Analysis of thermal stability and mechanical properties of nanocrystalline ultra-thin Ni–Co foils

In this study, nanocrystalline ultra-thin (NUT) Ni and Ni–Co foils were prepared using electrodeposition with varying Co content. Their composition and microstructure were characterized. A systematic investigation was conducted to the thermal stability and mechanical properties of NUT foils. The results indicated that with an increase in Co content, there was an increase in the driving force for grain growth and a decrease in grain boundary (GB) energy, resulting in unexpected improvement in the thermal stability despite the decrease in grain size. The incorporation of Co enhanced the strength of the NUT foils. However, the effect of increased Co content on the improvement in strength was relatively small. Due to its extremely thin thickness and nanocrystalline microstructure, the ultimate tensile strength (UTS) and elongation (EL) of the NUT foil fell below the theoretical strength, while the maximum UTS and EL were observed at a Co content of 17 %, which exhibited a UTS of 1337 MPa and an EL of 8.6 %. The tensile fracture exhibited a clear combination of brittle and ductile modes. With an increase in Co content, the NUT foil transitioned from predominantly brittle to ductile and then back to brittle fracture.