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XX3.08 - Hierarchical, Bicontinuous Refractory-Based Nanocomposites 
December 1, 2014   4:15pm - 4:30pm

Nanoporous metals formed via dealloying result in porous materials whose pores and ligaments can be tuned as small as 10 nm. An important structural feature is that dealloying retains the long-range crystalline order of the parent alloy, creating a polycrystalline material in which the individual grains are nanoporous. This structural hierarchy is interesting in the context of mechanics of materials because it can be modeled as a network of micropillars. Micropillars have increased yield stresses over their bulk counterparts, an effect attributed to a decrease in the source length, λ, for single arm dislocation sources. The operation stress for these sources is proportional to λ-1, and λ is proportional to the pillar diameter. As the pillar diameter decreases its yield strength increases and similar size trends have been seen in nanoporous metals. This strengthening effect is unique because the yield strength is increased without decreasing the ductility of the material. In contrast, material strengthening techniques, such as precipitation hardening or decreasing the grain size, impede dislocation movement and decrease ductility. There is a challenge in testing and studying this effect in nanoporous metals because they are 55-75% void, leading to easy crack propagation and thus appear to be macroscopically brittle. In this work the authors attempt to solve this issue by creating fully dense bicontinuous nanocomposites using a novel dealloying technique, liquid metal dealloying(LMD). LMD is analogous to electrochemical dealloying except that the electrolyte is replaced with a bath of molten metal, and a composite is formed in one step upon cooling. Our work focuses on Cu-X bicontinuous nanocomposites, where X is Ta or W. Parent alloys of Ti-X are immersed in a bath of molten Cu, which has high Ti solubility, but is immiscible with Ta and W. Upon cooling, the resulting nanocomposites are made up of features at several length scales: the ligaments of the refractory form single-crystal micropillar networks with tunable ligament diameter, the copper ligaments are nanocrystalline, and the copper phase also contains CuTi intermetallic precipitates. Mechanical testing of these composites using microtension and compression testing, nanoindentation, and microindentation indicate these materials have very high strengths, correlated to lengthscales within their nanostructure, yet retain significant ductility.

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