Full length articleTime-resolved atomic-scale observations of deformation and fracture of nanoporous gold under tension
Graphical abstract
We report in situ observations of the deformation and failure of dealloyed nanoporous gold by utilizing the state-of-the art aberration-corrected transmission electron microscopy and fast direct electron detection camera. Time-resolved atomic observations reveal that the brittle failure of the nanoporous gold originates from plastic instability of individual gold ligaments by the interplay between dislocation plasticity and stress-driving surface diffusion.
Introduction
Nanoporous gold (NPG) prepared by dealloying has a bicontinuous nanoporous structure. The interconnected nano-sized gold ligaments with a nearly cylindrical cross-section and random orientation form a three-dimensional (3D) network [[1], [2], [3], [4]]. Although NPG possesses a high surface-to-volume ratio with many desirable physical and chemical properties, the resultant nanoporous structure usually leads to the dramatic loss in ductility and strength under tension [[5], [6], [7], [8]], significantly affecting the stability and reliability of NPG for applications of electrodes, catalysts, sensors and actuators [[9], [10], [11]]. Nevertheless, how the nanopores lead to the brittle failure of ductile noble metals under tension has not been well understood in spite of extensive experimental and theoretical investigations on mechanical properties of dealloyed nanoporous metals [[12], [13], [14], [15]]. It has been reported that the strength of monolithic NPG strongly depends on the average size of gold ligaments by following Hall-Petch relationship when ligaments are larger than tens of nanometers [16] and approaches to the theoretical strength of gold as ligament sizes are smaller than ∼10 nm [[17], [18], [19]]. Under compression and nanoindentation tests, NPG can experience large plastic deformation by densification of porosity, but fails in a brittle manner under tension [20]. Moreover, there is an obvious brittle-to-ductile (BTD) transition with the increase of nanopore and ligament sizes [21]. Regardless of the macroscopic brittleness under tension, considerable local deformation to failure of individual NPG ligaments has been observed [7,8]. The inconsistence between local plasticity and macroscopic brittleness has been explained by short-lived dislocations on the basis of molecular dynamics (MD) simulations [[22], [23], [24], [25]]. However, it is technically challenging to experimentally characterize the dynamic process of NPG deformation and failure on the atomic scale under tension. In this study, we employed the state-of-the-art aberration-corrected transmission electron microscope (Cs-corrected TEM) equipped with a recently-developed fast direct electron detection camera to investigate in situ deformation and failure of NPG under tension, which provides microscopic and atomic insights into the mechanical behavior of NPG.
Section snippets
Sample preparation
Free-standing NPG films with a thickness of about 100 nm were fabricated by chemically dealloying Ag65Au35 (at.%) leaves in a 70 vol% HNO3 solution for 30 min at room temperature by which silver was selectively dissolved from the alloy and left behind a NPG membrane (Fig. 1a). The as-prepared NPG films were carefully rinsed with distilled water to remove the residual nitric acid. The size of gold ligaments, comparable to that of the nanopores, falls in a wide distribution range from ∼5 to 35 nm
Microscopic observations of the deformation and failure of NPG
Although the morphology of NPG looks complex, the gold ligaments in the observed region essentially have an identical crystal orientation [110] and thus the corresponding selected area electron diffraction pattern (SAED) can be indexed as a single crystal without detectable grain boundaries (the inset of Fig. 2a). Accordingly, the tensile direction of the in situ observations is determined along [001] as marked by the white arrowheads. As shown in Fig. 2 and Movie S1, prior to the formation of
Discussion
In the study we attempted to present a comprehensive description on the microscopic and atomic observations of the mechanical response of NPG subjected to macroscopic tensile loading. Apparently, the deformation and failure of individual ligaments depend on their sizes, crystallographic orientations, surface structures and surrounding pores in the complex and heterogeneous material. In general, the plastic deformation starts from small ligaments via dislocations nucleation and propagation,
Conclusion
In summary, in situ tensile TEM observations of NPG unveil the atomic mechanisms of nanopore induced embrittlement of ductile metals. The brittleness of dealloyed nanoporous metals under tension is associated with the plastic instability of individual gold ligaments caused by the combination of dislocation plasticity and stress-driven surface diffusion. Geometrically, the existence of small ligaments in the heterogeneous porous structure expedites the loss of the mechanical stability by quick
Acknowledgements
This work is sponsored by National Natural Science Foundation of China (51271113, 11327901, 11704245, 11234011); by Shanghai Pujiang Program (17PJ1403700); and by JST-CREST "Phase Interface Science for Highly Efficient Energy Utilization", JST, Japan. M. C. is sponsored by the Whiting School of Engineering, Johns Hopkins University.
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