Department of Mechanical Engineering, Northwestern University , Evanston, Illinois 60208-3111, United States.
Theoretical and Applied Mechanics Program, Northwestern University , Evanston, Illinois 60208-3111, United States.
Nano Lett. 2016 Jan 13;16(1):255-63. doi: 10.1021/acs.nanolett.5b03630. Epub 2015 Nov 16.
The characterization of nanomaterials under high strain rates is critical to understand their suitability for dynamic applications such as nanoresonators and nanoswitches. It is also of great theoretical importance to explore nanomechanics with dynamic and rate effects. Here, we report in situ scanning electron microscope (SEM) tensile testing of bicrystalline silver nanowires at strain rates up to 2/s, which is 2 orders of magnitude higher than previously reported in the literature. The experiments are enabled by a microelectromechanical system (MEMS) with fast response time. It was identified that the nanowire plastic deformation has a small activation volume (<10b(3)), suggesting dislocation nucleation as the rate controlling mechanism. Also, a remarkable brittle-to-ductile failure mode transition was observed at a threshold strain rate of 0.2/s. Transmission electron microscopy (TEM) revealed that along the nanowire, dislocation density and spatial distribution of plastic regions increase with increasing strain rate. Furthermore, molecular dynamic (MD) simulations show that deformation mechanisms such as grain boundary migration and dislocation interactions are responsible for such ductility. Finally, the MD and experimental results were interpreted using dislocation nucleation theory. The predicted yield stress values are in agreement with the experimental results for strain rates above 0.2/s when ductility is pronounced. At low strain rates, random imperfections on the nanowire surface trigger localized plasticity, leading to a brittle-like failure.
在高应变速率下对纳米材料进行特性描述对于理解它们在动态应用（如纳米谐振器和纳米开关）中的适用性至关重要。探索具有动态和速率效应的纳米力学也具有重要的理论意义。在这里，我们报告了在高达 2/s 的应变速率下进行的双晶银纳米线的原位扫描电子显微镜（SEM）拉伸测试，这比文献中以前报道的要高 2 个数量级。实验是通过具有快速响应时间的微机电系统（MEMS）实现的。结果表明，纳米线的塑性变形具有较小的激活体积（<10b(3)），表明位错成核是速率控制机制。此外，在 0.2/s 的阈值应变速率下观察到明显的脆性到韧性的失效模式转变。透射电子显微镜（TEM）显示，随着应变速率的增加，位错密度和塑性区的空间分布沿纳米线增加。此外，分子动力学（MD）模拟表明，诸如晶界迁移和位错相互作用等变形机制是这种延展性的原因。最后，使用位错成核理论解释了 MD 和实验结果。在明显呈现延展性的情况下，预测的屈服应力值与 0.2/s 以上应变率的实验结果一致。在低应变速率下，纳米线表面的随机缺陷会引发局部塑性，导致脆性失效。
