Effects of Nb addition on the microstructures and mechanical properties of a precipitation hardening Cu-9Ni-6Sn alloy

The effects of Nb addition on the microstructures and mechanical properties of the casting Cu-9Ni-6Sn alloy were investigated in the present work. Microstructures of the as-cast and heat treated samples were characterized by optical microscopy (OM), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The results demonstrate that, with the addition of Nb from 0.0 wt% to 0.35 wt%, the average grain size of Cu-9Ni-6Sn alloys in the as-cast condition is reduced from 570 µm to 156 µm due to the formation of Nb-bearing intermetallic compounds. The occurrence of NbNi3 particles after solution treatment is considered as an important factor that leads to the postponed hardening response to age treatment in the early stage. For the peak-aged samples with 0.35 wt% Nb addition, the tensile strength is increased from 768.9 to 913.3 MPa when compared to that of the Nb-free samples. In addition, for the peak-aged samples, the hardness increment is mainly attributed to the coherent stress generated by the ordered γ' phases, which results in practically identical hardness (~350 HV) regardless of the Nb additions. Furthermore, an optimized combination of UTS (719.5 MPa), elongation (13.76%) and electrical conductivity (11.37% IACS) of the Cu-9Ni-6Sn alloy can be achieved by the 0.35 wt% Nb addition accompanied with aging treatment at 375 ℃ for 1 h.