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Studying the theoretical strength of materials takes researchers down a new path.
Biting a gold ring is an age-old test of authenticity, but, in fact, extended line defects or dislocations within the atomic structure are what render this precious element so malleable. Using dislocation-free gold, workers could, at least in theory, build skyscrapers. For years, scientists have been studying the theoretical strength of metals, which can be 1,000 times greater than their typical strength because of defects and dislocations that prevent atoms from occupying their perfect positions. The smaller a sample, the closer the material comes to achieving maximum theoretical strength as the defects are, in effect, eliminated until all that remains is a very tiny, defect-free sample. Researchers have been intrigued, however, by the difficulty in demonstrating this theoretical strength using a recently developed method that employs a focused ion beam to hew micrometer-sized pillars, which are then compressed to test for strength. While the theoretical strength of metal whiskers was reached decades ago using a tensile test to pull each whisker apart , researchers have found that in compression testing many of the recently made pillars did not respond as expected. Easo George, an ORNL materials scientist with a part-time appointment at the University of Tennessee, says he and colleagues thought the focused ion beam technique might also introduce dislocations, thereby weakening the micro-pillars. While doing work on a "completely unrelated" project, the researchers stumbled upon an entirely new way of forming these tiny metallic shapes. Using the new method, the researchers produced micro-pillars using a xenon arc lamp to grow an in situ composite of nickel aluminide and molybdenum fibers through a process known as floating zone directional solidification. An acid wash then etches away the nickel aluminide matrix, leaving behind thousands of free-standing monocrystalline molybdenum pillars. "Now that we have these naturally made pillars , we do not need to do any additional work," says George. "We can change their size by altering the rate at which we grow them—a much cleaner process." ORNL researchers used an instrument called a nano-indenter to conduct compression tests on the new pillars and found that all the tested pillars yielded at the theoretical strength. "When we demonstrated this result, many of the people who were testing the pillars made by the ion beams took notice," George says. "This discovery now allows us to compare experimental results and theoretical predictions in a much better way and to carefully introduce dislocations into each pillar in order to study their effects in a well-controlled manner." While interesting at a basic discovery level, the research also has potential applications. As electronic devices, sensors and other technologies become tinier, predicting and testing the behavior of materials in very small amounts become increasingly important, George says. In addition, because nanoscale features are needed to strengthen even bulk materials, a growing need exists to isolate and characterize such features. "We cannot simply push toward the theoretical strength limit," George says. "We also must make materials tougher and more ductile. But with at least a factor of 100 between what is feasible theoretically and what is typical, we have a lot of room for improvement. Understanding the nature and behavior of dislocations will be important in pushing us toward the goal of dramatically improved properties of materials."—Larisa Brass |
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