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Systematic study of the dealloying of Au-Ag and Au(Pt)-Ag alloys shows that the dealloying occurs by two processes: a primary dealloying process that selectively dissolves Ag from the parent alloy and creates a nanoporous (np) structure, and a secondary dealloying process that occurs behind the corrosion front and further dissolves the residual Ag from the nano-ligaments. The secondary dealloying can occur during coarsening, and/or when a more anodic potential is applied. With suppressed np structure coarsening in Pt-containing samples, we found that the intrinsic np structure created by the primary dealloying contains small ligament diameter (3-7 nm) and high concentration of residual Ag (∼50 at.%), irrespective of the dealloying potentials. Dilatometry experiments show that the volume shrinkages are rather small during primary dealloying and are large during secondary dealloying. The primary dealloying can be explained by the percolation dissolution mechanism. Although the mechanism of the secondary dealloying (without coarsening) remains unclear, we point out that the kinetics of the secondary dealloying is decisive to some important characters of np metals, such as the crack formation and the final residual Ag concentrations.The dealloying is a corrosion process which selectively dissolves one or more reactive elements from a homogenous alloy (solid solution or inter-metallic compound). 1,2 While the dealloying remains as one of the major corrosion problems in industrial alloys, 3,4 in the past few years it has also emerged as a promising method for fabrication of a new class of materials -the nanoporous (np) metals that are potentially important for a variety of applications including catalysis, 5,6 sensing, 7 actuator, 8-11 super-capacitor, 12,13 and responsive materials with electrically tunable mechanical 14,15 and physical properties. 16,17 The mechanisms of the dealloying and np structure formation have been extensively studied in previous publications, 1,18-22 particularly in the prototypical Au-Ag alloy system which is dealloyed to form np gold (npg). Majority of these studies were conducted in the context of electrochemistry and corrosion science, with focuses on the understandings of how the corrosion initiates, 18,19,21,23 how the corrosion proceeds to form np structure, 1,18,24-26 and how to quantify and interpret the critical dealloying potentials 18,27-29 and the parting limits. 30 On the other end of the spectrum, substantial efforts have been made to characterize the structures of corrosion products (np metals), such as the ligament and pores sizes, the porosity, the residue of the reactive elements and the defects (e.g., native cracks). While porosity is predominantly determined by the initial composition of parent alloy, other characters are sensitive to various dealloying parameters. For npg, by controlling the dealloying conditions, the ligament size can be varied from 3 nm to submicron scale. 2,31-33 The residual Ag content, which is critical to catalytic, 6,34,35 optical 36 and mechanica...

Low-angle grain boundaries generally exist in the form of dislocation arrays, while high-angle grain boundaries (misorientation angle >15°) exist in the form of structural units in bulk metals. Here, through in situ atomic resolution aberration corrected electron microscopy observations, we report size-dependent grain-boundary structures improving both stabilities of electrical conductivity and mechanical properties in sub-10-nm-sized gold crystals. With the diameter of a nanocrystal decreasing below 10 nm, the high-angle grain boundary in the crystal exists as an array of dislocations. This size effect may be of importance to a new generation of interconnects applications.

Replacing parts of a solid with voids usually weakens a material. Here, we report that a material (Au–Pt–Ag alloy) becomes stronger by 7 times, its strength increases from 50 to 345 MPa, when ≈40% of solids (Ag) is dissolved and replaced by pores during dealloying. The corrosion‐induced strengthening originates from the formation of nanoporous structure, and the high strength of nanoscale ligaments due to size effect. The results also suggest that the connectivity of nanoporous structure decreases during coarsening, leading to additional decreases in strength and stiffness, which may explain the unexpected low strength and stiffness of previously reported nanoporous gold.

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