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" The formation and evolution of dislocation cell structures (patterning) is one of the most important aspects of the deformation process in ductile metals," according to Lyle Levine and colleagues at the National Institute of Standards and Technology (NIST), Oak Ridge National Laboratory (ORNL), and the University of Southern California. Metallurgists already know that this patterning process arises as clusters of dislocations interact with each other while recent diffraction-resolved studies have shown that the process is initiated by crystal lattice breakup. However, predicting exactly how a metal will behave under stress remains an unresolved problem. Now, Levine and his colleagues have turned to a twenty-year old model to help them out. When metals deform, the neat crystal structure breaks into a complex three-dimensional web of crystal defects called "dislocation walls" that enclose cells of dislocation-free material. These dislocation structures are directly responsible for a metal's mechanical behaviour. Twenty years ago, Häel Mughrabi suggested that stresses in the dislocation walls and the cell interiors would be different and have opposite signs. Levine's work provides direct evidence for Mughrabi's model. "Scientifically, these stress fluctuations are probably the single most significant finding of the work since no previous measurements even hinted at their existence," explains Levine, "A few researchers had speculated that such variations might exist but they had no clue about their size and distribution." The researchers used an X-ray probe at the Advanced Photon Source at Argonne National Laboratory to gather the first direct evidence for a twenty-year-old model of metallic stresses and strains in a copper sample. Their measurements are, however, anything but clear cut. The results confirm the model but also show that averages can be deceiving because they mask the extremely large variations in stresses that, until now, had gone undetected. Levine's experiments could have implications for important practical problems in sheet metal forming and the control of metal fatigue, which is responsible for many structural materials failures in everything from bridges and buildings to aircraft. "One big advantage to this method is that the results are completely definitive. We can make unambiguous, quantitative measurements from the submicron sample volumes most pertinent to metals deformation," Levine says. The new technique opens a detailed window into the microstructure of stress in metals and provides quantitative data to support computer models of mechanical stress. Related links:
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Levine, testing his metal
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