Nanocrystallization and Core-loss properties of Fe-rich FeSiBPNbCu nanocrystalline alloy

The present work investigates the structure, soft-magnetic and core-loss properties of 25 mm wide Fe83Si2B9P4Nb1Cu1 nanocrystalline ribbons. The melt-spun precursor ribbons of the thickness of 22 and 28 mu m were prepared using commercial raw materials under ambient atmosphere. The XRD and DSC results show predominantly amorphous and hetero-amorphous structure for 22 and 28 mu m ribbons in the as-quenched state. Further, the partial crystallization annealing leads to thickness dependent nanocrystallization process, wherein the thinner 22 mu m ribbon shows sluggish primary(alpha-Fe(Si)) and delayed secondary(Fe-3(B0.8P0.2)) crystallization process compared to 28 mu m ribbons. The difference is explained through as-quenched precursor matrix structure and selective solute re-distribution of the intergranular region during nanocrystallization. Moreover, the higher crystallite size (D) and volume fraction(V-cr) of alpha-Fe(Si) nanocrystallites are observed for the 28 mu m ribbon in the optimal annealing window. Under optimal annealing conditions, the 28 mu m nanocrystalline ribbon (733 K) shows better AC soft-magnetic properties including B-800 of 1.62 T, H-c of 15.7 A/m, B-r/B-s ratio > 0.8 compared to B-800 of 1.59 T, H-c of 17 A/m and B-r/B-s ratio > 0.7 of 22 mu m (743 K) ribbon. The 28 mu m ribbon shows a low core loss (P) of 0.34 W/kg under 50 Hz, 1.5 T compared to P > 0.65 W/kg for 22 mu m ribbons. The lower core-loss behaviour of thicker 28 mu m in the sub-kHz frequency regime has been explained based on the nanocrystalline microstructure and loss coefficients (hysteresis and eddy current). The work compares the AC core-loss properties with reported Fe-rich nanocrystalline alloys and also discusses the scope of improving B800 beyond 1.65 T for the present alloys.