Co-Ge-Ni phase equilibria and Ni/CoGe interfacial reactions

Background The Co-Ge-Ni ternary system is important for thermoelectric and electronic applications. This study determines the Co-Ge-Ni isothermal sections at 800°C and 600°C. Methods Alloys were prepared from pure constituent elements, equilibrated, and their equilibrium phases were identified. Furthermore, the interfacial reactions between CoGe and Ni at 600°C and 500°C were investigated. The reaction couples were prepared using an annealed CoGe substrate electroplated with Ni. Significant Findings No ternary compound is found. Two continuous solid-solution phases, (Co,Ni)₅Ge₃ and (α-Co,Ni), exist at both 800°C and 600°C. At 800°C, the stable phases along the Co-Ge side are (α-Co,Ni), ε-(Co), (Co,Ni)₅Ge₃, CoGe, CoGe₂, and (Ge), whereas those along the Ge-Ni side are liquid, NiGe, (Co,Ni)₅Ge₃, Ni₃Ge, and (α-Co,Ni). At 600°C, there is no liquid phase, and Co₅Ge₂ and Co₅Ge₇ are stable. The Ni₃Ge phase exhibits significant Co solubility but does not form a continuous solid solution extending to the Co-Ge side. The reaction path of the Ni/CoGe diffusion couples is Ni/Ni₃Ge/(Co,Ni)₅Ge₃/CoGe, and Ge is the dominant diffusion species. The reaction layer thickness is 163 μm at 600°C for 7 days, 56 μm at 500°C for 7 days, and only 1 μm at 300°C for 50 days.