Dislocation pileups in microcantilevers may be fully reversible
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Dislocation pileups in microcantilevers may be fully reversible
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s key components in microelectro mechanical structures, small cantilevers are often subject to mechanical stress. This can result in the development of dislocation pileups, which can have a major impact on the behavior of the cantilevers. However, how such dislocation pileups are affected by the structure of the cantilevers, and in particular by the presence of different interfaces is not well known. An article recently published in the Journal of Materials Research (JMR) investigated how different cantilever designs affect dislocation behavior, and in particular how this can cause a reversal of dislocation pileups. Cantilevers have been used for applications at both the macro- and microscale for centuries, says co-author Christoph Kirchlechner, a materials scientist at the Max Planck Institute in Germany. Anchored on one side, the free end of the cantilever is designed to support a structure as it comes under load. For microscale cantilevers, the mechanical properties, and especially the effects of increased flow stress, cannot totally be explained by similar properties on the macroscale. Specifically, it is not only down to the strain gradient, says Kirchlechner. Instead, research points toward a lack of dislocations and their arrangement at the micro- and smaller scales. “In bulk materials that are ultrafine grained, we can see that dislocation pileups are important,” says Kirchlechner. Dislocations moving toward an obstacle—like the neutral plane or a grain boundary in bulk materials—can jam. If they are not able to leave their slip plane, they will nicely align and form a dislocation pileup. It is known that the nature of dislocation pileups changes under stress, as expressed by the Bauschinger effect, where a change in flow stress is observed in forward and backward loading. However, what these dislocations were actually
doing in response to stress is less well known. “But they’ve never really been looked into at the mesoscale,” says Kirchlechner. “And now we have the capabilities to see them [in action] and understand the broad picture of what they are doing.” Part of why they A slotted bending have not been stud- © M.W. Kapp. ied in situ is due to measurement limitations. Dislocations are easily imaged post-use in a transmission electron microscope. For this reason, mechanical properties have always been measured before or after a cantilever had undergone stress loading. However, in situ loading and unloading is required to see changes to the dislocation structure happening while stress is occurring, says Kirchlechner. The researchers suggest that these measurements were not telling the whole story. During the loading of stress, the dislocation arrangement looks very different. “As both the extrinsic (i.e., sample size) and the intrinsic (i.e., microstructural) dimensions play a non-trivial role in the mechanical performance, it is essential to develop in-depth understanding of the mutual interplay between defects, like dislocations and gr
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