atoms realigned into lattices to form layered superstructures at the grain boundaries. These superstructures had previously been thought to exist only rarely in some alloys. Finding it at many different boundaries led the team to conclude that these superstructures are probably much more common than many people had thought.
The nature of these superstructures also explains what causes the alloy's weakness. While the bismuth atoms bind well to the nickel atoms, they don't bind nearly as well to other bismuth atoms. This means the layers of the superstructures at the grain boundaries between the two elements are connected weakly, making the overall alloy brittle.
For his contribution to the paper, Widom worked with his student Qin Gao to complement the electron microscope analysis using calculations of energy. At grain boundaries in general, some arrangements required more energy than others to maintain the necessary bonds, Widom said. "Low-energy boundaries will tend to stick together more," Widom explained, "because nature likes to drive itself toward low energies."
By computing the amounts of energy that would be required for different arrangements on various grain boundary surfaces, Widom was able to predict what form the superstructures would likely take in these places.
"We looked at several surfaces and predicted the structures that ought to occur, and those were the structures that were seen," Widom said.
While the weakness of this particular nickel-bismuth alloy was already known before this paper confirmed it, Widom sees this work as paving the way for future experiments that could save materials science researchers and manufacturers time and energy.
"The techniques that are used here can be used in principle to investigate other combinations theoretically before you actually make the materials and try them out," Widom said.
The paper was co-authored by Qin Gao and Gregory Rohrer of Carnegie Mellon; Zhiyang Yu, Denise Yin and Martin Harmer of Lehigh University; Patrick Cantwell of the Rose-Hulman Institute of Technology and Yuanyao Zhang, Naixie Zhou and Jian Luo of the University of California, San Diego. The research was funded by grants from the Office of Naval Research Multidisciplinary University Research Initiatives and the U.S. Department of Energy.