Effect of the Nb content on the amorphization process of the mechanically alloyed Fe–Co–Nb–B powders

Mechanical alloying was used to prepare Fe57Co21Nb7B15 (7Nb) and Fe61Co21Nb3B15 (3Nb) powder mixtures in a high-energy planetary ball mill under argon atmosphere. Thermal stability, structural and magnetic properties were followed by differential scanning calorimetry, X-ray diffraction, vibrating sample magnetometry and Mössbauer spectrometry. For the Fe57Co21Nb7B15 powders, an amorphous matrix (∼69%) where nanocrystalline bcc FeCo, bcc Nb(B), tetragonal Fe2B and orthorhombic Fe3B type phases were embedded is reached after 96 h of milling. However, for the Fe61Co21Nb3B15 mixture, the partially amorphous structure (∼75%) is achieved within 48 h of milling. Further milling time (up to 96 h) gives rise to the primary crystallization of α-Fe in addition to bcc FeCo, orthorhombic FeB, tetragonal Fe2B and tetragonal Fe3B type borides. Lower Nb content powders (3Nb) exhibit higher saturation magnetization, Ms, higher coercive field, Hc, and lower glass transition temperature values than those of the 7Nb powders.

► We report for the first time the mechanical recrystallization in the FeCoNbB alloys.
► Partially amorphous structure is obtained after 48 h for both 3Nb and 7Nb mixtures.
► 3Nb content powders exhibit higher Ms, higher Hc and lower Tg than 7Nb powders.
► Curie temperature of the amorphous phase is higher for 7Nb powders.