Crystallization and magnetic hardening behaviour of Fe-rich FeSiBNb(Cu) melt-spun alloys

The sequential, multi-stage crystallization and magnetic hardening behaviour of Fe82B14Si2Nb2, Fe83B13Si2Nb2,
Fe83B12Si2Nb2Cu1 and Fe85B13Nb2 melt-spun alloys have been investigated. The microstructure-crystallizationmagnetic
property relationship was established using X-ray diffractometry (XRD), differential scanning calorimetry
(DSC), magnetometry, transmission electron microscopy (TEM) and magneto-optical Kerr effect microscopy
(MOKE) techniques. The increase of Fe content (> 82 at%) of as-quenched ribbons imparts microstructural
heterogeneity across the ribbon cross–section; i.e., textured α-Fe crystals at the free surface to heteroamorphous
microstructure in the bulk matrix. The isochronal annealing of hetero-amorphous alloys depicts
simultaneous surface and bulk crystallization process occurring before and after the crystallization onset temperature
(Tx1) temperature. The annealing temperature range (Ta < Tx1) coinciding with the paramagnetic
region of the thermo-magnetic plot, induces irreversible magnetic hardening due to simultaneous coarsening of
pre-existing crystal nuclei and exchange de-coupling between nanocrystal and intergranular matrix. The onset of
primary crystallization in the pre-crystallized ribbons results in bimodal nanocrystallite distribution having an
average crystallite size exceeding the ferromagnetic exchange length of Fe-based alloys. The minor Cu addition
alters the growth morphology of pre-existing nuclei from dendrite-like to equiaxed, assisting heterogeneous
nucleation and improving intergranular amorphous stability by delaying boride precipitation. The soft-magnetic
property deterioration of partially crystallized ribbons is discussed within the framework of Extended-Random
Anisotropy models.