Mechanism of Cr enhancement on stress relaxation resistance in Cu-Ni-Co-Si-Mg alloys

Cu-Ni-Co-Si-Mg-xCr alloys (where x = 0, 0.15, 0.32, or 0.4 wt%) were prepared, and stress relaxation tests were conducted at 50 degrees C, 100 degrees C, and 150 degrees C for 48 h. The results showed that the addition of Cr considerably enhanced the stress relaxation resistance of the Cu-Ni-Co-Si-Mg alloys at 150 degrees C. Compared with that of the 0 % Cr alloy, the stress relaxation resistance improved by 29.7 %, 22.6 %, and 23.3 % as the Cr content increased. To understand the underlying mechanisms, the microstructure of the alloys, the morphology of the precipitates, and the dislocation density before and after the stress relaxation tests were analyzed via electron backscatter diffraction (EBSD), transmission electron microscopy (TEM), and X-ray diffraction (XRD). The differences in precipitates and grain size were the primary reasons for the variations in stress relaxation resistance. The delta-(Ni, Co)2Si phase constitutes the main precipitate in the alloys. Adding 0.15 % Cr can produce Cr precipitates with non-coherent orientation relationship with Cu matrix, which are effective in pinning dislocations. Moreover, the addition of Cr increased the grain size, which reduced the preferential orientation of the grains and the activation of the dislocation sources during microplastic deformation. Both factors suppressed dislocation movement, which is the fundamental reason for the enhanced stress relaxation resistance of the Cu-Ni-Co-Si-Mg-0.15Cr alloy.