Unexpected cellular growth of nanoporous gold during dealloying: Indication of vacancy injection?

Abstract

Dealloying can lead to stress corrosion cracking in engineering alloys or produce nanoporous materials for various functional applications. The role of bulk diffusion in dealloying has been debated since the 1960s when Pickering and Wagner proposed a vacancy-mediated lattice diffusion mechanism to explain the dealloying process. However, this mechanism was proven invalid because lattice diffusion is too slow to account for the rapid dealloying process at room temperature. Instead, it is now widely accepted that dealloying is dominated by the interfacial dissolution of more-reactive components, not necessarily involving lattice diffusion in the bulk. In this study, we report that the dealloying of Cu(Au) alloys at low potentials is incomplete, leaving some un-etched thin walls within the matrix of nanoporous gold. This results from the cellular growth of the nanoporous gold phase during dealloying, with cell walls composed of the precursor alloy phase slightly enriched with Au. This behavior is analogous to the cellular growth of solid phase during directional alloy solidification, suggesting that lattice diffusion may have occurred in the solid ahead of the dealloying front. Vacancy injection may lead to Au enrichment due to the inverse Kirkendall effect, enhancing the critical dealloying potential of the precursor ahead of the dealloying front, and triggering the unstable growth of the nanoporous gold phase. Although direct evidence is still lacking, our finding suggests that vacancy injection is involved in dealloying, which may have important implications on the design of corrosion-resistant alloys or novel nanoporous materials.