A non-equilibrium crystallization diagram for Co sub-surface layers of the Co75.26-xFe4.74(BSi)20+x amorphous alloy for x > 1 during magnetic field annealing is determined using Transmission Electron Microscopy (TEM). The diagram shows that, depending on the composition and temperature, the Co sub-layer exhibits five distinctively different microstructures when the metallic glass is heat treated below its bulk crystallization temperature. At high boron content, the structure is dominated by the FCC structure with a high degree of oxygen impurity faulting regardless of the annealing temperature. At low concentration, the Co sub-layer exhibits FCC or HCP structures, depending on the temperature. Also found was a two-phase region in which FCC and HCP Co co-existed. Such a diagram serves as a useful guide to obtaining the desired properties as the microstructure is closely linked to the magnetic properties of the material.