Diffusion of atoms in a crystalline lattice is a thermally activated process that can be strongly accelerated by defects such as grain boundaries or dislocations. When carried by dislocations, this elemental mechanism is known as "pipe diffusion." Pipe diffusion has been used to explain abnormal diffusion, Cottrell atmospheres, and dislocation-precipitate interactions during creep, although this rests more on conjecture than on direct demonstration. The motion of dislocations between silicon nanoprecipitates in an aluminum thin film was recently observed and controlled via in situ transmission electron microscopy. We observed the pipe diffusion phenomenon and measured the diffusivity along a single dislocation line. It is found that dislocations accelerate the diffusion of impurities by almost three orders of magnitude as compared with bulk diffusion.

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In this work, the mechanical behaviour of millimetre-scale, bulk single crystalline, nanoporous gold at room temperature is reported for the first time. Tension and compression tests were performed with a custom-designed test system that accommodates small-scale samples. The absence of grain boundaries in the specimens allowed measurement of the inherent strength of millimetre-scale nanoporous gold in tension. The elastic modulus and strength values in tension and compression were found to be significantly lower than values measured with nanoindentation-based techniques and previously reported in the literature, but close to those reported for millimetre-scale polycrystalline samples tested using traditional compression techniques. Fracture toughness was found to be very low, in agreement with the macroscopic brittleness of nanoporous gold, but this is due to the localization of deformation to a narrow zone of ligaments, which individually exhibit significant plasticity and necking

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Thin fi lms of nanoporous noble metals exhibit an interconnected, porous structure with ligament widths and pores on the order of 10 nm or higher. In this study, thin fi lm stress measurements and in-situ nanoindentation in a transmission-electron microscope were performed to investigate the effects of nanoscale geometric confi nement on the mechanical properties of metals and on dislocation-mediated plasticity. Although some fi lms exhibit macroscopic cracking, the deformation of individual ligaments is completely ductile and clearly involves dislocation activity, even in 10 nm wide ligaments. The stresses generated in these fi lms during thermal cycling correspond to bulk stresses that approach the theoretical strength of the metal. Film stress exhibits a dependence on fi lm thickness, even though the ligament width is much smaller and would presumably govern deformation.

Nanoporous gold (np-Au) thin films were fabricated from Au-Ag alloy films sputtered onto substrates. At several stages of dealloying, the evolution of the microstructure and Ag content were analyzed and stress in the np-Au thin films was measured. A nanoporous structure evolved almost immediately throughout the film thickness, and the ligament width coarsened during further dealloying, with a time dependence of t 1/8 . The initial alloy films, which contained 25 at. pct Au, became stress free after extended dealloying and during thermal cycling up to 200°C. Preferential dissolution caused cracking at grain boundaries, which accommodated a portion of the volume contraction from dealloying, but the films nonetheless remained attached to their substrates.

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