Intermetallic samarium cobalt deforms without dislocation activity
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nto an upgrade of the instruments used at the Advanced Photon Source, a synchrotron facility at the Argonne National Laboratory, such as the PtychoProbe. Orders-of-magnitude improvement in the x-ray flux from the soon-to-be upgraded synchrotron is anticipated. Jeff Gelb from Sigray, Inc., an industrial developer of laboratory x-ray systems, says that coherent diffraction
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Intermetallic samarium cobalt deforms without dislocation activity
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he bending of metals is mediated by dislocations. Half-planes of atoms shift around, interacting primarily with grain boundaries, but also with other microstructural defects. As deformation continues, these interactions pile up and frustrate dislocation motion, which both increases the material’s strength and reduces its ductility. The Hall–Petch relation, an empirical rule introduced in the 1950s, expresses this interplay between material strength, dislocation motion, and grain size. An international group of researchers has identified deformation that does not involve dislocations. This gives rise to an extended regime of inverse Hall–Petch behavior. In a recent issue of Nature Communications (doi:10.1038/s41467-019-11505-1), Izabela Szlufarska and Hubin Luo of the University of Wisconsin–Madison and their colleagues discussed the unusual deformation mechanism of samarium cobalt. SmCo5 is a hard magnetic intermetallic with hexagonal—but not closepacked—symmetry. “The initial goal of this project was to understand how we can control the grain size and texture of this material through plastic deformation,” says Szlufarska. In particular, the researchers were interested in controlling the material’s magnetic behavior through its microstructure, so Luo carried out molecular dynamics simulations to determine the active slip systems. However, no slip system had a low enough activation energy. Instead, the model predicted direct
imaging techniques, such as ptychography, are the future of synchrotron x-ray imaging. Gelb, who was not involved in this study, says the lensless approaches represent a major breakthrough in overcoming the limits imposed by x-ray optics, and that researchers have only begun to scratch the surface of what is possible with coherent radiation. Aashutosh Mistry
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Amorphous shear bands. (a–d) Displacements of atoms relative to their positions in the unstrained samples and distribution of von Mises stress in the same area around a triple junction for grain size of 23 nm under the strain of 6.2% (a, b), and the strain of 7.2% (c, d). (e) High-resolution transmission electron micrograph of a selected shear band and its surrounding regions. Fast Fourier transform patterns are shown in the insets. Scale bar, 10 nm. Credit: Hubin Luo and Hongliang Zhang.
amorphization along shear planes. “Our first reaction was to question these predictions,” Szlufarska says. “We spent a significant amount of time testing [them]
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