Characteristics of intermetallic phases in Cu/(Sn,Ni) diffusion couples annealed at 220 °C

The influence of Ni addition (5 at.%) on the morphology and chemical composition of the phases formedduring solid state reaction in Cu/(Sn,Ni) diffusion couples, annealed at 220 C for different periods oftime, 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 andcontinuous layer of the Cu$_3$Sn phase was observed. Moving towards the (Sn,Ni) end of the diffusioncouple, the (Cu$_{1x}$Ni$_x$)$_6$Sn$_5$ phase was identified. This phase was represented by two types of structures: adiscontinuous layer located close to the Cu$_3$Sn phase, and precipitates (needles or faced) within the(Sn,Ni) end. These structures of (Cu$_{1x}$Ni$_x$)$_6$Sn$_5$ also varied in chemical composition. The experiment withsynchrotron radiation demonstrated two crystallographic variants of the Cu$_6$Sn$_5$ phase: hightemperaturehexagonal $\eta$ and low-temperature monoclinic $\eta'$; however, only the hexagonal variantwas confirmed by TEM. Differences in the morphology and chemical composition of the (Cu$_{1x}$Ni$_x$)$_6$Sn$_5$ phase were attributed to various mechanisms of their formation. The precipitates with a higher contentof Ni were most probably transformed from the Ni$_3$Sn$_4$ 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. Afterthe annealing experiment, the (Ni$_{1x}$Cu$_x$)$_3$Sn$_4$ phase was observed beyond the interface area as small, irregularly distributed precipitates in the (Sn,Ni) end-member. TEM examination allowed for the precisephase characterisation of the mentioned intermetallics. Moreover, except for the strong reflectionsvisible in SADP fitted to the hexagonal $\eta$-Cu$_6$Sn$_5$ phase, additional reflections were observed andassigned to the cubic Cu$_9$NiSn$_3$ phase