Segregation-induced ordered superstructures
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 in their paper published in Science.
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. 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.
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