Structural Phase Evolution in the Equilibrium Immiscible Co–Cu System Studied by Solid-State Reaction/Ion Beam Mixing of Nano-Sized Multilayers

Unique structural phase evolution was observed in the nano-sized Co–Cu multilayers upon solid-state reaction/ion beam mixing and resulted in the formation of several metastable phases in the equilibrium immiscible Co–Cu system. Despite of a positive heat of formation of the Co–Cu system, a superlattice was obtained in the nano-sized Co 34 Cu 66 multilayered films upon 600 K thermal annealing, and it later evolved into a dodecagonal phase when the annealing temperature was risen up to 800 K. Comparatively, by ion beam mixing, other metastable phases were obtained, e.g. , an amorphous phase and a dodecagonal phase were obtained in the Co 15 Cu 85 films. The same dodecagonal phase was also formed in the Co 50 Cu 50 films and the ion mixed dodecagons tended to assemble themselves into a fourfold rotational symmetry at relatively high irradiation dose. In the present paper, thermodynamic modeling and the formation mechanism of the metastable phases were also discussed based on Miedima's model and consideration of the physical process driven far-from-equilibrium by ion beam mixing.