TEM-EDX observations of the microstructure of electrodeposited Ni-Sn alloys

The microstructure of Ni-Sn alloys electrodeposited at 65°C and in 50-5000 A/m ² from acidic aqueous solutions containing 1.25mol/L NiCl ₂ and 0.03-0.41 mol/L SnCl ₂ were examined by TEM-EDX. In the Ni-Sn alloy deposition, Ni and Sn were codeposited at an atomic ratio of 1: 1 under the wide ranges of solution compositions and current densities. EDX mapping images of a cross section of Ni-50 at%Sn alloy deposited at current densities more than 1000 A/m ² revealed a lamellar phase parallel to the interface between substrate and deposit. The lamellar phase were identified as Ni ₃Sn ₄ and Ni ₃Sn ₂ by EDX line analysis. It was concluded that the Ni-50 at%Sn alloys were not composed of the metastable-phase NiSn single alloy reported previously, but consisted of two Ni ₃Sn ₄ and Ni ₃Sn ₄ alloy phases in a thermodynamically stable instead.