Effect of lanthanide element on glass-forming ability and local atomic structure of Fe-Co-(Ln)-B amorphous alloys

The glass-forming ability and the local ordering structures of Fe₇₀Co₁₀B₂₀ and Fe₆₇Co₁₀Ln₃B₂₀ (Ln=Sm, Tb or Dy) alloys have been studied by differential scanning calorimetry (DSC), density measurement and X-ray diffraction method. The supercooled liquid region, defined by the temperature interval ΔT_x between glass transition temperature T_g and onset crystallization temperature T_x, appears and is extended by the addition of lanthanide element in Fe-Co-B systems. ΔT_x (ΔT_x=T_x-T_g) is 32 K for Fe₆₇Co₁₀Sm₃B₂₀, 36 K for Fe₆₇Co₁₀Tb₃B₂₀ and 38 K for Fe₆₇Co₁₀Dy₃B₂₀. The packing fraction, defined by the ratio of total volume of atoms to the volume of a cell, increases by a few percent in these alloys. The least-squares refining analysis for interference functions as well as the usual radial distribution function analysis shows that the coordination number of transition metals around Fe decreases by the addition of the lanthanide element whereas the change in the coordination number of transition metals around B is not clearly observed within the experimental uncertainty. The lanthanide element is considered to play an important role in forming a dense packing structure so as to rearrange the network of local ordering units of (Fe, Co)-B in these alloys.