Evolution of the microstructure and properties of a Cu–Cr-(Mg) Alloy upon thermomechanical treatment

Highlights: • The softening temperature of Cu–Cr alloy was increased by 30 °C due to Mg addition. • The contribution of precipitates coarsening and recrystallization to softening were estimated by calculation. • The Mg addition could inhibit the recrystallization and the precipitation coarsening. -- Abstract: Cu–Cr and Cu–Cr–Mg alloy wires were prepared by atmospheric melt casting, hot extrusion, multi-pass drawing and heat-treatment. The effect of Mg addition on the hardness, the strength, the electrical conductivity, and the softening resistance were explored. The types and morphology of the nano-precipitates of the two alloys were compared by transmission (TEM) and high-resolution (HREM) electron microscopy. The effect of Mg on the recrystallization of peak-aged Cu–Cr alloy was analyzed by electron back-scatter diffraction (EBSD). The results show that the addition of Mg significantly improves the strength, the hardness, and the softening resistance of aged Cu–Cr alloy. The conductivity of the aged Cu–Cr–Mg alloy can be higher than 80% IACS. The observation of the microstructure showed that the main aging strengthening phases in the two alloys was Cr nano-precipitates. The addition of Mg significantly inhibits the recrystallization process of the aged Cu–Cr alloy and the coarsening of the precipitates of the peak-aged alloy at high holding temperature. The size of precipitates and the volume fraction of recrystallization in Cu-0.4Cr alloy treated by peak aging at 500 °C and heat-treated at 600 °C for 1 h were about 5.6 nm and 25.93%, while that in Cu-0.44Cr-0.17 Mg alloy were 4.5 nm and 7.42%. The results showed that reduction of dislocation density and the coarsening of precipitate are the main reasons for the decrease in the hardness of peak-aged alloys at high holding temperature.