Size Effects in the Mechanical Properties of Bulk Bicontinuous Ta/Cu Nanocomposites Made by Liquid Metal Dealloying

Abstract

Control of microstructural length scales is central to the design and fabrication of strong and tough metals. [1][2][3] Most strengthening strategies in metals, such as precipitation hardening, grain refinement, and work hardening, uniformly control the intrinsic microstructure and generally work to inhibit dislocation motion. These methods increase the stress required to move dislocations but they also tend to lower ductility. As the physical dimensions are reduced below that of a material's internal microstructural obstacles, a number of interesting size effects come into play. For example, the strength of metallic thin films scales with the inverse of the film thickness, [4] dislocation-free whiskers are many times stronger than conventional wires, [5] the hardness of metals increases with decreasing indentation size, [6] and the compressive strength of micropillars is much greater than that measured in the bulk. [7][8][9][10] There is general recognition that dislocation activity is different in reduced volumes where interactions with free surfaces can lead to dislocation starvation, source truncation, or pileup. [11][12][13][14] Although the exact origin of size-dependent strengthening is still debated and may vary from system to system, experimental observations of smaller-is-stronger show promising new metallic materials. It is compelling to consider how to translate these kinds of mechanical responses into bulk materials.One class of bulk-nanostructured materials that exhibit size effects on their mechanical properties are dealloyed metals, for which nanoporous gold (NPG) is a model system due to its ease of fabrication. [15][16][17][18][19] NPG is made via electrochemical dealloying, immersing a Ag-rich Ag-Au parent alloy in acid, selectively dissolving away the silver atoms while leaving the gold atoms behind to diffuse along the metal/electrolyte interface, self-organizing into a porous structure with welldefined length scales (e.g., average ligament or pore diameter). [20][21][22] Dealloyed metals are porous at scales smaller than the parent alloy grain size and the dealloyed phase is a complex, 3D network of interconnected beams without any crystal defects that did not exist prior to dealloying. Indentation and micropillar compression tests show that with smaller feature size, the strength of individual NPG ligaments approach the theoretical strength of gold, but testing of bulk samples generally results in brittle failure, a feature common to most nanoporous metals (NPMs). [23,24] Weissmuller recently showed that if the pores of nanoporous gold are impregnated with a polymer, then the mechanical properties of the composite are greatly improved compared to the porous component alone, having both good strength and high plastic strain in compression. [25] The challenge with electrochemically dealloyed materials is that they tend to be precious metals, whose low melting points make it difficult to make strong materials for structural applications. Kato introduced a possible solution by developing a new ...

Dealloyed nanoporous materials with interface-controlled behavior

“…The mechanical behavior of these materials can be significantly improved when the pore space is infiltrated with a second phase, such as epoxy resin 41 or a second metal. 42 Details of the topology in the corrosion-made network of nanoscale ligaments are crucial for their mechanical behavior. Contrary to truss structures in which the network geometry can be designed, nanoscale pattern formation during dealloying is self-organized and microstructural evolution during the subsequent coarsening proceeds spontaneously.…”

Kinetics and morphological evolution of liquid metal dealloying

“…Liquid metal dealloying (LMD) has recently emerged as a facile technique to fabricate bulk nanoporous and nanocomposite materials for a variety of applications including electrolytic capacitors, Li-ion battery anodes, and high-strength structural materials [1][2][3][4][5]. Dealloying is a corrosion process in metallic systems where one or more components are selectively dissolved from an alloy [6].…”

Topology-generating interfacial pattern formation during liquid metal dealloying

“…In a second step, one of the phase can be etched away to obtain a nanoporous structure with a high specific area. Several new materials with outstanding properties have been fabricated with this new technique: nanoporous Si for battery anodes with extremely long cycle fatigue 14 , ultra-high surface area non-porous Nb for electrolytic capacitors 15 and Cu–Ta nanocomposites with outstanding material properties 16 .…”