Controls of microstructure to enhance mechanical properties of nanoporous gold

Abstract

To enhance mechanical properties of solid metals, defects such as grain boundary, twin boundary, and initial dislocation density are generally controlled. Simliary, we have developed processes to control microstructure in the ligaments of nanoporous gold (np-Au). we developed fabrication processes of nanocrystalline, prestrained and nanotwinned np-Au by controlling internal microstructure of precursor. Microstructures of each sample were investigated using electron microscopes before and after dealloying. Mechanical properties were measured by using nanoindentation, flexural test, compression test, and in-situ tensile test. Issues on characterizations of mechanical properties of np-Au have been studied. Time-dependent deformation of np-Au was investigated with spherical nanoindentations, and indentation size effect (ISE) in np-Au was studied. In many previous studies, it has been shown that hardness of np-Au is increased with decreasing ligament size, which is called ligament size effect, and with decreasing indentation depth, which is in line with indentation size effect (ISE) for solid materials. Regarding ISE of np-Au, there would be different mechanism from solid materials whose ISE mechanism is described with geometrically necessary dislocation because dislocations can easily glide and escape to free surface in np-Au. To discuss this, we fabricated np-Au samples with different ligament size by controlling conditions of freecorrosion dealloying and post heat treatments and investigated their structural similarity with 3D reconstruction by serial sectioning using focused ion beam. Nanoindentation, uniaxial compression and shear tests were conducted and nanomechanics modeling for indentation on np-Au is suggested to describe ISE in np-Au.