Non-equilibrium crystalline phases formed in the Co-Ta system by ion beam mixing

Co-rich Co-Ta solid solutions of the high-temperature Co FCC structure were synthesized by 200 keV room-temperature xenon ion mixing. The experimental results revealed that these FCC solid solutions were formed gradually from the Co hexagonal solid solutions through a HCP → FCC transition, that the solid solubility was greatly beyond the equilibrium value up to 20 at.% Ta, and that their lattice parameters followed the prediction of Vegard's law. Interestingly, a new metastable crystalline phase of FCC structure with a = 4.43 A was formed in two Ta-rich films, i.e. Co30Ta70 and Co25Ta75 multilayered films, at different irradiation stages. The thermal stabilities of the Co-rich and the Ta-rich metastable crystalline phases were also studied by thermal annealing. (author)