Achieving superplasticity in ultrafine-grained copper: Influence of Zn and Zr additions

Equal-channel angular pressing (ECAP) was applied to samples of pure Cu, a Cu-30%Zn alloy and alloys of Cu-0.18%Zr and Cu-30%Zn-0.13%Zr. All materials exhibited ultrafine grain sizes in the range of ∼ 0.1-0.4 μm after ECAP but the microstructures were inhomogeneous and the grains were elongated in the as-pressed condition. Tensile testing showed that superplasticity was not achieved in any of these materials at a temperature of 573 K but at 673 K the Cu-Zn-Zr alloy exhibited superplastic elongations at strain rates at and below ∼ 10^-3 s^-1. The results demonstrate that the presence of Zn and Zr are both beneficial in promoting the occurrence of superplastic ductilities. Zirconium is needed because it increases the recrystallization temperature, inhibits grain growth and, therefore, serves to retain a small grain size at elevated temperatures, and zinc is beneficial because it introduces solute atoms into the matrix so that dislocation creep is inhibited and superplastic flow can occur more easily.