Atomic-Scale Observation of Deformation in Nanoscale Body Center Cubic (BCC) Crystals

在室温下，用于小型设备的机械部件的金属通常具有面心立方特征。它们的机械性能通常是众所周知的。然而，在高温下，这些FCC金属变软。因此，它们不适合高温应用。在这种情况下，体心立方（BCC）纳米结构金属提供了一种替代方案。 这些金属具有潜在的所需高温强度。它们有望在未来的高温器件，如微/纳米机电系统（MEMS/NMMS）的组件。虽然大尺寸BCC金属的力学行为是众所周知的，但由于尺寸效应，这些宏观尺度的性质不能直接用于纳米尺寸的结构。为了分析小型器件，需要了解体心立方金属在小尺度下的力学行为。然而，在小尺度下的变形过程缺乏实验数据，也没有了解BCC金属在纳米尺度下的变形行为。本研究将使用特殊高分辨率透射电子显微镜（HRTEM）内的原位机械测试方法。这为在原子尺度上研究体心立方金属材料的力学行为提供了一种新的途径。通过本研究获得的纳米尺寸BCC晶体的力学行为的理解将直接影响到高温MEMS/NEMS的设计和制造。原位高分辨透射电镜的研究有望为直接观察机械应力作用下原子尺度的形变开辟一条新途径。研究结果有望为实验力学和纳米材料的发展做出贡献。本研究将利用高解析度透射电子显微镜（HRTEM）这一最先进的仪器，采用原位拉伸技术来揭示纳米尺寸BCC金属试样的变形过程。首先，将原位制备纳米尺寸的高强度BCC金属试样。其次，利用高分辨透射电子显微镜（HRTEM）对这些BCC试样进行原位拉伸/压缩实验，记录其在室温和高温下的变形行为;第三，观察变形过程中的晶格扰动、位错偶极子形核以及滑移和孪晶之间的竞争。将对以下关键问题进行分子动力学建模：a）位错偶极子形成; B）孪晶和位错的成核; c）孪晶和滑移的竞争作为晶体取向的函数。实验将通过位于华盛顿州里奇兰的西北太平洋国家实验室环境分子科学实验室（EMSL）的国家用户设施进行。这项合作将建立国家研究基础设施。

The metals used in mechanical components for small-scale devices at room temperature are normally of face center cubic character. Their mechanical behavior is generally well-known. However, at high temperature, these FCC metals become soft. Therefore, they are not suitable for high temperature application. In that case, body center cubic (BCC) nanostructured metals offer an alternative. These metals possess potentially desired high temperature strength. They are expected to serve in components in future high temperature device such as micro/nano electro-mechanical systems (MEMS/NMMS). Although the mechanical behavior of large-sized BCC metals is well-known, these macroscale properties cannot be directly used for nanometer-sized structures due to size effects. For the analysis of the small devices, it is necessary to know the mechanical behavior of BCC metals at small scales. However, there is lack of experimental data for the deformation process at small scales, and also there is no understanding of the deformation behavior of BCC metals at the nanometer scale. An in-situ mechanical testing approach inside a special high resolution transmission electron microscope (HRTEM) will be used in this research. This constitutes a new approach for studying the mechanical behavior at atomistic scale for nanometer-sized BCC metal specimens. The understanding of the mechanical behavior of nanometer-sized BCC crystals gained from this research will have direct impact on the design and fabrication of the high temperature MEMS/NEMS. The research on the in-situ HRTEM is expected to open a new approach to directly observe atomic-scaled deformation under mechanical stress. The results from the research are expected to contribute to the advancement of experimental mechanics and nanomaterials. The research will employ an in-situ tensile technique utilizing the most advanced instrument of high resolution transmission electron microscope (HRTEM) to reveal the deformation process in nanometer-sized BCC metal specimens. Firstly, nanometer-sized high strength BCC metal specimens will be fabricated in-situ. Secondly, tensile/compression experiment in-situ in the HRTEM will be conducted on these BCC specimens to documents deformation behavior at room temperature and high temperatures; Thirdly, lattice disturbance, dislocation dipole nucleation and competition between slip and twinning in the deformation process will be observed. Molecular dynamics modeling on key issues with a) dislocation dipole formation; b) nucleation of twinning and dislocation and c) competition of twinning and slip as function of crystal orientation will be carried out. The experiments will be carried out via national user facilities at the Environmental Molecular Sciences Laboratory (EMSL), Pacific Northwest National Laboratory, Richland, WA. This collaboration will build national research infrastructure.