Mechanical annealing of Cu-Si nanowires during high-cycle fatigue
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esearch Letters
Mechanical annealing of Cu–Si nanowires during high-cycle fatigue Charlotte Ensslen, Oliver Kraft, and Reiner Mönig, Institute for Applied Materials, Karlsruhe Institute of Technology, Hermann-von-HelmholtzPlatz 1, 76344 Eggenstein-Leopoldshafen, Germany Jin Xu and Guang-Ping Zhang, Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, 110016 Shenyang, China Reinhard Schneider, Laboratorium für Elektronenmikroskopie, Karlsruhe Institute of Technology, 76128 Karlsruhe, Germany Address all correspondence to Reiner Mönig at [email protected] (Received 27 January 2014; accepted 5 June 2014)
Abstract Monotonic and cyclic tension–tension tests with an upper stress in the GPa regime have been performed on Cu–Si nanowires. The results show that the exceptional high strength of these nanomaterials is maintained or even improved upon cyclic loading. Post-mortem transmission electron microscopy gives insight in the microstructural evolution. Fatigue-induced grain growth correlates with an observed increase in compliance, the formation of dislocation networks, and an increase in tensile strength.
Nanostructured materials typically possess superior mechanical properties compared with their bulk counterparts and thus are of great interest for the applied and fundamental scientific research. For example, Au nanopillars and nanowhiskers yield under uniaxial stress close to the theoretical strength at several GPa,[1,2] whereas bulk polycrystalline faced-centered cubic (fcc) metals yield at a few tens of MPa.[3] These two very different examples constitute the boundaries of strength of a material. In between, strength values can scale with dimensions such as sample size or microstructural length,[4,5] a phenomenon that is known as the mechanical size effect. In recent decades, the mechanical properties of small-scale materials, with dimensions well below 1 µm, have been intensively investigated using different nanomechanical tests, such as uniaxial deformation[6] and bending.[7] In this respect, monotonic uniaxial tensile or compression testing of cylindrical specimen geometries, such as pillars and whiskers, minimizes strain gradients and simplifies the interpretation of data. Supplementary insight into the influence of size effects on the deformation behavior is obtained by atomistic simulations.[8] Based on this research, it has been argued that the multiplication of dislocations becomes increasingly difficult in the submicrometer regime causing the size effect. To date, the behavior of nanoscale materials under cyclic load has only marginally been investigated. The data available include fatigue testing of different specimen geometries, e.g., nanocrystalline bulk materials,[9,10] thin films,[11] and microwires.[12–14] An overview is given in the literature.[15,16] A common result of these studies is that nanostructured materials show an increased fatigue life, especially in the high-cycle regime. This can be related to their increased
yield strength, which
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