Fracture toughness of nanoporous gold with hierarchical grain structure

Abstract

Nanoporous gold (np-Au) is a material with sponge-like structure, composed of continuous ligament and pore at the nanoscale. Due to low density and high surface area-to-volume ratio, there have been many researches to apply np-Au as catalyst, actuator, and sensor. However, brittle behavior of np-Au unlike ductile gold at bulk scale remains as an issue to be overcome. Previous researches have shown that the brittleness of np-Au appears by stress concentration on pore surface and catastrophic crack propagation through grain boundary, which had been formed under Au-Ag precursor alloy state. Here, we focus on crack propagation through the grain boundary and investigate dependency of fracture toughness on grain boundary structure. We fabricated well-annealed, cold-rolled, and hot-rolled AuAg precursor alloys. Well-annealed precursor alloys had grain size of micro-scale, cold rolled precursor alloys had grain size of nano-scale, and hot rolled precursor alloys had anisotropic grain structure with grain size of micro-scale. By free corrosion dealloying in nitric acid, Ag is selectively etched from precursor alloy and np-Au is formed with grain boundary structure of well-annealed, cold-rolled, and hotrolled precursor alloy. Microstructure of precursor alloys and np-Au were observed by SEM (scanning electron microscope) and EBSD (electron back-scattered diffraction). Dependency of fracture toughness, KIc on grain boundary structure is investigated under tension with nano-UTM, and effect of grain boundary density and grain boundary structure on fracture toughness and crack propagation path are discussed.