|M.Sc Student||David Omer|
|Subject||Strength and Ductility of Twinned Metallic Nanowires|
|Department||Department of Mechanical Engineering||Supervisor||Professor Dan Mordehai|
|Full Thesis text|
In recent years, there are increasing efforts to minimize devices and designing their properties using micro- and nano-mechanical systems. The increasing use of such systems requires a deeper understanding of the mechanical properties of their components and the influence of their size on their mechanical behavior. Recently, there is a growing recognition that specimens of the same material and geometry may behave mechanically differently owing to differences in their initial microstructure. Therefore, developing an understanding of plasticity at the sub-micrometer scale, which takes into account the governing mechanisms that control it, is necessary for appropriate design.
Metallic nanowires are important building blocks in a wide range of future devices. They are characterized by unique mechanical properties and plastic deformation mechanisms, compared to their bulk counterparts. In this research, we study computationally how a single pre-existing longitudinal twin boundary in Au nanowires influences the deformation process under tension. For this study, we employed Molecular Dynamics simulations, which allow us to investigate the interactions of the twin boundary with dislocations during the deformation. For comparison, we study mechanical properties of both single- and bi-crystalline Au nanowires.
We found that in the single-crystalline nanowires, deformation twinning is the governing mechanism during tensile loading. This mode of deformation leads to reorientation of the whole nanowire from an axis in the <110> lattice orientation to a <100> direction. Although similar mechanisms were identified in the bi-crystalline nanowires, the twin boundary acts as a barrier for dislocation glide, and hence detwinning and reorientation of the nanowire require deformation twinning on both grains simultaneously, a process for which we coined the term coordinated-deformation twinning. As it turns out, the ability to dominate the elongation by coordinated-deformation twinning is limited and therefore the twin regions are thinner in the bi-crystalline twinned nanowires, with regard to the nanowires without a pre-existing longitudinal twin boundary.
In both cases, the elongation via deformation twinning diminishes at large plastic strains and the relative contribution of ordinary dislocation plasticity increases. Once the latter plastic mechanism prevails, necking developed in the twinned regions, which eventually lead to fracture there. Since the coordinated-deformation twinning and detwinning of the longitudinal twin boundary in the bi-crystalline nanowires was limited, the ductility of the bi-crystalline nanowires was proposed to be smaller than the single-crystalline ones.
We also compared the MD simulation results with experimental observations, as part of a research collaboration with the University of Göttingen. The synergy between the computational and the experimental observations supported the mechanisms detailed above but also provided an important insight into the relationship between the quality of the nanowire’s surface and their strength. We found that the strength at the onset of plasticity relates to the perfection of the surface. With that being said, it is important to note that beyond the yield point, further deformation and the mechanisms identified above were not influenced by the quality of the surface.