This work aimed to improve age-hardenable Cu-Ni-Sn=alloys that have high strengths, excellent stress relaxation resistances, and moderate electrical conductivities. A Cu-9wt%Ni-9wt%Sn alloy with proposed compositions of Ni and Sn, and Cu-9wt%Ni-6wt%Sn, Cu-15wt%Ni-8wt%Sn, and Cu-21wt%Ni-5.5wt%Sn alloys with almost the same compositions of Ni and Sn as commercial Cu-Ni-Sn alloys were first peak-aged at 400°C, then cold-rolled to 60% or 80%, and finally annealed at 400°C to improve the stress relaxation resistance of the four cold-rolled alloys ((9-9), (9-6), (15-8), and (21-5.5) alloys). The (15-8) and (21-5.5) alloys exhibited higher tensile strengths (σu≈1300 MPa) than the (9-9) and (9-6) alloys (σu≈1200 MPa), but lower electrical conductivities (E =9 and 6%IACS) than the latter alloys (E =12 and 11%IACS). The differences in the σu between the four alloys were caused by the differences in the dislocation density, deformation twin boundary spacing, and inter-precipitate spacing between them. The four alloys had good stress relaxation resistances. This was attributable to both the decrease in the density of mobile dislocations by the formation of Cottrell atmospheres of Sn atoms on the dislocations introduced by cold rolling during annealing, and the increase in resistance to moving dislocations by Cottrell atmospheres of Ni and Sn atoms on the moving dislocations caused by their chemical bonding. The stress relaxation resistance of the (21-5.5) alloy was the best of the four alloys (the lowest stress relaxation rate R=9%), and the (15-8) alloy exhibited the highest value of R=19. The differences in the R between the four alloys were mainly ascribed to the differences in the amounts of dissolution of Ni and Sn in the Cu matrix between them.
