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 Fe(67)Co(10)Ln(3)B(20) (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 Delta T-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. Delta T-x (Delta T-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.