Monolithic nanoporous copper ribbons from Mg-Cu alloys with copper contents below 33 at.%: Fabrication, structure evolution and coarsening behavior along the thickness direction

Monolithic nanoporous copper (NPC) ribbons with unimodal or bimodal channel size distributions can be fabricated through chemical dealloying of Mg-Cu alloys with Cu content below 33 at.% in an acidic solution at room temperature. The microstructure of these NPC ribbons was characterized using X-ray diffraction, scanning electron microscopy, energy dispersive X-ray analysis, transmission electron microscopy, and high-resolution transmission electron microscopy. The experimental results show that with increasing Cu content from 13.5 at.% up to 32 at.%, NPC with unimodal channel size distributions gradually changes into that with bimodal channel size distributions upon the dealloying; meanwhile we found cracking to be unavoidable. For the Mg-Cu alloys composed of α-Mg and Mg ₂Cu, the microstructure of the NPC ribbons strongly depends upon phase proportions and distributions in the initial alloys. The Mg-Cu alloys with 13.5-25 at.% Cu can be fully dealloyed and result in the formation of NPC with a homogenous structure, while the separate dealloying of α-Mg and Mg ₂Cu in the Mg-32 Cu alloy leads to the formation of NPC with bimodal pore size distributions. Moreover, the microstructures of two distinct surfaces of the NPC ribbon from the Mg-32 Cu alloy make an essential difference due to the great disparity in the phase distributions, which can be attributed to the different cooling rates on the free and quenching surfaces during rapid solidification. In addition, the evolution process of porous structure along the thickness direction of alloy ribbons during the dealloying is from the interior to exterior, which is just contrary to the coarsening process during the post-dealloying. The evolution and coarsening mechanisms are discussed in detail based upon percolation theory and chloride ion effect.