Evidence for new crystalline phases formed during early stages of crystallization of amorphous FeCuNbSiB alloys

Abstract

Amorphous FeCuNbSiB alloys with composition near Fe_73.5Cu₁Nb₃Si_13.5B₉ (the well-studied FINEMET alloy) were annealed at 520 °C for different durations of time, t_A, varying from 3 to 20 min. Detailed investigation of the structure and composition of crystalline phases formed during the initial stages of crystallization in these alloys using techniques such as x-ray diffraction, Mossbauer spectroscopy, scanning electron microscopy, atomic force microscopy and energy dispersive absorption of x-rays, revealed the following. New crystalline phases, tetragonal Fe₃B and hexagonal Fe₂Si, not reported previously, exist in all the nanocrystalline alloys in question in contrast to the well-documented cubic Fe-Si phase with the DO₃ structure which coexists with the Fe₃B and Fe₂Si phases in some compositions only. The crystallization of Fe₃B and Fe₂Si nanocrystalline grains starts at the surface of the ribbons and then proceeds to the bulk whereas the crystallization of DO₃ Fe-Si gets initiated within the bulk. The average size of the nanocrystalline grains of the Fe₃B and Fe₂Si and cubic DO₃ Fe-Si structures in the residual amorphous matrix is around 20 nm but their volume fractions are as low as ≈ 5%, 10%, and 7%, respectively. The cubic Fe-Si nanocrystals of the DO₃ structure have a silicon concentration in the range 15-20 at.%. The magnetic moments in the amorphous precursor point, on average, are 40° out of the ribbon plane while in the nanocrystalline alloys this angle varies between 2° and 19° depending on the Fe concentration.