Strengthening mechanisms in nanoporous alloy foams with a hierarchical structure

Nanoporous metallic foams (NMFs) of Cu, Cu–Zn, and Cu–Ni alloys with relative densities between 0.5 and 14% were created by polymer templating followed by thermal treatments for calcination and reduction. The mechanical strength of NMFs can be dominated by chemistry and morphological parameters, including the relative density, ligament size, and chemistry. An analysis by Pearson correlation coefficients found the predominant factors in the strength, as measured with nanoindentation, of the alloys NMFs were largely controlled by the relative density. The modulus of a Cu75Ni25 alloy with a relative density of 14% was 57% greater and its hardness was 64% greater than the same alloy with a 0.5% relative density. When foams with similar ligament size and relative density but different chemistries were compared, second phase particles in the form of ZnO in Cu–Zn alloys strengthened NMFs alloys significantly more than solid solution strengthening in Cu–Zn or Cu–Ni alloys. This suggests solid solution strengthening will have limited effectiveness in increasing the strength of NMFs, and precipitation hardening should be more impactful in creating stronger NMFs.