Microsope and Mathematics Put Atomic Theories to the Test
By Michael J. Martinez
April 23 — For centuries people have tried to strengthen metals by combining them: creating alloys of two or more metals in hopes of producing a stronger, lighter and more malleable material.
Until now that process has proceeded by trial and error, though
there have been notable successes, like the titanium-aluminium alloys
used in the hulls of airplanes. But what if scientists could develop
a mathematical model for creating the strongest, lightest alloys
The electron microscope at Johns Hopkins sends a stream of electrons through an atom and then through a prism filter. By measuring the differences in how the electrons react, researchers can identify the atom in question. Click on the interactive box to see a history of the microscope's development. (ABCNEWS.com)
Using a new electron microscope, researchers at Johns Hopkins
University in Baltimore and at Northwestern University in Chicago are
studying how individual atoms in an alloy are connected, and how
those connections change when the metal is bent and shaped. With that
information they may be able to develop just such a mathematical
model for the creation of alloys.
Every Atom in Its Place
Under a grant from the U.S. Air Force, the two universities are working on new materials for eventual use in fighter jets and other aircraft.
Views of titanium aluminide alloy at increasing magnifications. The top images shows a small grain of the alloy about 10mm in diameter. In the bottom image, individual atoms can be seen. (John Balk/Hemker Lab)
The first step, according to Johns Hopkins researcher Kevin Hemker, is to identify the atoms and their crystalline connections within the alloy. To do that, Hemker’s team uses an electron microscope with a field-emission gun, developed by Philips Electron Optics.
There are no more than two dozen electron microscopes in the country, Hemker points out. “It’s the field-emission gun that makes this one special.”
The gun sends an extremely narrow beam of electrons — less than a nanometer (one billionth of a meter) across — through a single atom and then through a prism. The electrons that pass through the atom are slowed considerably. When the electrons then pass through the prism, the paths of the slower electrons are altered much more than the paths of those that did not pass through the atom. By measuring those changes, the team can identify the atom under examination.
The next step is to study how those atoms arrange themselves into crystals, and how those crystals interact when the metal is bent and shaped into objects. Bending an object causes defects in the crystals called “dislocations” to occur. Those dislocations are difficult to correct.
“Think of a coat hanger,” Hemker says. “When you bend it, it’s relatively easy. But try to bend it back into the shape it held before. That’s a lot harder, and that’s because the atoms were packed tighter around where you bent it, and aren’t as willing to bend back.”
The John Hopkins team plans to study different alloys — aluminumtitanium and copper-gold, for example — to see not only how they’re formed on an atomic level, but how their atomic structures are affected when the material is bent.
Mathematics and Metallurgy
Identifying and altering individual atoms in an alloy crystal is one thing — figuring out what to do with the information is something else. That’s where the Materials Research Center at Northwestern comes in.
A team of researchers led by physics professor Art Freeman has been preparing mathematical computer models of different alloys, to show how distortions rearrange the crystalline latticework of atoms. Using mathematical values for each alloy, they will then compare those findings with Hemker’s results.
“If they agree with our predictions, we say ‘A-ha!,’ the theory is really developed and is on the right track,” says Freeman. “If not, we have to go back and find out which assumptions were incorrect.”
In the end, they are hoping to create a purely mathematical system to determine which atoms would make the best alloys. In this way, Hemker says, the trial-and-error metallurgy of centuries past could give way to computer simulations of new and promising alloys.
“We would be able to see how any element, introduced into an alloy structure, would change it.”
With millions of possible combinations, scientists eventually hope to find the perfect alloy — stronger than steel, lighter than down.
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S U M M A R Y
A powerful new electron microscope and some new mathematical models may aid researchers in the quest for the ultimate alloy.
“If they agree with our predictions, we say ‘A-ha!,’ the theory is really developed and is on the right track. If not, we have to go back and find out which assumptions were incorrect.” Art Freeman, Northwestern University
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