Effect of Mg on the stress relaxation resistance of Cu–Cr alloys

Cu–Cr and Cu–Cr–Mg alloys were prepared to determine the stress relaxation mechanism and the effect of Mg on the stress relaxation resistance of Cu–Cr alloys by analyzing the microstructure, morphological characteristics of the precipitates and dislocation densities before and after the stress relaxation test using electron backscatter diffraction (EBSD), transmission electron microscopy (TEM) and X-ray diffraction (XRD). The formation of Cottrell atmospheres in the Cu–Cr–Mg was verified by theoretical calculations and the serrations in the stress-strain curves; thus, the way that Mg improved the stress relaxation resistance of the Cu–Cr alloy was identified. The stress decreased rapidly at first, dropped slowly and then gradually stabilized during the stress relaxation of the two alloys. The kinetic equations of both alloys were obtained. The stress relaxation behavior was very sensitive to the temperature, and an increase in the temperature accelerated the rate of stress reduction. Recrystallization and a decrease in the dislocation density were accompanied by the stress relaxation of both alloys, but the changes in the morphological characteristics of the precipitates were negligible. The recrystallization was the main factor behind the stress relaxation of both alloys. Mg obviously improved the stress relaxation resistance of the Cu–Cr alloy. There were no Cottrell atmospheres at room temperature, and the Mg and mobile dislocations formed Cottrell atmospheres only at 100 °C and above. The increase in the stress relaxation resistance of the Cu–Cr–Mg alloy was attributed to the interactions among the fine grains, precipitates, Mg atoms and dislocations.