The effect of Co addition on the modulated structure coarsening and discontinuous precipitation growth kinetics of Cu–15Ni–8Sn alloy

To understand the effect of Co addition on the modulated structure coarsening and discontinuous precipitation (DP) growth kinetics of Cu–15Ni–8Sn alloy, the experiments are carried out using optical microscope (OM), scanning electron microscope (SEM) and transmission electron microscopy (TEM). Our results showed that the addition of Co in Cu–15Ni–8Sn alloy did not recover with a new second-phase associated with Co. During the aging treatment process, the addition of Co reduced the coarsening kinetics of the modulated structure, which was related to the equilibrium solid solubility (Xe) of Ni and Sn in Cu, diffusion coefficient of Ni and Sn in Cu (D), and to the interfacial energy (σ) of α-Cu matrix/L12. The decrease in values of Xe, D and σ were resulted in a slowdown of the coarsening kinetics. Further, the suitable addition of Co significantly inhibited the nucleation and growth of discontinuous precipitation (DP). The growth kinetics of DP for Cu–15Ni–8Sn-xCo alloy were evaluated using the Johnson-Mehl-Avrami-Kolmogorov equation. With the combination of the Arrhenius equation, the activation energies (Q) of the samples were 105.45 kJ/mol, 150.76 kJ/mol, 178.44 kJ/mol, 251.53 kJ/mol, and 148.89 kJ. Compared with cobalt-free alloy, cobalt-containing alloys exhibited greater activation energy, which suggested that the addition of Co can greatly inhibit the formation of DP. Furthermore, the addition of Co led to decrease in the thickness of the grain boundary and increase in the interlamellar spacing of the DP, which were considered to be the main reasons for suppressing the growth of DP.