Spontaneous formation of Au-Co interface via a Low-Temperature reduction process

Eutectic alloys manufactured by metallurgical methods are widely used in many industrial applications due to their special properties provided by the multi-phase eutectic microstructure. Previous studies introduced the preparation of metals and alloys through the chemical reduction of metal complex salts. We employed this technique to prepare the equiatomic composition in the binary eutectic system Au-Co through a low-temperature solid-gas phase reduction of two precursors: the bi-metallic complex salt [Co(NH₃)₆][AuCl₄]Cl₂, and an equimolar mixture of H[AuCl₄] and pre-reduced cobalt. Complete reduction at 350 °C resulted in the formation of a close-to-equilibrium two-phase composition of fcc gold and hcp cobalt. The structure of the fully reduced metallic materials was composed of interconnected cobalt ligaments and bi-metallic Au-Co (gold-cobalt) particles. Specifically, these spontaneously formed particles featured fcc cobalt islands interfacing with fcc gold at a specific orientation relationship. Under heating, the fully reduced Au-Co products underwent melting that corresponds to the equilibrium eutectic melting point at 996.5 °C. These results imply that spontaneous lattice self-adjustment can occur in multi-component eutectic-type systems at low temperatures directly in the solid state. This may ease the technologies of joining such as epitaxial growth, additive manufacturing, and powder metallurgy.