Atom probe analysis and magnetic properties of nanocrystalline Fe84.3Si4B8P3Cu0.7

We have investigated the microstructure and the magnetic properties of as-cast and flash annealed (4 s at 420 °C-560 °C) Fe84.3Si4B8P3Cu0.7 melt-spun ribbons. The scanning electron microscopy-electron backscattered diffraction (SEM-EBSD) analyses proved the existence of crystalline layer with the thickness of 1 μm in the as-cast state on both the wheel-contacted and the free surfaces of ribbons. Transmission electron microscopy (TEM) and X-ray diffraction (XRD) patterns showed that the crystalline surfaces were textured. Moreover, the microstructure, composition and phase evolution of surface crystallization were studied using TEM and 3DAP which the spherulitic microstructure on the surface was demonstrated. Flash annealing above 420 °C led to a nanocrystalline microstructure of containing 10-15 nm α-Fe (Si) crystallites embedded in a residual amorphous matrix containing Fe, Si, B and P. Three-dimensional atom probe (3DAP) and high resolution transmission electron microscopy (HRTEM) analysis revealed that boron and phosphorous were rejected from α-Fe (Si) phase and enriched in the residual amorphous phase. The nucleation of the nanocrystals occurs heterogeneously from the 3-5 nm α-Fe crystals that were present in the as-cast state. The saturation magnetic induction (Bs) increases from Bs = 1.55 T in the amorphous state to Bs = 1.76 T after the crystallization. The saturation magnetostriction constant, λs, decreased from originally 35 ppm in the as-cast state to about 14 ppm with coercive fields in the range of Hc = 20-30 A/m.