Mechanical response of nanoporous metals: A story of size, surface stress, and severed struts
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uction Studies of the mechanical properties of nanoporous (NP) metals have a twofold motivation. First, their performance as functional or structural materials requires strength, plastic deformability, and high values of either stiffness or compliance, depending on the application. Documenting and understanding the mechanical behavior is mandatory for establishing the required mechanical performance. Second, and of equal relevance, dealloying-made NP metals offer exciting opportunities for exploring and unraveling the mechanical behavior of small-scale systems in general. High sample-to-sample reproducibility and the availability of samples in the size regime from millimeter to centimeter, suitable for reliable macroscopic testing schemes, enable particularly meaningful investigations of small-scale plasticity. Furthermore, in-electrolyte tests afford an in situ variation of the surface state, providing unique access to the signatures of surface contributions to the elastic, plastic, and fracture behavior.
Fundamental observations of stiffness, strength, and fracture Compression tests of nanoporous gold Figure 1 summarizes the plastic deformation behavior of nanoporous gold (NPG). As seen in Figure 1a (inset), the material can be deformed continuously to large plastic strain
in compression without the formation of cracks.1 The experimental compression stress–strain curves show the systematic strengthening at small size and the pronounced strain hardening.1 This particular data set also exhibits unload–load sections that allow a precise measurement of Young’s modulus in each deformation state. The yield strength is also typically extracted from such data.
Uniform deformation, no-crush bands The compression stress–strain curve of conventional low-density open-cell metal foams typically shows a linear elasticity region, followed by a long plateau that is associated with the propagation of localized collapse or crush bands, and finally, a densification region where almost all pores have collapsed and the stress rises steeply.2 Figure 1a illustrates that NPG exhibits a different compression behavior:1,3–5 the “elasticity” region is often nonlinear and involves pre-yielding; the stress plateau is replaced by a hardening region in which the stress increases gradually with strain; the onset of densification is also ill-defined. The continuous hardening in NPG is associated with uniform densification in compression, as observed in electron backscatter diffraction (EBSD) images of compressed NPG samples.3 Localized crush bands are not observed.3 Most pores
Hai-Jun Jin, Institute of Metal Research, Chinese Academy of Sciences, China; [email protected] Jörg Weissmüller, Institute of Materials Physics and Technology, Hamburg University of Technology; and Hybrid Materials Systems Group, Helmholtz-Zentrum Geesthacht, Germany; [email protected] Diana Farkas, Department of Materials Science and Engineering, Virginia Tech, USA; [email protected] doi:10.1557/mrs.2017.302
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