Capillary shrinkage induced ductile–brittle transition in nanoporous gold: Crack healing via cold welding

When a brittle nanoporous material infiltrated with water is dried in air, the capillarity-induced large volume shrinkage often leads to cracking within the material. In consequence, porous material may become more fragile or even shatter after drying. In this paper, we prepared nanoporous gold (NPG) with a low relative density by dealloying dilute Ag(Au) precursor alloys. Drying of this sample in air leads to a large volume contraction up to 22 ∼ 38%. Meanwhile, the material undergoes a ductile–brittle transition during drying: The wet sample of as-dealloyed NPG, while tested in water, shows a surprisingly large tensile strain up to 8%–10% before fracture; After drying, the fracture strain of the same material drops to around 1% in tension. This ductile-to-brittle transition is not caused by the formation of cracks as usual. Instead, it can be linked to the self-healing of microcracks during drying. We found that the plasticity of as-dealloyed NPG arises from the presence of high-density native microcracks. The bridging or deflection of these microcracks in tension leads to a diffuse failure, which accounts for the large irreversible tensile strain of the wet, as-dealloyed NPG. Further examination reveals that these native microcracks can be healed during drying, owing to the cold welding of nano-ligaments under capillary contraction force. Healing of these microcracks suppresses the diffuse failure induced by crack bridging or deflection, leading to a brittle fracture of the dried NPG in tension. Cold welding of nano-ligaments, which accounts for the anomalous crack healing and ductile–brittle transition of NPG in drying, might occur frequently in nanoporous metals under external load and contribute significantly to their mechanical performance.