The Micromechanisms of Deformation in Nanoporous Gold
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The Micromechanisms of Deformation in Nanoporous Gold Rui Dou and Brian Derby School of Materials, University of Manchester, Grosvenor Street, Manchester, M1 7HS, UK. ABSTRACT We have carried out a TEM investigation of the micromechanisms of deformation in these nanoporous gold specimens after compression testing. We find that the nanoporous specimens show deformation localized to the nodes between the ligaments of the foamed structure, with very high densities of microtwins and Shockley partial dislocations in these regions. These deformation structures are very different from those seen after solid nanowires are tested in compression, which show very low dislocation densities and a few sparsely distributed twins. However, similar dislocation structures to those found in the nanoporous specimens are observed in the larger nanowires when they are deformed in bending. The currently accepted model for the deformation of nanoporous gold, implicitly assumes that the deformation of these structures is by bending near the nodes where ligaments intersect. We hypothesis that the much higher dislocation densities seen in both the nanoporous gold and the nanowires deformed in bending are evidence for the presence of geometrically necessary dislocations in these deformed structures. INTRODUCTION In polycrystalline metals there is a well known dependence of yield strength on grain size. Small single crystal metal micropillars or nanowires show a similar but unrelated size effect and at the smallest diameters these are considerably stronger than the corresponding bulk metal. [1-3] Studies of the mechanical properties of nanoporous metallic foams (chiefly gold) also report an increase in yield strength with decreasing ligament diameter with the smallest diameter ligaments having strength in excess of 1 GPa. [4-10] Despite this apparent similarity in the mechanical behavior of metallic micropillars/nanowires and nanoporous gold, the geometry of plastic deformation is different in the two cases. Most of the studies of metal micropillars have reported compression tests of isolated pillars fabricated by focused ion beam machining (FIB). These structures are seen to deform by single or multiple slip, localized to a few slip planes or shear bands, and display characteristic surface steps. Nanoporous gold, however, is believed to deform by mechanisms originally ascribed to the plastic collapse of much larger scale foams, through the formation of plastic hinges at nodes in the structure. [11, 12] There is currently no universal consensus as to the mechanisms that result in the scale dependent behavior observed with the deformation of micropillars or nanoporous gold. Greer and Nix [13] have proposed a mechanism in which the close proximity of a free surface and the associated image forces remove dislocations from the structure, requiring continuous nucleation of new dislocations to maintain deformation. Recent in situ deformation work in the transmission electron microscope (TEM) has observed dislocation generation an annihi
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