Improvement of properties in Cu–Ag composites by doping induced microstructural refinement

Highlights: • Nb doping increased strength in Cu–Ag, mainly by refining precipitate spacing. • Continuous precipitates (CPs) have finer spacing than discontinuous precipitates (DPs). • Nb doping increased the activation energy of precipitation and promoted CPs. • Nb doping increased volume fraction of CPs, decreased that of DPs. • Delayed discontinuous precipitation allowed increased driving force for continuous precipitation. • Dissolved Nb marginally decreased electrical conductivity in Cu–Ag. The microstructure, precipitation kinetics, electrical conductivity, and mechanical properties of Cu-7.9 wt%Ag and Cu-5.8 wt%Ag doped with 0.6 wt%Nb were investigated. As-cast microstructure revealed both discontinuous and continuous precipitation of Ag in Cu matrix. Differential scanning calorimetry data indicated that continuous precipitation occurred at higher temperatures with higher activation energy. Both micro-sized and nano-sized Nb particles were observed in the Nb-doped Cu–Ag alloy. The doping of Nb significantly suppressed the growth of discontinuous Ag precipitates and consequently increased the activation energy and the volume fraction of continuous Ag precipitates. Because of larger volume fraction of finer-spaced continuous Ag precipitates, the Nb-doped alloy showed higher hardness and strength than the un-doped Cu–Ag. After cold-rolling, the strength of the Nb-doped alloy remained higher than that of the un-doped Cu–Ag.