Nucleation-Controlled Distributed Plasticity In Penta-Twinned Silver Nanowires
Horacio Espinosa (Northwestern University), Tobin Filleter (), Senghua Ryu (), Keonwook Kang (), Jie Yin (), Rodrigo Bernal (), Kwon Sohn (), Jiaxing Huang (), Wei Cai ()
Prager Medal Symposium in honor of George Weng: Micromechanics, Composites and Multifunctional Materials
Mon 2:40 - 4:00
MacMillan 117
As the dimension in metallic thin films and micro-pillars decreases from 10 micrometers to ~100 nm, flow stress displays an increasing trend. This behavior has been attributed to dislocation motion/multiplication mechanisms, primarily through the “source shut-down” mechanism. In contrast, face-centered-cubic (FCC) metal nanowires (NWs) with diameters below 100 nm are dislocation free prior to deformation, resulting in plastic deformation mechanisms that are controlled by dislocation nucleation. The nucleation-controlled nature of plastic deformation, atomistically predicted in single crystal metallic NWs, typically leads to high strength accompanied by limited strain hardening and ductility. Although there have been many MD simulations on the plastic deformation of metal nanowires, most of the predictions have not been tested experimentally due to the difficulty of conducting in-situ TEM uniaxial measurements, which can directly identify plastic deformation mechanisms. To address this challenge, we present a combined study of in-situ TEM tensile testing and MD simulations conducted on penta-twinned Ag NWs. We demonstrate that the coherent internal twin boundaries present in the NWs lead to unique size-dependent strain hardening that achieves both high strength and ductility. We find that thin Ag NWs deform via the surface nucleation of stacking fault decahedrons (SFDs) in plastic zones distributed along the NW. The internal twin boundaries, which run along the axis of the penta-twinned NWs, act as barriers for dislocation propagation leading to the formation of SFD chains. In the case of thin nanowires, surface imperfections promote distributed nucleation of SFD along the specimen length. In contrast, thick NWs exhibit lower flow stress accompanied by a reduction in the number of distributed plastic zones due to the onset of necking, which can be understood by the formation of more irregular and complex dislocation structures as observed in MD simulations.