Cu-Ni-Si alloy with a different Co content was prepared by inductive melting and hot rolling. The alloy was solution treated at 950 °C for 1.5 h and aged at 450 °C, 500 °C, and 550 °C for different times. The phase diagram calculation and transmission electron microscopy was used to investigate the effect of Co addition on the aging precipitation behavior of the Cu-Ni-Si alloy. The phase transformation kinetics equation was calculated as well. The results show that, with the increase of aging temperature, the two-phase region of Fcc + Ni₂Si in the Cu-Ni-Si ternary diagram would get wider. Some Ni_xSi_y phases would also form in the Cu-rich isothermal section. The addition of Co would replace part of Ni to form the (Ni, Co)₂Si phase, which inhibits the spinodal decomposition process of the Cu-Ni-Si alloy during the aging process. The precipitated phase of the Cu-Ni-Si alloy with a high content of the Co element is more likely to grow with the extension of aging time. The phase transformation kinetic equations of the Cu-Ni-Si alloy at 450 °C and 500 °C showed good agreement with the experimental results. Furthermore, it can be seen from the precipitation kinetic curve the addition of the Co element accelerates precipitation in the aging process.

Stress relaxation tests in cantilever bending were performed on the C7025 and C7035 alloys at 298 K and 393 K, respectively. The effect of stress-relief treatments on stress relaxation properties was investigated. The structural changes associated with the stress relaxation process were examined using transmission electron microscopy. The stress relaxation curve fits well to empirical formula σ* = [K’ln(t + α₀) + C]⁻ⁿ for stress relaxation. The curves can be split into two stages. The stress drops fast at first and then it gets slower in the second stage, and tends towards a certain limited value after a long time. The curve and microstructure reveal that the C7035 alloy has a lower rate of stress relaxation and a higher anti-stress relaxation capacity than the C7025. The first reason is that the movement of vacancies required by spinodal decomposition is inhibited, and the quantity of cobalt-containing vacancies decreases dramatically in the C7035 alloy. The other reason is that the precipitated phases became uniformly diffused in the C7035 alloy. The precipitate phase is uniformly distributed in the grain boundaries and the matrix, during the relaxed condition, and thus the dislocation movement is blocked by the precipitate.