AbstractThe influence of Ni addition (5 at.%) on the morphology and chemical composition of the phases formed during solid state reaction in Cu/(Sn,Ni) diffusion couples, annealed at 220 °C for different periods of time, was investigated. Chemical analysis of the reaction zone performed using scanning electron microscopy (SEM/EDS) identified several intermetallic phases. Near to the copper substrate, a thin and continuous layer of the Cu3Sn phase was observed. Moving towards the (Sn,Ni) end of the diffusion couple, the (Cu1−xNix)6Sn5 phase was identified. This phase was represented by two types of structures: a discontinuous layer located close to the Cu3Sn phase, and precipitates (needles or faced) within the (Sn,Ni) end. These structures of (Cu1−xNix)6Sn5 also varied in chemical composition. The experiment with synchrotron radiation demonstrated two crystallographic variants of the Cu6Sn5 phase: high-temperature hexagonal η and low-temperature monoclinic η′; however, only the hexagonal variant was confirmed by TEM. Differences in the morphology and chemical composition of the (Cu1−xNix)6Sn5 phase were attributed to various mechanisms of their formation. The precipitates with a higher content of Ni were most probably transformed from the Ni3Sn4 phase present in the initial (Ni,Sn) end-member, while the formation of the Ni-poor layer took place as a result of diffusion at the initial interface. After the annealing experiment, the (Ni1−xCux)3Sn4 phase was observed beyond the interface area as small, irregularly distributed precipitates in the (Sn,Ni) end-member. TEM examination allowed for the precise phase characterisation of the mentioned intermetallics. Moreover, except for the strong reflections visible in SADP fitted to the hexagonal η-Cu6Sn5 phase, additional reflections were observed and assigned to the cubic Cu9NiSn3 phase.